Patent Publication Number: US-2020291359-A1

Title: Ectodermal mesenchymal stem cells and method for producing same

Description:
TECHNICAL FIELD 
     The present invention relates to an ectomesenchymal stem cell and a method for producing the same. The present invention also relates to a method for screening for a multipotent stem cell inducer. 
     BACKGROUND ART 
     Mesenchymal stem cells (MSCs) contained in bone marrow fluids and the like have differentiation potency into various tissues, such as bone, cartilage, fat, muscle, nerve, and epithelium (multi-lineage differentiation potency). Thus, attempts to provide regenerative medicine (cell transplant therapy) using MSCs have become widespread in recent years. However, it is known that MSCs previously used in regenerative medicine gradually lose their proliferation ability and multi-lineage differentiation potency when they are continuously passaged in vitro. It is thus required to find a cell that has higher ability to promote tissue regeneration than common MSCs or a substance that has the effect of activating/inducing the cell in vivo, and to provide a therapeutic method that is more effective than conventional regenerative medicine. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: WO 2012/147470 
     Non Patent Literature 
     Non Patent Literature 1: PNAS 2011 Apr 19; 108(16): 6609-14 
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present invention is to provide an ectomesenchymal stem cell and a method for producing the same. Another object of the present invention is to provide a method for screening for a multipotent stem cell inducer. 
     Solution to Problem 
     The present inventors have found that ectomesenchymal stem cells (EMSCs) induced by necrotic tissue injury and circulating in peripheral blood contribute to the regeneration of injured tissues. This finding is based on the discovery, previously reported by the present inventors (PNAS 2011 Apr. 19; 108(16): 6609-14), of a mechanism whereby “HMGB1 released from necrotic tissue in the exfoliated epidermis in epidermolysis bullosa causes bone marrow mesenchymal stem cells to accumulate in the exfoliated epidermis site via peripheral blood circulation to induce regeneration of the injured skin,” and further based on the results obtained this time, that are “when a skin of an epidermolysis bullosa mouse is grafted on one side of the parabiosis model and a fragment peptide of HMGB1 is administered on the other side, PDGFRα lineage-positive cells accumulate in the skin graft site to regenerate epidermis” and “when a cartilage injury is created in a mouse and a fragment peptide of HMGB1 is administered, PO lineage-positive cells accumulate in the cartilage injury site via peripheral blood circulation to regenerate a cartilage tissue”, and the like. The present inventors also obtained experimental results suggesting that the source of EMSC in peripheral blood may be a certain PDGFRα-positive cell in the bone marrow and that EMSC in peripheral blood may be a cell whose embryological origin is ectomesenchyme generated from the epidermal side of the cranial neural fold. Based on such discoveries, the present inventors have completed the inventions of an ectomesenchymal stem cell, a method for producing the same, and a method for screening for a substance having multipotent stem cell inducing activity, using a cell in peripheral blood induced by a necrotic tissue injury as an indicator. 
     The present inventors have previously found that a peptide consisting of the amino acid sequence of positions 1-44 (MGKGDPKKPRGKMSSYAFFVQTCREEHKKKHPDASVNFSEFSKK) (hereinafter referred to as “HA1-44 peptide”) at the N-terminus of A-box of HMGB1 protein mobilizes multipotent stem cells such as mesenchymal stem cells (MSCs) from bone marrow into peripheral blood to exert a therapeutic effect in various disease models. This time, the present inventors have newly found that administration of the HA1-44 peptide causes reactions in an organism ((i) a change in the configuration of the PDGFRα-positive cell population in peripheral blood, (ii) an increase in the number of PDGFRα-positive cells in peripheral blood, and (iii) a change in gene expression of PDGFRα-positive cells in vertebral bone marrow). Then, the present inventors have found a method for screening for a substance having similar activity to the HA1-44 peptide, i.e., a multipotent stem cell inducer, using these reactions as an indicator, and completed the present invention. That is, the present invention provides a method for determining whether a test substance has similar activity to an HA1-44 peptide (the activity of promoting induction and/or mobilization of multipotent stem cells and/or promoting tissue regeneration), based on whether the same reaction as any of (i) to (iii) above occurs by administering a test substance to a subject such as an animal. 
     The present inventors have also analyzed details of cells that are mobilized by the HA1-44 peptide into peripheral blood and injured tissues and contribute to regeneration of the tissues. As a result, the present inventors have found that (i) cells in peripheral blood that contribute to promoting tissue regeneration by the HAl-44 peptide are PDGFRα-positive cells derived from vertebral bone marrow, (ii) PDGFRα-positive cells derived from vertebral bone marrow have greater differentiation potency into bone, cartilage, and/or fat than PDGFRα-positive cells derived from bone marrows of other bones, and (iii) a small amount of PDGFRα-positive cells having the developmental lineage (Prx1 lineage-negative) same as PDGFRα-positive cells derived from vertebral bone marrow are also present in bone marrows from other bones. Based on these findings, the present inventors have found that, by culturing a cell population derived from a biological tissue containing MSCs, such as a bone marrow or peripheral blood on a dish to form a colony, subcloning each colony, and selecting a cell clone exhibiting high differentiation potency to bone, cartilage, and/or fat, multipotent stem cells having a higher ability to promote tissue regeneration than MSCs obtained by conventional methods (which collect a bone marrow and culture on a dish) can be obtained, and have completed the present invention. 
     Specifically, the present invention relates to the followings:
     A) An ectomesenchymal stem cell.   B) A method for producing an ectomesenchymal stem cell.   C) A cell obtained by the method according to item B).   D) A cell or cell population in peripheral blood, induced by an MSC in-blood-mobilizing substance.   E) A cell or cell population in the vertebral bone marrow, induced by a peptide consisting of the amino acid sequence of positions 1-44 (SEQ ID NO: 1) at the N-terminus of A-box of the high-mobility group box 1 (HMGB1) protein (hereinafter also referred to as “HA1-44 peptide”).   F) A method for producing the cell or cell population according to item D) or E).   G) A method for obtaining, isolating, and/or enriching a cell having a high tissue regeneration promoting ability similar to a PDGFRα-positive cell in a vertebral bone marrow, from a biological tissue containing a mesenchymal stem cell (MSC).   H) A cell or cell population obtained by the method according to item G).   I) A composition for use in promoting tissue regeneration, containing an ectomesenchymal stem cell.   J) A method for screening for a substance having inducing activity of a multipotent stem cell, using a cell in peripheral blood induced by a necrotic tissue injury as an indicator.   K) A method for screening for a substance having inducing activity of a multipotent stem cell, using an HA1-44 peptide as a positive control and a reaction of multipotent stem cells that contribute to tissue regeneration in vivo as an indicator.   L) A method for determining an expected tissue regeneration promoting effect in a subject to which an MSC in-blood-mobilizing substance has been administered, using a colony-forming Pα cell in peripheral blood as an indicator.   

     More specifically, the present invention relates to the followings.
     a) A colony-forming PDGFR-positive cell having characteristic i) and characteristics ii) and/or iii) below:   

     i) having differentiation potency into an osteoblast, an adipocyte, and a chondrocyte; 
     ii) having differentiation potency into an epidermal cell; 
     iii) being P0 lineage-positive.
     b) The cell according to item a), wherein the cell is PDGFRα-positive.   c) A cell according to item a) or b), wherein the cell has one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − .   d) A vertebral bone marrow-derived cell that is PDGFRα-positive, CD34-positive, and Sca 1 -negative.   e) A method for producing a colony-forming PDGFR-positive cell, comprising any one of steps 1) to 4) below:   

     1) collecting peripheral blood from a subject having a necrotic tissue injury, and culturing the peripheral blood on a solid phase; 
     2) collecting peripheral blood from a subject having a necrotic tissue injury, culturing the peripheral blood on a solid phase, and then selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , p0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − ; 
     3) collecting peripheral blood from a subject having a necrotic tissue injury, and selectively recovering a cell having one or more characteristics selected from pα + , Pα lin+ , p0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  from the peripheral blood; 
     4) collecting peripheral blood from a subject having a necrotic tissue injury, selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , p0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  from the peripheral blood, and culturing the cell on a solid phase.
     f) A method for producing a colony-forming PDGFR-positive cell, comprising any one of steps 1) to 4) below:   

     1) culturing peripheral blood collected from a subject having a necrotic tissue injury on a solid phase; 
     2) culturing peripheral blood collected from a subject having a necrotic tissue injury on a solid phase, and then selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , p0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − ; 
     3) selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , p0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  from peripheral blood collected from a subject having a necrotic tissue injury; 
     4) selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , p0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  from peripheral blood collected from a subject having a necrotic tissue injury, and culturing the cell on a solid phase.
     g) A method for producing a colony-forming PDGFR-positive cell, comprising any one of steps 1) to 4) below:   

     1) collecting a vertebral bone marrow from a subject, and culturing the vertebral bone marrow on a solid phase; 
     2) collecting a vertebral bone marrow from a subject, culturing the vertebral bone marrow on a solid phase, and then selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin− ; 
     3) collecting a vertebral bone marrow from a subject, and selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin−  from the vertebral bone marrow; 
     4) collecting a vertebral bone marrow from a subject, selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− ,and Sox1 lin−  from the vertebral bone marrow, and culturing the cell on a solid phase.
     h) A method for producing a colony-forming PDGFR-positive cell, comprising any one of steps 1) to 4) below:   

     1) culturing a vertebral bone marrow collected from a subject on a solid phase; 
     2) culturing a vertebral bone marrow collected from a subject on a solid phase, and then selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin− ; 
     3) selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin−  from a vertebral bone marrow collected from a subject; 
     4) selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin−  from a vertebral bone marrow collected from a subject, and culturing the cell on a solid phase.
     i) A cell population obtained by administering an MSC in-blood-mobilizing substance to a subject, collecting peripheral blood from the subject, and culturing the collected peripheral blood on a solid phase.   j) The cell population according to item i), wherein the MSC in-blood-mobilizing substance is an HA1-44 peptide.   k) A method for producing a cell, comprising a step of administering an MSC in-blood-mobilizing substance to a subject, collecting peripheral blood from the subject, and culturing the collected peripheral blood on a solid phase.   l) A method for producing a cell, comprising a step of culturing peripheral blood collected from a subject to which an MSC in-blood-mobilizing substance has been administered on a solid phase.   m) The method according to item k) or l), wherein the MSC in-blood-mobilizing substance is an HA1-44 peptide.   n) A cell population obtained by 1) administering an HA1-44 peptide to a subject, 2) collecting a vertebral bone marrow from the subject, and 3) culturing the collected bone marrow on a solid phase or sorting a PDGFRα-positive cell from the collected bone marrow.   

     o) A method for producing a cell, comprising the steps of 1) administering an HA1-44 peptide to a subject, 2) collecting a vertebral bone marrow from the subject, and 3) culturing the collected bone marrow on a solid phase or sorting a PDGFRα-positive cell from the collected bone marrow.
     p) A method for producing a cell, comprising a step of culturing a vertebral bone marrow collected from a subject to which an HA1-44 peptide has been administered on a solid phase or sorting a PDGFRα-positive cell from the collected bone marrow.   q) A method for producing a cell population, comprising the steps of:   

     1) culturing a cell population from a biological tissue containing MSC on a solid phase; 
     2) subcloning a colony obtained in step 1); 
     3) culturing a portion of cells obtained by the subcloning in a differentiation-inducing medium into bone, cartilage, and/or fat, and measuring an expression level of a differentiation marker of bone, cartilage, and/or fat; and 
     4) selecting a cell clone showing a high expression level compared to the expression level of a differentiation marker of bone, cartilage, and/or fat in case that MSC obtained by culturing a femoral bone marrow on a solid phase are cultured in a differentiation-inducing medium into bone, cartilage, and/or fat.
     r) A method for producing a cell population, comprising the steps of:   

     1) culturing a cell population derived from a biological tissue containing MSC on a solid phase; and 2) selecting a colony having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − .
     s) The method according to item r), wherein step 2) is a step of selecting a Prx1 lineage-negative colony.   t) A method for producing a cell population, comprising a step of selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  from a cell population derived from a biological tissue containing MSC.   u) A method for producing a cell population, comprising the steps of:   

     1) selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  from a cell population derived from a biological tissue containing MSC; and 
     2) culturing the cell recovered in step 1) on a solid phase.
     v) A cell or cell population obtained by the method according to claims *to*.   w) A composition for use in promoting tissue regeneration, comprising a colony-forming PDGFR-positive cell having characteristic i) and characteristics ii) and/or iii) below:   

     i) having differentiation potency into an osteoblast, an adipocyte and a chondrocyte; 
     ii) having differentiation potency into an epidermal cell; 
     iii) being P0 lineage-positive.
     x) The composition according to claim*, for use in promoting regeneration of a tissue derived from mesoderm or ectoderm.   y) A method for screening for a multipotent stem cell inducer, comprising the steps of:   

     1) collecting peripheral blood from a subject, and counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in the peripheral blood; 
     2) collecting peripheral blood from a subject to which a test substance has been administered, and counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in the peripheral blood; and
     3) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the number of cells counted in step 2) is larger than the number of cells counted in step 1).   z) A method for screening for a multipotent stem cell inducer, comprising the steps of:   

     1) counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in peripheral blood collected from a subject; 
     2) counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in peripheral blood collected from a subject to which a test substance has been administered; and 
     3) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the number of cells counted in step 2) is larger than the number of cells counted in step 1).
     aa) A method for screening for a multipotent stem cell inducer, comprising the steps of:   

     1) collecting peripheral blood from a subject, and culturing the peripheral blood on a solid phase to obtain an adhesive cell population; 
     2) performing an exhaustive gene expression analysis on the cell population obtained in step 1 on a colony or single-cell basis; 
     3) administering a peptide consisting of an amino acid sequence of SEQ ID NO: 1 (HA1-44 peptide) to a subject, collecting peripheral blood, and culturing the peripheral blood on a solid phase to obtain an adhesive cell population; 
     4) performing an exhaustive gene expression analysis on the cell population obtained in step 3 on a colony or single-cell basis; 
     5) administering a test substance to a subject, collecting peripheral blood, and culturing the peripheral blood on a solid phase to obtain an adhesive cell population; 
     6) performing an exhaustive gene expression analysis on the cell population obtained in step 5 on a colony or single-cell basis; 
     7) pooling gene expression data obtained in steps 2 and 4, and performing a clustering analysis; 
     8) pooling gene expression data obtained in steps 2 and 6, and performing a clustering analysis; and 9) comparing an analysis result of step 7 to an analysis result of step 8, and selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the cell population obtained in step 5 (test substance administration group) has the same cluster configuration as the cell population obtained in step 3 (HA1-44 peptide administration group).
     ab) The method according to item aa), wherein the test substance is administered in place of the HA1-44 peptide in step 3 and the HA1-44 peptide is administered in place of the test substance in step 5.   ac) The method according to item aa) or ab), wherein the exhaustive gene expression analysis is RNA sequencing (RNA-seq).   ad) The method according to any one of items aa) to ac), wherein clustering analysis is performed using an iterative clustering and guide-gene selection (ICGS) algorithm.   ae) A method for screening for a multipotent stem cell inducer, comprising the steps of:   

     1) culturing peripheral blood collected from a subject on a solid phase to obtain an adhesive cell population; 
     2) performing an exhaustive gene expression analysis on the cell population obtained in step 1) on a colony or single-cell basis; 
     3) culturing peripheral blood collected from a subject to which a peptide consisting of an amino acid sequence of SEQ ID NO: 1 (HA1-44 peptide) has been administered on a solid phase to obtain an adhesive cell population; 
     4) performing an exhaustive gene expression analysis on the cell population obtained in step 3) on a colony or single-cell basis; 
     5) culturing peripheral blood collected from a subject to which a test substance has been administered on a solid phase to obtain an adhesive cell population; 
     6) performing an exhaustive gene expression analysis on the cell population obtained in step 5) on a colony or single-cell basis; 
     7) pooling gene expression data obtained in steps 2) and 4), and performing a clustering analysis; 
     8) pooling gene expression data obtained in steps 2) and 6), and performing a clustering analysis; and 
     9) comparing an analysis result of step 7) to an analysis result of step 8), and selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the cell population obtained in step 5) has the same cluster configuration as the cell population obtained in step 3).
     af) A method for screening for a multipotent stem cell inducer, comprising the steps of:   

     1) collecting peripheral blood from a subject, and culturing the peripheral blood on a solid phase to obtain an adhesive cell population; 
     2) counting the number of colonies obtained in step 1); 
     3) administering a test substance to a subject, collecting peripheral blood, and culturing the peripheral blood on a solid phase to obtain an adhesive cell population; 
     4) counting the number of colonies obtained in step 3); and
     5) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the number of colonies counted in step 4) is larger than the number of colonies counted in step 2).   ag) A method for screening for a multipotent stem cell inducer, comprising the steps of:   

     1) culturing peripheral blood collected from a subject on a solid phase to obtain an adhesive cell population; 
     2) counting the number of colonies obtained in step 1); 
     3) culturing peripheral blood collected from a subject to which a test substance has been administered on a solid phase to obtain an adhesive cell population; 
     4) counting the number of colonies obtained in step 3); and 
     5) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the number of colonies counted in step 4) is larger than the number of colonies counted in step 2).
     ah) The screening method according to item ae) or af), wherein the colony counted in steps 2) and 4) is a colony having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − .   ai) A method for screening for a multipotent stem cell inducer, comprising the steps of:   

     1) collecting a bone marrow from a vertebra of a subject, and obtaining a PDGFRα-positive cell population by culturing on a solid phase or cell-sorting; 
     2) performing an exhaustive gene expression analysis on the cell population obtained in step 1 on a colony or single-cell basis; 
     3) administering a test substance to a subject, collecting a bone marrow from vertebra, and obtaining a PDGFRα-positive cell population by culturing on a solid phase or cell-sorting; 
     4) performing an exhaustive gene expression analysis on the cell population obtained in step 3 on a colony or single-cell basis; 
     5) pooling gene expression data obtained in steps 2 and 4, and performing a pathway analysis; and 
     6) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when, as a result of the analysis of step 5, (i) a pathway associated with EIF2 signaling, regulation of eIF4 and p70S6K signaling, and/or mTOR signaling is activated or (ii) expression of a cell death-related gene is suppressed, in the cell population obtained in step 3 (test substance administration group) compared to the cell population obtained in step 1 (untreated group). 
     aj) A method for screening for a multipotent stem cell inducer, comprising the steps of: 
     1) obtaining a PDGFRα-positive cell population by culturing on a solid phase or cell-sorting from a bone marrow collected from a vertebra of a subject; 
     2) performing an exhaustive gene expression analysis on the cell population obtained in step 1) on a colony or single-cell basis; 
     3) obtaining a PDGFRα-positive cell population by culturing on a solid phase or cell-sorting from a bone marrow collected from a vertebra of a subject to which a test substance has been administered; 
     4) performing an exhaustive gene expression analysis on the cell population obtained in step 3) on a colony or single-cell basis; 
     5) pooling gene expression data obtained in steps 2) and 4), and performing a pathway analysis; and 
     6) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when, as a result of the analysis of step 5), (i) a pathway associated with EIF2 signaling, regulation of eIF4 and p70S6K signaling, and/or mTOR signaling is activated or (ii) expression of a cell death-related gene is suppressed, in the cell population obtained in step 3) compared to the cell population obtained in step 1).
     ak) The method according to item ai) or aj), wherein the exhaustive gene expression analysis is RNA sequencing (RNA-seq).   al) A method for determining a tissue regeneration-promoting effect of an MSC in-blood-mobilizing substance, comprising the steps of:   

     1) counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in peripheral blood collected from a subject before administering an MSC in-blood-mobilizing substance; and 
     2) counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in peripheral blood collected from the subject after administering an MSC in-blood-mobilizing substance, 
     wherein tissue regeneration is suggested to be promoted in the subject when the number of cells counted in step 2) is larger than the number of cells counted in step 1). 
     Advantageous Effects of Invention 
     According to the present invention, an ectomesenchymal stem cell and a method for producing the same can be provided. Furthermore, a method for screening for a multipotent stem cell inducer can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram plotting the number of Pα cells in peripheral blood and HMGB1 concentration for each of the skin flap-created mouse and the skin flap-not created mouse. 
         FIG. 2  is photographs of a colony obtained by culturing peripheral blood of mice, and a graph showing CFU activity converted per mL of peripheral blood. 
         FIG. 3  is photographs showing results of differentiation induction of iCFPα cells obtained from mouse peripheral blood into osteoblasts, adipocytes, chondrocytes, and keratin-5 expressing cells. Osteoblasts were detected by ALP staining, adipocytes were detected by Oil Red-O staining, chondrocytes were detected by Toluidine blue staining, and keratin-5 expressing cells were detected by fluorescence of reporter protein tdTomato. 
         FIG. 4  is a diagram showing the results of performing single-cell transcriptome analysis on iCFPα cells and performing clustering analysis based on the obtained data. The cell types shown on the left are predicted cell types based on gene expression profiles (such as high expression of a particular gene set). 
         FIG. 5  is a diagram showing the results of performing transcriptome analysis on iCFPα cells on a colony basis and performing clustering analysis. The cell types shown on the left are predicted cell types based on gene expression profiles (such as high expression of a particular gene set). In the diagram, one column corresponds to one colony. 
         FIG. 6  is a photograph of colony-forming cells obtained by culturing peripheral blood of Pα-H2B-GFP mice. 
         FIG. 7  is a graph showing the results of examining negative/positive rates of Pα lineage, P0 lineage, Prx1 lineage, Sox1 lineage, and LepR lineage for iCFPα cells. 
         FIG. 8  is a graph showing the results of CFU activity of colony-forming cells obtained from peripheral blood assessed by Prx1 lineage for each of the skin flap-created mouse and the skin flap-not created mouse. 
         FIG. 9  is photographs showing the results of detecting Pα expression and Prx1 lineage for cells present in bone marrow tissue of the femur, vertebra, sternum, ilium, hip joint (femoral head and lumbar lid) and skull of Pα-H2B-GFP::Prx1-Cre::Rosa26-tdTomato mice. 
         FIG. 10  is a diagram showing the results of FACS analysis on bone marrow cells in the femur, vertebra, sternum and ilium of Pα-H2B-GFP::Prx1-Cre::Rosa26-tdTomato mice. 
         FIG. 11  is a graph showing the results of examining P0 lineage, Prx1 lineage, Sox1 lineage and LepR lineage for CFPα cells derived from vertebral bone marrow. 
         FIG. 12  is a graph showing the results of examining P0 lineage, Prx1 lineage, Sox1 lineage and LepR lineage for CFPα cells derived from femoral bone marrow. 
         FIG. 13  shows (a) photographs of colonies, (b) colony numbers, and (c) growth curves for vertebral and femoral bone marrow-derived CFPα cells. The transverse axis of (c) indicates the passage number (P0=primary culture). 
         FIG. 14  is graphs showing the results of culturing vertebral and femoral bone marrow-derived CFPα cells under a differentiation-inducing condition into adipocytes, and examining the expression of differentiation markers of adipocytes. 
         FIG. 15  is graphs showing the results of culturing vertebral and femoral bone marrow-derived CFPα cells under a differentiation-inducing condition into osteoblasts, and examining the expression of differentiation markers of osteoblasts. 
         FIG. 16  is (a) graphs showing the results of culturing vertebral and femoral bone marrow-derived CFPα cells under a differentiation-inducing condition into chondrocytes, and examining the expression of differentiation markers of chondrocytes, and diagrams showing (b) photographs of formed chondropellets and (c) the weight of chondropellets. 
         FIG. 17  is photographs of K5-expressing cells obtained by culturing vertebral bone marrow-derived CFPα cells under a differentiation-inducing condition into keratinocytes. 
         FIG. 18  is photographs of K5-expressing cells obtained by culturing femoral bone marrow-derived CFPα cells under a differentiation-inducing condition into keratinocytes. 
         FIG. 19  is diagrams showing the results of performing single-cell transcriptome analysis on Pα cells in vertebral and femoral bone marrows and performing clustering analysis based on the obtained data. 
         FIG. 20  is a graph showing the results of separating Pα cells in vertebral bone marrow into four populations with FACS using Sca1 and CD34 expressions as indicators, and performing a CFU assay on the four populations. 
         FIG. 21  is a diagram showing the results of subjecting iCFPα cells in peripheral blood to clustering analysis with vertebral and femoral Pα cells. 
         FIG. 22  is a diagram showing the expression of Procr in cells of the S34-MSC cluster in bone marrow. 
         FIG. 23  is a graph showing the amount of Sca1 + CD34 +  cells present in the bone marrow of cervical vertebra, thoracic vertebra, lumbar vertebra and femur of Pα-H2B-GFP mice (as a percentage relative to PDGFRα + CD45 −  live cells). 
         FIG. 24  is (a) photographs of colony-forming cells obtained by culturing peripheral blood, and (b) a graph showing CFU activity of the cells converted per mL of peripheral blood. 
         FIG. 25  is (a) a diagram showing a schematic of the parabiosis model, and (b) photographs showing the observation results of grafted skin tissue after administration of the HA1-44 peptide. Pα cells were detected with YFP fluorescence, and type 7 collagen was detected with antibodies. 
         FIG. 26  is (a) a diagram showing a schematic of a parabiosis model, (b) photographs showing observation results of grafted skin tissue after HA1-44 peptide administration, and (c) a graph showing a percentage of PDGFRα +  cells in the grafted skin tissue. PDGFRα expression was detected by fluorescence of GFP, and Prx1 lineage was detected by fluorescence of a reporter protein tdTomato. 
         FIG. 27  is (a) a diagram showing a schematic of the parabiosis model, and (b) photographs showing the tissue observation results of the knee cartilage injury site in the control group (saline administration) and the HA1-44 peptide administration group. Cells of P0 lin+  were detected with fluorescence of reporter protein tdTomato. 
         FIG. 28  is photographs showing tissue observation results (safranin O staining) of knee cartilage injury sites in the control group (saline administration) and the HA1-44 peptide administration group at 2, 4, 8 and 12 weeks after knee cartilage defect creation. The arrowheads indicate the part where regeneration of the hyaline cartilage was observed. 
         FIG. 29  is a diagram showing the results of performing transcriptome analysis on cells obtained by culturing peripheral blood of mice and performing clustering analysis, on a colony basis. The cell types shown on the left are predicted cell types based on gene expression profiles (such as high expression of a particular gene set). One column corresponds to one colony. Squares were displayed under columns corresponding to colonies derived from mice in the HA1-44 peptide administration group. 
         FIG. 30  is a table simplifying clustering analysis results of colonies derived from mouse peripheral blood. 
         FIG. 31  is a graph showing the results of a pathway analysis performed based on transcriptome analysis data of vertebral Pα cells of mice in the HA1-44 peptide administration group and the saline administration group. 
         FIG. 32  is a graph showing the results of a pathway analysis (function analysis) performed based on transcriptome analysis data of vertebra Pα cells of mice in the HA1-44 peptide administration group and the saline administration group. 
         FIG. 33  is a graph showing the percentages of CD45-negative, TER-119-negative, and PDGFRβ-positive cells in human peripheral blood mononuclear cell fractions collected before, 8 hours after, and 24 hours after administration of the HA1-44 peptide. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As used herein, the “cell” means one cell or plural cells depending on the context. For example, a cell in the present application may be a cell population consisting of one type of cell or a cell population containing plural types of cells. For example, the expression “cells having differentiation potency into an osteoblast, an adipocyte and a chondrocyte” includes not only a case where one cell/one type of cell (or a homogeneous cell population derived from the cell) has differentiation potency into these three cell types, but also a case where a cell population containing plural cells exerts differentiation potency into the three cell types as a whole cell population. 
     In the present application, an ectomesenchymal stem cell (EMSC) means a PDGFR-positive cell having colony-forming ability and differentiation potency into mesenchymal three lineages (osteoblasts, adipocytes, chondrocytes) and being suggested to be an ectoderm-derived cell. The cell is suggested to be an ectoderm-derived cell, when, for example, the cell is P0 lin+ . In one aspect, EMSC has also differentiation potency into an epidermal cell (specifically, K5-positive keratinocyte). Examples of the epidermal cells into which EMSC can differentiate include, but are not limited to, keratinocytes, cells expressing keratin 5 (K5) (K5-positive cells), and keratinocytes expressing K5 (K5-positive keratinocytes). For example, whether or not the cell has the differentiation potency into K5-positive keratinocytes can be determined by whether or not the cell can be differentiated into a cell expressing K5 when the cell is cultured under a differentiation-inducing condition into keratinocytes. 
     Examples of markers (including cell lineage markers) which characterize EMSCs include Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − . 
     Examples of EMSC include:
     (a) a colony-forming Pα cell whose amount of presence in peripheral blood increases in response to necrotic tissue injury (such as a skin flap), in other words, a necrotic injury-induced colony-forming Pα cell (hereinafter, the cell is also referred to as “iCFPα cell”); and   (b) a colony-forming Pα cell contained in the vertebral bone marrow (hereinafter, the cell is also referred to as a “vertebral CFPα cell” or a “vertebra-derived CFPα cell”).   

     The iCFPα cell has i) colony-forming ability and ii) differentiation potency into osteoblasts, adipocytes and chondrocytes. Thus, it can be said that the iCFPα cell has properties of a mesenchymal stem cell. Furthermore, the iCFPα cell has differentiation potency into an epidermal cell (K5-positive keratinocyte) and is P0 lin+ . Thus, it can be said that the iCFPα cell is an ectomesenchymal stem cell. 
     Examples of markers that characterize iCFPα cells include Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − . 
     The vertebra-derived CFPα cell has i) colony-forming ability and ii) differentiation potency into osteoblasts, adipocytes and chondrocytes. Thus, it can be said that the vertebra-derived CFPα cell has properties of a mesenchymal stem cell. Furthermore, the vertebra-derived CFPα cell has differentiation potency into an epidermal cell (K5-positive keratinocyte) and is P0 lin+ . Thus, it can be said that the vertebra-derived CFPα cell is an ectomesenchymal stem cell. 
     Examples of markers that characterize the vertebra-derived CFPα cell include Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin− . In addition, the vertebra-derived CFPα cell includes a LepR lin+  cell and a LepR lin−  cell. Of these, the LepR lin−  cell is considered to exhibit properties closer to the iCFPα cell in peripheral blood. 
     The present application provides, as one aspect of EMSC, a colony-forming PDGFR-positive cell having characteristic i) and characteristics ii) and/or iii) below: 
     i) having differentiation potency into an osteoblast, an adipocyte and a chondrocyte; 
     ii) having differentiation potency into an epidermal cell; 
     iii) being P0 lineage-positive. 
     In one embodiment, the colony-forming PDGFR-positive cell is a PDGFRα-positive cell. In another embodiment, the colony-forming PDGFR-positive cell is a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − . 
     Examples of further characteristics of the colony-forming PDGFR-positive cell include the following:
     being Pα + ;   being CD34 + ;   being Sca1 − ;   being CD34 + , and Sca1 − ;   being CD34 + , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being Sca1 − , and having one or more characteristics selected from Pα lin+ , Pα lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being CD34 + , and Sca1 − , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being P0 lin+ , and Prx1 lin− ;   being P0 lin+ , Prx1 lin− , and Sox1 lin− ;   being P0 lin+ , Prx1 lin− , and LepR lin− ;   being P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being Pα lin+ , P0 lin+ , and Prx1 lin− ;   being Pα lin+ , P0 lin+ , Prx1 lin− , and Sox1 lin− ;   being Pα lin+ , P0 lin+ , Prx1 lin− , and LepR lin− ;   being Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and CD34 + ;   being Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and Sca1 − ;   being Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − ;   being Pα + , and CD34 + ;   being Pα + , and Sca1 − ;   being Pα + , CD34 + , and Sca1 − ;   being Pα + , and CD34 + , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being Pα + , and Sca1 − , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being Pα + , CD34 + , and Sca1 − , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being Pα + , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being Pα lin+ , P0 lin+ , and Prx1 lin− ;   being Pα lin+ , P0 lin+ , Prx1 lin− , and Sox1 lin− ;   being Pα lin+ , P0 lin+ , Prx1 lin− , and LepR lin− ;   being Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being Pα + , P α   lin+ , P0 lin+ , and Prx1 lin− ;   being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , and Sox1 lin− ;   being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , and LepR lin− ;   being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− ;   being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and CD34 + ;   being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and Sca1 − ;   being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and CD34 + , and Sca1 − .   

     In one embodiment, the present invention relates to a vertebral bone marrow-derived cell that is PDGFRα-positive, CD34-positive, and Sca1-negative. The cell is presumed as a cell equivalent to an iCFPα cell in peripheral blood due to marker commonality. It is thus expected that the vertebral bone marrow-derived cell exhibits properties similar to those of the iCFPα cell. The vertebral bone marrow-derived cell can be obtained, for example, by collecting vertebral bone marrow from a subject and selectively recovering a cell of Pα + , CD34 + , and Sca1 − . 
     The present application also provides a bone marrow-derived Pα+CD34 + Sca1 +  cell. Furthermore, the present application provides a method for producing a cell, comprising a step of selectively recovering a cell of Pα + , CD34 + , and Sca1 +  from a bone marrow. 
     Examples of the bone marrow that may be used as a source of Pα + CD34 + Sca1 +  cells include a bone marrow of vertebra (cervical, thoracic, or lumbar vertebra) and femur. In one aspect, the bone marrow that may be used as a source of Pα + CD34 + Sca1 +  cells is a bone marrow of vertebra. In another aspect, the bone marrow that may be used as a source of Pα + CD34 + Sca1 +  cells is a bone marrow of cervical vertebra. 
     The present inventors have also found that many Pα + CD34 + Sca1 +  cells are also present in bone marrow of the bone whose embryological origin is ectomesenchyme. Accordingly, the present application provides a Pα + CD34 + Sca1 +  cell derived from bone marrow of the bone whose embryological origin is ectomesenchyme. The present application also provides a method for producing a cell, comprising a step of selectively recovering a cell of Pα + , CD34 + , and Sca1 +  from bone marrow of the bone whose embryological origin is ectomesenchyme. Examples of the bone whose embryological origin is ectomesenchyme include frontal skull, nasal bone, zygomatic bone, maxillary bone, palate bone, and mandibular bone. 
     In one embodiment, the present invention relates to a method for producing a colony-forming PDGFR-positive cell, comprising any one of steps 1) to 4) below: 
     1) collecting peripheral blood from a subject having a necrotic tissue injury, and culturing the peripheral blood on a solid phase; 
     2) collecting peripheral blood from a subject having a necrotic tissue injury, and culturing the peripheral blood on a solid phase, and then selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − ; 
     3) collecting peripheral blood from a subject having a necrotic tissue injury, and selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  from the peripheral blood; 
     4) collecting peripheral blood from a subject having a necrotic tissue injury, selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  from the peripheral blood, and culturing the cell on a solid phase. 
     In one embodiment, the present invention relates to a method for producing a colony-forming PDGFR-positive cell, comprising any one of steps 1) to 4) below: 
     1) culturing peripheral blood collected from a subject having a necrotic tissue injury on a solid phase; 
     2) culturing peripheral blood collected from a subject having a necrotic tissue injury on a solid phase, and then selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − ; 
     3) selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − from peripheral blood collected from a subject having a necrotic tissue injury; 
     4) selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  from peripheral blood collected from a subject having a necrotic tissue injury, and culturing the cell on a solid phase. 
     Examples of the necrotic tissue injury include, but are not limited to, a skin flap, and an epidermis exfoliation in epidermolysis bullosa. In the skin flap, blood supply to the tip part of the flap is insufficient to lead to an ischemic state, resulting in necrosis of the cell/tissue. In epidermolysis bullosa, necrosis occurs in an exfoliated epidermal tissue. 
     When culturing peripheral blood on a solid phase, red blood cells may be removed from peripheral blood before culturing. Removal of red blood cells may be carried out by a method using a hemolysis reagent known to those skilled in the art, a method for treating peripheral blood with hetastarch and recovering supernatant containing a nuclear cell, or the like. 
     Examples of the cells to be selectively recovered in step 2), 3) or 4) of the method for producing a colony-forming PDGFR-positive cell include the following:
     Cells being Pα +     Cells being CD34 +     Cells being Sca1 −     Cells being CD34 + , and Sca1 −     Cells being CD34 + , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin−  and LepR lin−     Cells being Sca1 − , and having one or more characteristics selected from , P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin−  and LepR lin−     Cells being CD34 + , and Sca1 − , and having one or more characteristics selected from P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin−  and LepR lin−     Cells having one or more characteristics selected from Pα lin+ , P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin−  and LepR lin−     Cells being P0 lin+ , and Prx1 lin−     Cells being P0 lin+ , and Prx1 lin− , and Sox1 lin−     Cells being P0 lin+ , and Prx1 lin− , and LepR lin−     Cells being P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin−  and LepR lin−     Cells being Pα lin+ , P0 lin+ , and Prx1 lin−     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , and Sox1 lin−     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , and LepR lin−     Cells being Pα lin+ , P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin−  and LepR lin−     Cells being Pα lin+ , P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin−  and LepR lin− , and CD34 +     Cells being Pα lin+ , P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and Sca1 −     Cells being Pα lin+ , P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −     Cells being Pα + , and CD34 +     Cells being Pα + , and Sca1 −     Cells being Pα + , CD34 + , and Sca1 −     Cells being Pα + , and CD34 + , and having one or more characteristics selected from Pα lin+ , P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , and Sca1 − , and having one or more characteristics selected from Pα lin+ , P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , CD34 + , and Sca1 − , and having one or more characteristics selected from Pα lin+ , P0 lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , P0 lin+ , and Prx1 lin−     Cells being Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin−     Cells being Pα + , P0 lin+ , Prx1 lin− , and LepR lin−     Cells being Pα + , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , Pα lin+ , P0 lin+ , and Prx1 lin−     Cells being Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , and Sox1 lin−     Cells being Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , and LepR lin−     Cells being Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and CD34 +     Cells being Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and Sca1 −     Cells being Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − .   

     Herein, examples of the method for “selectively recovering” a cell include the following: 
     (1) a method for “sorting” a cell expressing a desired marker molecule with a cell sorter or the like; 
     (2) a method of “recovering”, “selecting”, “separating”, “isolating” or “enriching” a cell/colony expressing a desired marker molecule visually or based on a result of gene expression analysis. 
     Examples of the marker molecule include a surface marker (cell surface antigen) and a reporter protein of a lineage marker gene. 
     In one embodiment, the present invention relates to a method for producing a colony-forming PDGFR-positive cell, comprising any one of steps 1) to 4) below: 
     1) collecting a vertebral bone marrow from a subject and culturing the vertebral bone marrow on a solid phase; 
     2) collecting a vertebral bone marrow from a subject, culturing the vertebral bone marrow on a solid phase, and then selectively recovering a cell having one or more characteristics selected from Pα + , Prx1 lin− , and Sox1 lin− ; 
     3) collecting a vertebral bone marrow from a subject, and selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin−  from the vertebral bone marrow; 
     4) collecting a vertebral bone marrow from a subject, selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin−  from the vertebral bone marrow, and culturing the cell on a solid phase. 
     In one embodiment, the present invention relates to a method for producing a colony-forming PDGFR-positive cell, comprising any one of steps 1) to 4) below: 
     1) culturing a vertebral bone marrow collected from a subject on a solid phase; 
     2) culturing a vertebral bone marrow collected from a subject on a solid phase, and then selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin− ; 
     3) selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin−  from a vertebral bone marrow collected from a subject; 
     4) selectively recovering a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin−  from a vertebral bone marrow collected from a subject, and culturing the cell on a solid phase. 
     In one embodiment, in step 2), 3), or 4) of the method for producing the colony-forming PDGFR-positive cell, a cell having one or more characteristics selected from Pα + , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−  may be selectively recovered. 
     Examples of the cells to be selectively recovered in step 2), 3) or 4) of the method for producing a colony-forming PDGFR-positive cell include the followings:
     Cells being P0 lin+     Cells being Prx1 lin−     Cells being Sox1 lin−     Cells being P0 lin+ , and Prx1 lin−     Cells being P0 lin+ , and Sox1 lin−     Cells being Prx1 lin− , and Sox1 lin−     Cells being P0 lin+ , Prx1 lin− , and Sox1 lin−     Cells being Pα + , and P0 lin+     Cells being Pα + , and Prx1 lin−     Cells being Pα + , and Sox1 lin−     Cells being Pα + , P0 lin+ , and Prx1 lin−     Cells being Pα + , P0 lin+ , and Sox1 lin−     Cells being Pα + , P0 lin+ , and Sox1 lin−     Cells being Pα + , P0 lin+ , Prx1 lin− , and Sox1 lin−     Cells being LepR lin−     Cells being P0 lin+ , and LepR lin−     Cells being Prx1 lin− , and LepR lin−     Cells being Sox1 lin− , and LepR lin−     Cells being P0 lin+ , Prx1 lin− , and LepR lin−     Cells being P0 lin+ , Sox1 lin− , and LepR lin−     Cells being Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , P0 lin+ , and LepR lin−     Cells being Pα + , Prx1 lin− , and LepR lin−     Cells being Pα + , Sox1 lin− , and LepR lin−     Cells being Pα + , P0 lin+ , Prx1 lin− , and LepR lin−     Cells being Pα + , P0 lin+ , Sox1 lin− , and LepR lin−     Cells being Pα + , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin− .   

     The vertebra that may be used as a source for colony-forming PDGFR-positive cells include cervical, thoracic, and lumbar vertebrae. In one aspect, the vertebra used as a source for a colony-forming PDGFR-positive cell is cervical vertebra. 
     In addition, the present inventors have confirmed in experiments conducted so far that colonies obtained by culturing vertebral bone marrow on a solid phase are all PDGFR-positive. 
     As used herein, the “bone marrow” collected from a subject means a bone marrow tissue containing various bone marrow cells. 
     In one aspect, the present invention relates to a method for screening for a substance having inducing activity of a multipotent stem cell, using a cell in peripheral blood induced by a necrotic tissue injury as an indicator. 
     The present inventors have found that iCFPα cells in peripheral blood are increased by a necrotic tissue injury (for example, a skin flap), and that Pα + P0 lin+ Prx lin−  cells (i.e., a cell population containing iCFPα cells) in peripheral blood are increased by administering the HA1-44 peptide. Thus, using increase of iCFPα cells in peripheral blood as an indicator, a substance having an effect of increasing the amount of presence of multipotent stem cells (e.g., MSC) having proliferative ability (colony-forming ability) and multi-lineage differentiation potency in peripheral blood (hereinafter, the substance is also referred to as a multipotent stem cell mobilizing substance, or a multipotent stem cell inducer) can be screened. 
     In one embodiment, the present invention relates to a method for screening for a multipotent stem cell inducer, comprising the steps of: 
     1) collecting peripheral blood from a subject, and counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in the peripheral blood; 
     2) collecting peripheral blood from a subject to which a test substance has been administered, and counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in the peripheral blood; and 
     3) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the number of cells counted in step 2) is larger than the number of cells counted in step 1). 
     In one embodiment, the present invention relates to a method for screening for a multipotent stem cell inducer, comprising the steps of: 
     1) counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in peripheral blood collected from a subject; 
     2) counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in peripheral blood collected from a subject to which a test substance has been administered; and 
     3) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the number of cells counted in step 2) is larger than the number of cells counted in step 1). 
     For the characteristics of the cells to be counted in the screening method described above, surface markers (Pα, CD34, Sca1) can be detected using antibodies or the like. Alternatively, when an experimental animal in which a reporter gene is incorporated downstream of a promoter of the surface marker gene is used, the product of the reporter gene (such as fluorescent protein) can be detected as an indicator. Lineage markers (Pα, P0, Prx1, Sox1, LepR) can be detected by using a transgenic animal having a DNA structure/construct (such as Cre-loxP system) that allows lineage tracing of a gene of interest. 
     Examples of the cells to be counted in steps 1) and 2) of the screening method described above include the followings:
     being Pα + ;   being CD34 + ;   being Sca1 − ;   being CD34 + , and Sca1 − ;   Cells being and CD34 + , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Sca1 − , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being CD34 + , and Sca1 − , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being P0 lin+ , and Prx1 lin−     Cells being P0 lin+ , Prx1 lin− , and Sox1 lin−     Cells being P0 lin+ , Prx1 lin− , and LepR lin−     Cells being P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα lin+ , P0 lin+ , and Prx1 lin−     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , and Sox1 lin−     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , and LepR lin−     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and CD34 +     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and Sca1 −     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −     Cells being Pα + , and CD34 +     Cells being Pα + , and Sca1 −     Cells being Pα + , CD34 + , and Sca1 −     Cells being Pα + , and CD34 + , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , and Sca1 − , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , CD34 + , and Sca1 − , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , and having one or more characteristics selected from Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα lin+ , P0 lin+ , and Prx1 lin−     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , and Sox1 lin−     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , and LepR lin−     Cells being Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , P α   lin+ , P0 lin+ , and Prx1 lin−     Cells being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , and Sox1 lin−     Cells being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , and LepR lin−     Cells being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , and LepR lin−     Cells being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and CD34 +     Cells being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and Sca1 −     Cells being Pα + , P α   lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , and CD34 + , and Sca1 − .   

     In one aspect, the present invention relates to a method for screening for a substance having inducing activity of a multipotent stem cell, using the HA1-44 peptide as a positive control, and using the reaction of a multipotent stem cell that contributes to tissue regeneration in vivo as an indicator. 
     In one embodiment, the present invention relates to a method for screening for a multipotent stem cell inducer, comprising the steps of: 
     1) collecting peripheral blood from a subject, and culturing the peripheral blood on a solid phase to obtain an adhesive cell population; 
     2) performing an exhaustive gene expression analysis on the cell population obtained in step 1 on a colony or single-cell basis; 
     3) administering a peptide consisting of an amino acid sequence of SEQ ID NO: 1 (HA1-44 peptide) to a subject, collecting peripheral blood, and culturing the peripheral blood on a solid phase to obtain an adhesive cell population; 
     4) performing an exhaustive gene expression analysis on the cell population obtained in step 3 on a colony or single-cell basis; 
     5) administering a test substance to a subject, collecting peripheral blood, and culturing the peripheral blood on a solid phase to obtain an adhesive cell population; 
     6) performing an exhaustive gene expression analysis on the cell population obtained in step 5 on a colony or single-cell basis; 
     7) pooling gene expression data obtained in steps 2 and 4, and performing a clustering analysis; 
     8) pooling gene expression data obtained in steps 2 and 6, and performing a clustering analysis; and 
     9) comparing an analysis result of step 7 to an analysis result of step 8, and selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the cell population obtained in step 5 (test substance administration group) has the same cluster configuration as the cell population obtained in step 3 (HA1-44 peptide administration group). 
     In other embodiments, the test substance is administered in place of the HA1-44 peptide in step 3, and the HA1-44 peptide is administered in place of the test substance in step 5. That is, either the HA1-44 peptide or the test substance may be administered to the subject first. Subjects in steps 1, 3 and 5 may be the same individual or another individual. For example, the method may be performed by preparing three animals of the same strain, and administering no substance (or administering solvent only) to one, administering an HA1-44 peptide to another one, and administering a test substance to the remaining one, and then collecting peripheral blood from each individual to obtain an adhesive cell population, and perform an exhaustive gene expression analysis and a clustering analysis. The subject in step 1 may be a subject to which only the solvent has been administered which is the same as the solvent used in administering the HA1-44 peptide and the test substance in steps 3 and 5, respectively. 
     In another embodiment, a variant or modified of the HA1-44 peptide or a tagged HA1-44 peptide is used instead of the HA1-44 peptide. The variant has an amino acid sequence in which several, e.g., 1 to 5, preferably 1 to 4, 1 to 3, more preferably 1 to 2, even more preferably 1 amino acid is substituted, inserted, deleted and/or added in the amino acid sequence of the HA1-44 peptide. For example, the variant is a peptide having an amino acid sequence which has a 50% or more, preferably 60% or more, further preferably 70% or more, more preferably 80% or more, more preferably 85% or more, and particularly preferably 90% or more (e.g., 91%, 92%, 93%, 94%, 95%, 96%, 97%, or 98%) homology to the amino acid sequence of the HA1-44 peptide when performing a local alignment. The homology of amino acid sequences can be measured, for example, using FASTA, BLAST, DNASIS (manufactured by Hitachi Software Engineering Co., Ltd.), or GENETYX (manufactured by GENETYX CORPORATION). Alternatively, the sequences can be simply compared and calculated. The modified has an amino acid sequence in which an amino acid residue of several, e.g., 1 to 5, preferably 1 to 4, 1 to 3, further preferably 1 to 2, more preferably 1 amino acid in the amino acid sequence of the HA1-44 peptide is modified. When the tagged HA1-44 peptide is used, examples of the tag include, but are not limited to, an His 6 -tag, a FLAG tag, an myc tag, and a GST tag. The tag may be added to either the N-terminus or the C-terminus of the amino acid sequence. 
     In the screening method, the exhaustive gene expression analysis may be RNA sequencing (RNA-seq). In the screening method, clustering analysis may be performed using an iterative clustering and guide-gene selection (ICGS) algorithm. 
     In another embodiment, the present invention relates to a method for screening for a multipotent stem cell inducer, comprising the steps of: 
     1) culturing peripheral blood collected from a subject on a solid phase to obtain an adhesive cell population; 
     2) performing an exhaustive gene expression analysis on the cell population obtained in step 1) on a colony or single-cell basis; 
     3) culturing peripheral blood collected from a subject to which a peptide consisting of an amino acid sequence of SEQ ID NO: 1 (HA1-44 peptide) has been administered on a solid phase to obtain an adhesive cell population; 
     4) performing an exhaustive gene expression analysis on the cell population obtained in step 3) on a colony or single-cell basis; 
     5) culturing peripheral blood collected from a subject to which a test substance has been administered on a solid phase to obtain an adhesive cell population; 
     6) performing an exhaustive gene expression analysis on the cell population obtained in step 5) on a colony or single-cell basis; 
     7) pooling gene expression data obtained in steps 2) and 4), and performing a clustering analysis; 
     8) pooling gene expression data obtained in steps 2) and 6), and performing a clustering analysis; and 
     9) comparing an analysis result of step 7) to an analysis result of step 8), and selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the cell population obtained in step 5) has the same cluster configuration as the cell population obtained in step 3). 
     In another embodiment, the present invention relates to a method for screening for a multipotent stem cell inducer, comprising the steps of: 
     1) collecting peripheral blood from a subject, and culturing the peripheral blood on a solid phase to obtain an adhesive cell population; 
     2) counting the number of colonies obtained in step 1); 
     3) administering a test substance to a subject, collecting peripheral blood, and culturing the peripheral blood on a solid phase to obtain an adhesive cell population; 
     4) counting the number of colonies obtained in step 3); and 
     5) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the number of colonies counted in step 4) is larger than the number of colonies counted in step 2). 
     The subject in step 1) may be a subject to which only the solvent has been administered which is the same as the solvent used in administering the test substance in step 3). 
     The colony counted in steps 2) and 4) may be a colony having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − . 
     In another embodiment, the present invention relates to a method for screening for a multipotent stem cell inducer, comprising the steps of: 
     1) culturing peripheral blood collected from a subject on a solid phase to obtain an adhesive cell population; 
     2) counting the number of colonies obtained in step 1); 
     3) culturing peripheral blood collected from a subject to which a test substance has been administered on a solid phase to obtain an adhesive cell population; 
     4) counting the number of colonies obtained in step 3); and 
     5) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when the number of colonies counted in step 4) is larger than the number of colonies counted in step 2). 
     The subject in step 1) may be a subject to which only the solvent has been administered which is the same as the solvent used in administering the test substance in step 3). 
     The colony counted in steps 2) and 4) may be a colony having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − . 
     In another embodiment, the present invention relates to a method for screening for a multipotent stem cell inducer, comprising the steps of: 
     1) collecting a bone marrow from a vertebra of a subject, and obtaining a PDGFRα-positive cell population by culturing on a solid phase or cell-sorting; 
     2) performing an exhaustive gene expression analysis on the cell population obtained in step 1 on a colony or single-cell basis; 
     3) administering a test substance to a subject, collecting a bone marrow from vertebra, and obtaining a PDGFRα-positive cell population by culturing on a solid phase or cell-sorting; 
     4) performing an exhaustive gene expression analysis on the cell population obtained in step 3 on a colony or single-cell basis; 
     5) pooling gene expression data obtained in steps 2 and 4, and performing a pathway analysis; and 
     6) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when, as a result of the analysis of step 5, (i) a pathway associated with EIF2 signaling, regulation of eIF4 and p70S6K signaling, and/or mTOR signaling is activated or (ii) expression of a cell death-related gene is suppressed, in the cell population obtained in step 3 (test substance administration group) compared to the cell population obtained in step 1 (untreated group). 
     The subject in step 1 may be a subject to which only the solvent has been administered which is the same as the solvent used in administering the test substance in step 3. 
     In the screening method, the exhaustive gene expression analysis may be RNA sequencing (RNA-seq). In the screening method, the pathway analysis may be performed using an Ingenuity Pathway Analysis (IPA) software (https://www.qiagenbioinformatics.com). 
     In another embodiment, the present invention relates to a method for screening for a multipotent stem cell inducer, comprising the steps of: 
     1) obtaining a PDGFRα-positive cell population by culturing on a solid phase or cell-sorting from a bone marrow collected from a vertebra of a subject; 
     2) performing an exhaustive gene expression analysis on the cell population obtained in step 1) on a colony or single-cell basis; 
     3) obtaining a PDGFRα-positive cell population by culturing on a solid phase or cell-sorting from a bone marrow collected from a vertebra of a subject to which a test substance has been administered; 
     4) performing an exhaustive gene expression analysis on the cell population obtained in step 3) on a colony or single-cell basis; 
     5) pooling gene expression data obtained in steps 2) and 4), and performing a pathway analysis; and 
     6) selecting the test substance as a candidate for a substance having multipotent stem cell-inducing activity when, as a result of the analysis of step 5), (i) a pathway associated with EIF2 signaling, regulation of eIF4 and p70S6K signaling, and/or mTOR signaling is activated or (ii) expression of a cell death-related gene is suppressed, in the cell population obtained in step 3) compared to the cell population obtained in step 1). 
     In the screening method, the exhaustive gene expression analysis may be RNA sequencing (RNA-seq). In the screening method, the pathway analysis may be performed using Ingenuity Pathway Analysis (IPA) software (https://www.qiagenbioinformatics.com). 
     In other aspects, the present invention relates to a cell or cell population in peripheral blood, induced by an MSC in-blood-mobilizing substance. The present invention also relates to a cell or cell population in the vertebral bone marrow induced by the HA1-44 peptide. 
     As used herein, the “MSC in-blood-mobilizing substance” means a substance having the activity of a mobilizing mesenchymal stem cell (MSC) into peripheral blood or increasing the amount of MSC present in peripheral blood. Examples of the MSC in-blood-mobilizing substance include, but are not limited to, an HMGB1 protein, an HMGB2 protein, and an HMGB3 protein (e.g., those described in WO 2008/053892 and WO 2009/133939), an S100A8 protein and an S100A9 protein (e.g., those described in WO 2009/133940 and WO 2011/052668), and various HMGB1 peptides (e.g., a peptide consisting of amino acid residues 1-44 of the HMGB1 protein (the HA1-44 peptide in the present application)) described in International Application WO 2012/147470 by the present inventors. 
     In one embodiment, the present invention relates to a cell population obtained by administering an MSC in-blood-mobilizing substance to a subject, collecting peripheral blood from the subject, and culturing the collected peripheral blood on a solid phase. In one embodiment, the MSC in-blood-mobilizing substance may be an HA1-44 peptide. 
     In other embodiments, the present invention relates to a cell population obtained by 1) administering an HA1-44 peptide to a subject, 2) collecting a vertebral bone marrow from the subject, and 3) culturing the collected bone marrow on a solid phase, or sorting a PDGFRα-positive cell from the collected bone marrow. 
     In another aspect, the present invention relates to a method for producing the cell or cell population described above. 
     In one embodiment, the present invention relates to a method for producing a cell, comprising a step of administering an MSC in-blood-mobilizing substance to a subject, collecting peripheral blood from the subject, and culturing the collected peripheral blood on a solid phase. In other embodiments, the present invention relates to a method for producing a cell, comprising a step of culturing peripheral blood collected from a subject to which an MSC in-blood-mobilizing substance has been administered on a solid phase. In one embodiment of these production methods, the MSC in-blood-mobilizing substance may be an HA1-44 peptide. 
     In other embodiments, the present invention relates to a method for producing a cell, comprising the steps of 1) administering an HA1-44 peptide to a subject, 2) collecting a vertebral bone marrow from the subject, and 3) culturing the collected bone marrow on a solid phase, or sorting a PDGFRα-positive cell from the collected bone marrow. 
     In yet another aspect, the present invention relates to a method for obtaining, isolating, and/or enriching a cell having a high tissue regeneration promoting ability similar to a PDGFRα-positive cell in a vertebral bone marrow from a biological tissue containing a mesenchymal stem cell (MSC). In yet another aspect, the present invention relates to a cell or cell population obtained by the method for obtaining, isolating and/or enriching described above. 
     In one embodiment, the present invention relates to a method for producing a cell population, comprising the steps of: 
     1) culturing a cell population from a biological tissue containing a mesenchymal stem cell (MSC) on a solid phase; 
     2) subcloning a colony obtained in step 1); 
     3) culturing a portion of cells obtained by the subcloning in a differentiation-inducing medium into bone, cartilage, and/or fat, and measuring an expression level of a differentiation marker of bone, cartilage, and/or fat; and 
     4) selecting a cell clone showing a high expression level compared to the expression level of a differentiation marker of bone, cartilage, and/or fat in case that MSC obtained by culturing a femur bone marrow on a solid phase are cultured in a differentiation-inducing medium into bone, cartilage, and/or fat. 
     In other embodiments, the present invention relates to a method for producing a cell population, comprising the steps of: 
     1) culturing a cell population derived from a biological tissue containing MSC on a solid phase; and 
     2) selecting a colony having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − . In one embodiment, step 2 may be a step of selecting a Prx1 lineage-negative colony. 
     In another embodiment, the present invention relates to a method for producing a cell population, comprising a step of selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − . from a cell population derived from a biological tissue containing MSC. 
     In other embodiments, the present invention relates to a method for producing a cell population, comprising the steps of: 
     1) selectively recovering a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 − . from a cell population derived from a biological tissue containing MSC; and 
     2) culturing the cell recovered in step 1) on a solid phase. 
     In the method for producing a cell population of the present application, the biological tissue containing MSC includes, but are not limited to, bone marrow, umbilical cord, umbilical cord blood, placenta, adipose tissue, dental cord, periosteum, synovial membrane, ovary membrane, and peripheral blood. Examples of the bone marrow includes, but are not limited to, bone marrows of femur, vertebra, sternum, ilium, and skull. 
     In yet another aspect, the present invention relates to a composition for use in promoting tissue regeneration, containing an ectomesenchymal stem cell. In one embodiment, the present invention relates to a composition for use in promoting tissue regeneration, containing a colony-forming PDGFR-positive cell having characteristic i) and characteristics ii) and/or iii) below: 
     i) having differentiation potency into an osteoblast, an adipocyte and a chondrocyte; 
     ii) having differentiation potency into an epidermal cell; 
     iii) being P0 lineage-positive. 
     In one embodiment, the composition is a composition for use in promoting regeneration of a tissue derived from mesoderm or ectoderm. Examples of the tissue derived from mesoderm include, but are not limited to, bone, cartilage, muscle, and vascular endothelium. Examples of the tissue derived from ectoderm include, but are not limited to, an epithelial tissue (e.g., epidermis), and a neural tissue. 
     The composition for use in promoting tissue regeneration of the present invention may contain a pharmaceutically acceptable carrier, a diluent and/or an excipient. In the composition for use in promoting tissue regeneration of the present invention, the amount of ectomesenchymal stem cells contained, the dosage form of the composition, the frequency of administration, or the like can be appropriately selected depending on the condition such as the type of tissue to be regenerated and/or the condition of the subject to be administered. 
     In yet another aspect, the present invention relates to a method for promoting tissue regeneration in a subject, comprising administering an ectomesenchymal stem cell. In one embodiment, the present invention relates to a method for promoting tissue regeneration in a subject, comprising administering to the subject a colony-forming PDGFR-positive cell having characteristic i) and characteristics ii) and/or iii) below: 
     i) having differentiation potency into an osteoblast, an adipocyte and a chondrocyte; 
     ii) having differentiation potency into an epidermal cell; 
     iii) being P0 lineage-positive. 
     The method for administering the cell can be appropriately selected depending on the condition such as the type of tissue to have regeneration promotion and/or the condition of the subject to be administered. Examples of the method for administering the cell include, but are not limited to, intradermal administration, subcutaneous administration, intramuscular administration, intravenous administration, nasal administration, oral administration, and suppositories. 
     In yet another aspect, the present invention relates to an ectomesenchymal stem cell for use in promoting tissue regeneration in a subject. 
     In yet another aspect, the present invention relates to a use of an ectomesenchymal stem cell for the manufacture of a medicament for promoting tissue regeneration in a subject. 
     It is believed that iCFPα cells in peripheral blood can be beneficial biomarkers for assessing EMSC-mediated tissue regenerative activity, in cases where necrotic tissue injury is generated. Accordingly, the present application provides a method for determining an expected tissue regeneration promoting effect in a subject to which an MSC in-blood-mobilizing substance has been administered, using an iCFPα cell in peripheral blood as an indicator. 
     In one embodiment, the present invention relates to a method for determining a tissue regeneration-promoting effect of an MSC in-blood-mobilizing substance, comprising the steps of: 
     1) counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in peripheral blood collected from a subject before administering an MSC in-blood-mobilizing substance; and 
     2) counting a cell having one or more characteristics selected from Pα + , Pα lin+ , P0 lin+ , Prx1 lin− , Sox1 lin− , LepR lin− , CD34 + , and Sca1 −  contained in peripheral blood collected from the subject after administering an MSC in-blood-mobilizing substance, 
     wherein tissue regeneration is suggested to be promoted in the subject when the number of cells counted in step 2) is larger than the number of cells counted in step 1). 
     In the present application, the “subject” may be either a human or a non-human animal. In one aspect, the subject is a non-human animal. Examples of the non-human animal include, but are not limited to, mouse, rat, monkey, pig, dog, rabbit, hamster, guinea pig, horse, and sheep. 
     The timing of collecting peripheral blood from a subject is not particularly limited with respect to a method for producing a cell (or cell population), a method for screening, and a method for determining a tissue regeneration promoting effect of an MSC in-blood-mobilizing substance, provided by the present application. When artificially creating a necrotic tissue injury or administering an MSC in-blood-mobilizing substance, a method in which peripheral blood is collected, for example, 2 to 24 hours after creating the necrotic tissue injury or administering the MSC in-blood-mobilizing substance can be included. In one aspect, the timing of collecting peripheral blood from the subject may be 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24 hours after creating the necrotic tissue injury or administering the MSC in-blood-mobilizing substance. In another aspect, the timing of collecting peripheral blood from the subject may be 4 to 24 hours, 8 to 24 hours, 8 to 16 hours, or 10 to 14 hours after creating the necrotic tissue injury or administering the MSC in-blood-mobilizing substance. 
     In one aspect of the cell, the method for producing a cell (or cell population), the composition comprising the cell, and the screening methods provided herein, the PDGFR-positive cell is a PDGFRα-positive cell. 
     It should be noted that all the prior art literature cited herein is incorporated herein by reference. This application also claims priority based on U.S. Provisional Patent Application No. 62/593310, filed on Dec. 1, 2017 to the United States Patent and Trademark Office, the contents of which are incorporated herein by reference. 
     Hereinafter, the present invention will be described in further detail. Note that the present invention is described in further detail below, but the present invention is not limited to the aspects described below. 
     EXAMPLES 
     Materials and Methods 
     1. Mouse 
     Pα-H2B-GFP mice capable of confirming PDGFRα expression with GFP fluorescence (The Jackson Laboratory, Stock No: 007669) and various cell lineage tracing mice utilizing the Cre-loxP system were used for experiments. 
     The cell lineage tracing mice utilizing the Cre-loxP system can be made by crossing a Cre driver mouse with a Cre reporter mouse. The Cre driver mouse is a transgenic mouse having a DNA structure in which a coding sequence of Cre recombinase is introduced downstream of a promoter sequence of a desired gene. The Cre reporter mouse is a transgenic mouse into which a DNA sequence having the structure “promoter (such as a CAG promoter)-loxP-stop cassette-loxP-desired reporter gene (EYFP or tdTomato in the Examples of the present application)” is introduced at a locus such as ROSA26. 
     In the Examples herein, the following mice were prepared as Cre driver mice.
     Pα-Cre mouse (The Jackson Laboratory, Stock No: 013148)   P0-Cre mouse (The Jackson Laboratory, Stock No: 017927)   Prx1-Cre mouse (The Jackson Laboratory, Stock No: 005584)   Sox1-Cre mouse (RIKEN BioResource Research Center, Accession No. CDB0525K)   LepR-Cre mouse (The Jackson Laboratory, Stock No: 008320)   Krt5-Cre mouse (MGI ID: 1926815, K5 Cre transgenic mouse described in Proc Natl Acad Sci U S A. 1997 Jul 8; 94(14):7400-5)   

     The following mice were prepared as Cre reporter mice.
     Rosa26-EYFP mouse (The Jackson Laboratory, Stock No: 006148)   Rosa26-tdTomato mouse (The Jackson Laboratory, Stock No: 007909)   

     The following cell lineage tracing mice were generated by crossing the six driver mice described each with the Rosa26-tdTomato reporter mouse.
     Pα-Cre::Rosa26-tdTomato mouse   P0-Cre::Rosa26-tdTomato mouse   Prx1-Cre::Rosa26-tdTomato mouse   Sox1-Cre::Rosa26-tdTomato mouse   LepR-Cre::Rosa26-tdTomato mouse   Krt5-Cre::Rosa26-tdTomato mouse   

     Furthermore, Pα-Cre::Rosa26-EYFP mouse was also generated by crossing the Pα-Cre driver mouse with the Rosa26-EYFP reporter mouse. 
     Pα-H2B-GFP mouse is a mouse in which a sequence encoding a fusion protein of histone H2B and eGFP has been knocked in downstream of a promoter of the PDGFRα gene. The Pα-H2B-GFP mouse was crossed with the Prx1-Cre::Rosa26-tdTomato mouse to produce Pα-H2B-GFP::Prx1-Cre::Rosa26-tdTomato mouse. 
     2. Creation of Skin Flap 
     In the examples of the present application, a skin flap was created as a method for causing a necrotic tissue injury. Specifically, the method for creating the skin flap was as follows. 
     Male mice (20-25 g) of 8-10 week old were shaved on the back under 1.5-2.0% (v/v) isoflurane inhalation anesthesia. A skin flap of 2.0 cm wide×4.0 cm long across the center of the back was created with a razor so that the skin continuity was maintained only on the tail side (which led to an ischemic state of the tip part away from the root of the flap, resulting in causing a necrotic injury to skin tissue). The affected area after the skin flap creation was protected with a bandage of sufficient size. Cardiac blood collection, separation of vertebra and femoral bone marrow cells, and creation of frozen sections of vertebra and femur were performed 12 hours after the skin flap creation for each. 
     3. Creation of Parabiosis 
     A parabiosis model was created using a 6-week old male wild-type mouse and a 6-week old male cell lineage tracing mouse with reference to Kamran P et al. J Vis Exp. 2013 Oct. 6; (80). Two mice were simultaneously subjected to general anesthetic induction with isoflurane, and positioned on a heat pad. The body side surfaces opposite to each other of the two mice were shaved, and the skin was removed approximately 5 mm wide from the elbow joint to the knee joint. The dorsal skin was continuously sutured with 5-0 VICRYL. The elbow joints and knee joints each opposite to each other were sutured with 3-0 VICRYL. The abdominal skin was also continuously sutured with 5-0 VICRYL. The mice were left on the heat pad until awakened from anesthesia. To avoid restricting activity, up to one pair were raised per cage, and scattered food was offered. 
     4. Tissue Staining and Observation 
     After vertebral and femoral bones were collected, fixation with 4% paraformaldehyde, deashing with 0.5 M EDTA solution, and replacement with 30% sucrose solution were performed, then a frozen block was created using a frozen embedding agent for Kawamoto method, Super Cryoembedding Medium (SCEM) (Leica). The block was sliced with Cryofilm type 2C(9) (Leica) at a thickness of 10 μm, and immunostaining was performed with each antibody (with a primary antibody at 4° C. overnight, and with a secondary antibody at 4° C. for 1 hour). Confocal microscopy was used for tissue observation. 
     5. Acquisition and Culture of Cells in Peripheral Blood, Vertebra and Femur 
     The following method was used to recover cells from peripheral blood. Approximately 800 to 1000 μL of peripheral blood was collected from the heart under general anesthesia (using a 1 mL syringe containing heparin). To remove red blood cells, Hetasep (STEMCELL Technologies, Inc., Cat No. ST-07906) at equal amount of the collected blood was added, the mixture was centrifuged at 100 G for 2 minutes, and incubated at room temperature for 15 minutes, then the supernatant was recovered. The supernatant was subjected to the next experiment as a sample containing nuclear cells in peripheral blood. 
     The following method was used to recover cells from bone marrow. The vertebrae and femur collected under general anesthesia were immersed in 0.2% Collagenase A solution (Sigma-Aldrich Corp., Cat#10103578001) and incubated at 37° C. for 1 hour. Bone marrow cells were extruded in a breast bowl, recovered by pipetting and passed through a 40 μm cell strainer. The supernatant was centrifuged at 300 G for 5 minutes for removal, and hemolyzed with RBC Lysis buffer (BioLegend, Inc., cat #420301) to remove red blood cells, then the resultant was subjected to the following experiments as bone marrow cells. 
     6. Colony Assay of Peripheral Blood 
     The supernatant obtained by the above procedure (sample containing nucleated cells in peripheral blood) was seeded in a 6-well plate coated with collagen I (Corning Incorporated, Cat No. 356400), and cultured for 10 days under a condition of 37° C., 5% CO2, 5% 02 using a medium containing 1% L-glutamine (NACALAI TESQUE, INC.), 10 μM ROCK inhibitor (Y27632, Tocris Bioscience) and 1% penicillin/streptomycin (NACALAI TESQUE, INC.) in an expansion medium prepared using a MesenCult Expansion Kit (STEMCELL Technologies, Inc., Cat No. ST-05513) according to the manual of the kit (all concentrations described are final concentrations). The medium was replaced with a fresh medium twice a week during the culture period. On day 10 of the culture, cells on plates were stained with a Differential Quik Stain Kit (Sysmex Corporation, Cat No. 16920), and the number of colonies containing not fewer than 50 cells was counted. 
     7. Differentiation Induction Into Osteoblast/Adipocyte/Chondrocyte 
     Viable cells of passages 3-5 were seeded in a 12-well plate at 50,000 cells/well, and cultured with 10% FBS/DMEM until subconfluency. Subsequently, fat differentiation induction was performed for 14 days using 10% FBS/DMEM containing 100 nM dexamethasone (Sigma-Aldrich Corp.), 0.5 mM isobutylmethylxanthine (Sigma-Aldrich Corp.), 50 mM indomethacin (Wako Pure Chemical Industries, Ltd.) and 10 μg/ml insulin (Sigma-Aldrich Corp.) for differentiation into adipocytes. 
     Osteoblast differentiation induction was performed for 21 days using 10% FBS/DMEM including 1 nM dexamethasone, 20 mM β-glycerol phosphate (Wako Pure Chemical Industries, Ltd.) and 50 μg/ml ascorbate-2-phosphate (Sigma-Aldrich Corp.) for differentiation into osteoblasts. After each differentiation induction, fixation with 4% PFA was performed, then adipocytes were stained with oil red-0 and osteoblasts were ALP stained with an ALP activity assay kit (TAKARA BIO Inc., Kusatsu, Japan), and observed with microscope. 
     In cartilage differentiation induction, 300,000 cells were first centrifuged in a 15 ml tube at 300 g, for 5 minutes. Cartilage differentiation induction was performed for 21 days using 10% FBS/DMEM containing 40 ng/ml proline (Sigma-Aldrich Corp.), 50 μg/ml ascorbic acid 2-phosphate, x100 ITS mix (BD Biosciences), 2 μg/ml fluocinolone (Tokyo Chemical Industry Co., Ltd., Tokyo, Japan), 5 ng/ml transforming growth factor-b3 (R&amp;D Systems, Minneapolis, Minn.), and 100 nM dexamethasone (Sigma-Aldrich Corp.). The completed chondropellets were paraffin-embedded, thinly sliced at a thickness of 6 μm, and stained with toluidine blue. 
     8. Differentiation Induction into Keratinocytes—Animal 
     Krt5-Cre::Rosa26-tdTomato mice were raised until they were 8-10 weeks old, and used for experiments. 
     Collection of Materials 
     A skin flap was created on the back of the mouse the day before the collection of materials, and 12 hours after the creation, peripheral blood was collected by cardiac blood collection under anesthesia. Peripheral blood was also collected in the same way from the mouse in which no skin flap was created. Femurs and vertebra were collected after blood collection. 
     Cell Conditioning 
     Red blood cells were removed from the peripheral blood with HetaSep, and one portion of blood corresponding to one mouse was seeded into one well of a 6-well plate coated with collagen I and cultured under a condition of 5% O 2 , 5% CO 2 , and 37° C. The medium used was MesenCult or 20% FBS/MEMα (both containing Rock inhibitor and 1% Penicillin-Streptomycin). 
     After both bone ends were cut out, the collected femur was divided into longitudinal halves. The collected vertebra was divided into longitudinal halves. Both were then treated with 0.2% Collagenase A/DMEM (10 mM HEPES, 1% Penicillin-Streptomycin) (37° C. water bath, for 1 hour). After Collagenase A treatment, the bone marrow cells were recovered in a breast bowl, dispersed into single cells with 40 um cell strainer, hemolytized with 1 x RBC Lysis solution, then seeded, and cultured under a condition of 5% O 2 , 5% CO 2 , and 37° C. The medium used was MesenCult or 20% FBS/MEMα (both containing Rock, inhibitor and 1% Penicillin-Streptomycin). 
     Differentiation Induction 
     After confirmation of colony formation, medium containing 1 uM retinoic acid (Sigma-Aldrich Corp.) and 25 ng/mL BMP4 (R&amp;D Systems) was added to the well and cultured at 5% CO 2 , 37° C. to induce differentiation into keratinocytes. An all-in-one microscope (KEYENCE CORPORATION) was used to observe Tomato-positive/negative cells. 
     9. Transcriptome Analysis (RNA-seq of Cell Populations) 
     RNA was extracted from the cell population, and an RNA-seq library was created according to Smart-seq2 protocol (Nature Protocols 9, 171-181 (2014) doi: 10.1038/nprot.2014.006). The obtained library was sequenced with Nextseq 500 (Illumina, Inc.) using a nextseq high output kit (37 bp pair end reads). Using bcl2fastq v2.17.1.14 of Illumina, Inc. with default parameters and optional --no-lane-splitting, conversion of base calls to fastq format and demultiplex were performed. Reads were trimmed using TrimGalore (http://www.bioinformatics.babraham.ac.uk/projects/trimg alore/), and mapping and counting (quantification) were performed using RSEM (Li et al., BMC Bioinformatics 12, 323 (2011); version STAR-2.5.2b). Differential expression analysis among samples was performed using DESeq2 (Love et al., Genome Biol. 15, 550 (2014)). Differentially Expressed Genes (DEGs) were uploaded to the Ingenuity Pathway Analysis (IPA) software (https://www.qiagenbioinformatics.com) to extract the most relevant biological pathways and functions to DEGs. Clustering analysis was performed using ICGS algorithm of AltAnalyzev.2.1.0-Py based on log2 converted values of 1-added TPM (Transcripts Per kilobase Million) counts obtained with RSEM (log 2(TPM+1)). 
     10. Transcriptome Analysis (Single-Cell RNA-seq) 
     Single cell suspension was prepared, and cell viability was evaluated with an automated cell counter TC20 (BioRad). A single-cell RNA-seq library was created using ddSEQ Single-Cell Isolator and SureCell WTA 3′ Library prep kit according to the manufacturer&#39;s protocol. The obtained library was sequenced with Nextseq 500 (Illumina, Inc.) using a Nextseq high output kit (Read 1: 68 bp, Read 2: 75 bp). Using bc12fastq v2.17.1.14 of Illumina, Inc. with default parameters and optional—no-lane-splitting, conversion of base calls to fastq format and demultiplex were performed. Any synthetic and sequencing errors that may occur in the cell barcode region were corrected with Edit distance (ED)&lt;2. Reads were analyzed (by mapping and quantification) using Drop-Seq Tools v1.12 (STAR version: STAR-2.5.2b) to generate a digital expression matrix. The resulting matrix was standardized with voom (limma 3.32.10). Clustering analysis with ICGS algorithm was performed using AltAnalyze v.2.1.0-Py with default settings based on the standardized matrix. Based on the standardized matrix, dimensional compression with tSNE algorithm (using Rtsne), and clustering analysis using hclust (method=ward.D2) were also performed, and the results were plotted with ggplot2 or plot.ly. DESingle was used for Differential expression analysis. 
     11. FACS Analysis 
     FACS analysis was performed on bone marrow cells collected from vertebra and femur with fluorescently labeled antibodies against various surface molecules. A series of processes including fluorescence detection, sorting, and the like were performed using BD FACS Aria III system, and analysis of the obtained data was performed with FlowJo software Ver. 6.3.3 (Tree Star, Ashland, Oreg.). 
     12. Parabiosis and Cartilage Defect Models 
     A parabiosis model was created using a 6-week old male wild-type mouse and a 6-week old male P0-Cre::Rosa26-tdTomato mouse using the method described in 3. above. After completion of the blood chimera at 4 weeks after operation, a 0.5×0.5×0.5 mm cartilage defect was created in the knee joint of the wild-type mouse using a 0.5 mm-diameter hand-turned drill (manufactured by MEISINGER USA, L.L.C.). Shortly after the cartilage defect was created, 100 μg of HA1-44 peptide diluted with 100 μL of saline was administered from the tail vein and subsequently administered at the same dose twice a week until 4 weeks after operation. To the control group, 100 μL of saline was administered to each mouse from the tail vein on the same schedule as in the HA1-44 peptide administration group. A knee joint was collected 12 weeks after knee cartilage defect creation, then fixation with 4% paraformaldehyde (overnight), deashing with 0.5 M EDTA solution (3 days), and 30% sucrose replacement (1 day) were performed to create a frozen block. It was sliced at a thickness of 10 μm using a cryostat, and the distribution of Tomato-positive cells was analyzed by confocal microscopy. A cartilage defect was also created in the knee joint using an 8-week old male wild-type mouse in the same way as described above, and the HA1-44 peptide was administered at the same dose and schedule as described above, then the knee joint was collected 2, 4, 8 and 12 weeks after knee cartilage defect creation to stain the tissue sections with safranin 0. 
     Abbreviation 
     Abbreviations for markers (XX, YY are the desired gene/protein name)
     XX + : XX-positive   XX − : XX-negative   YY lin+ : YY lineage-positive   YY lin− : YY lineage negative   PDGFR: platelet-derived growth factor receptor   Pα: platelet-derived growth factor receptor alpha (PDGFRα)   Pα cell: PDGFRα-positive cell   MSC: mesenchymal stem cell   EMSC: ectomesenchymal stem cell   CFPα cells: colony-forming Pα cells   iCFPα cells: necrotic injury-induced colony-forming Pα cells   CFU: colony-forming unit   LepR: Leptin receptor   

     Example 1 
     Example 1 
     Properties of iCFPα Cells in Peripheral Blood 
     A skin flap was created on the back of Pα-H2B-GFP mice, and 12 hours later, peripheral blood was collected, and the number of Pα cells contained in the peripheral blood was examined. As a result, Pα cells were significantly increased in the skin flap group compared to the control group in which no skin flap was created. The increase of Pα cells was correlated with HMGE1 concentration increase in peripheral blood ( FIG. 1 ). A colony assay was also performed by culturing the peripheral blood of Pα-H2B-GFP mice. As the result, the skin flap group had significantly more colonies (all Pα-positive cells) than the control group, showing higher CFU activity ( FIG. 2 ). Such results indicate that a necrotic tissue injury causes an increase of colony-forming Pα cells in peripheral blood. 
     Furthermore, colony-forming Pα cells (i.e., iCFPα cells) obtained by culturing the peripheral blood of mice in which a skin flap was created exhibited differentiation potency into osteoblasts, adipocytes and chondrocytes, and further exhibited differentiation potency into cells expressing Keratin 5 (Krt5, K5) under a differentiation-inducing condition into keratinocytes ( FIG. 3 ). 
     Single-cell transcriptome analysis was performed on CFPα cells derived from peripheral blood after skin flap creation (iCFPα cells). As the result, most cells showed high expression of gene groups corresponding to cell types including MSC ( FIG. 4 ). Cells expressing genes characteristic of epidermal cells such as Krt8 and Krt18 were also included. Furthermore, transcriptome analysis on a colony basis was performed on CFPα cells derived from peripheral blood in the control and the skin flap groups, and clustering analysis with the ICGS algorithm was performed. As the result, in clusters where CFPα cells after skin flap creation accounted for majority, expression of gene groups characteristic of cell types including bone marrow stem cells and MSC was high. Especially, expression of HoxA2 gene was characteristically high ( FIG. 5 ). 
     Example 2 
     Example 2 
     Surface Markers of iCFPα Cells in Peripheral Blood 
     In both the control group and the skin flap group, all colonies obtained by culturing peripheral blood were Pα-positive ( FIG. 6 ). Furthermore, a single cell transcriptome analysis of peripheral blood CFPα cells (iCFPα cells) in the skin flap group resulted in CD34-positive and Sca1-negative. 
     Example 3 
     Example 3 
     Cell Lineage Markers of iCFPα Cells in Peripheral Blood 
     Peripheral blood-derived CFPα cells of skin-flap-created mouse (iCFPα cells) were lineage-traced for Pα lineage, P0 lineage, Prx1 lineage, Sox1 lineage, and LepR lineages using a transgenic mouse utilizing a Cre-loxP system. As a result, all iCFPα cells were Pα lineage-positive, P0 lineage-positive, Sox1 lineage-negative, and LepR lineage-negative ( FIG. 7 ). For Prx1-lineage, 93% of iCFPα cells were negative ( FIG. 7 ). 
     Although a small amount of Prx1 lineage-positive and negative colony-forming cells are present in blood even at normal state, only Prx1 lineage-negative cells were increased by a skin flap ( FIG. 8 ). Thus, iCFPα cells can be defined as Prx1 lineage-negative. 
     It is believed that iCFPα cells in peripheral blood are derived from ectoderm, because they were P0 lin+  and Prx1 lin− . Furthermore, considering that they were Sox1 lin−  and that HoxA2 gene was highly expressed, it is suggested that they are cells whose embryological origin is cranial neural fold. 
     Example 4 
     Example 4 
     Exploration of Ectodermal Derived Mesenchymal Cell 
     Upon examination of bone marrow tissue of the femur, vertebra, sternum, ilium, hip joint (femoral head and lumbar lid), and skull collected from Pα-H2B-GFP::Prx1-Cre::Rosa26-tdTomato mouse, Pα +  and Prx1 lin−  cells were specifically present in the vertebra (within the scope of this investigation) ( FIGS. 9 and 10 ). 
     Example 5 
     Example 5 
     Lineage Markers of CFPα Cells in Vertebra and Femur 
     For CFPα cells obtained by culturing vertebral and femoral bone marrow, P0 lineage, Prx1 lineage, Sox1 lineage, and LepR lineage were examined using a cell lineage tracing mouse. As the result, CFPα cells in the vertebra were P0 lin+ , Prx1 lin− , and Sox1 lin− , and about 60% of them were negative for LepRlin ( FIG. 11 ). CFPα cells in the femur were P0 lin+ , Prx1 lin+ , and Sox1 lin− , and about 80% of them were positive for LepRlin ( FIG. 12 ). Such results suggest that a source of iCFPα cells (Pα lin+ , P0 lin+ , Sox1 lin− , LepR lin− ) in peripheral blood is present in vertebra. 
     Example 6 
     Example 6 
     Properties of CFPα Cells in Vertebra and Femur 
     Colony-forming ability and differentiation potency into osteoblasts, adipocytes and chondrocytes were compared between CFPα cells obtained by culturing vertebral bone marrow and those obtained by culturing femoral bone marrows. As the result, vertebral CFPα cells showed higher abilities in both colony-forming ability and differentiation potency ( FIGS. 13 to 16 ). Furthermore, differentiation induction of CFPα cells of vertebra and femur was performed with all-transletinoic acid (ATRA) and BMP-4. As the result, a colony expressing Keratin 5 was observed ( FIGS. 17 and 18 ), confirming that CFPα cells in vertebrae and femur include cells having differentiation potency into K5-positive cells. 
     Example 7 
     Example 7 
     Transcriptome Analysis of Pα Cells in Vertebra and Femur 
     Pα cells were sorted from vertebral and femoral bone marrow cells, and single-cell transcriptome analysis was performed. As the result of clustering analysis, Pα cells in the bone marrow were divided into six clusters ( FIG. 19 ). The six clusters were defined as (1) S34-MSC (Sca1 and CD34-expressing), (2) osteoprogenitor (Osteomodulin and Wnt16-expressing), (3) osteoblast (osterix and osteocalcin-expressing), (4) osteocyte(PHEX and DMP1-expressing), (5) CAR cell (CXCL12 and LepR-expressing), and (6) CD45-expressing cell, based on the genes specifically expressed by the cells of each cluster. Comparing between vertebra and femur, it can be seen that the vertebra has more S34-MSC cluster cells and fewer CAR cluster cells, whereas the femur has more CAR cluster cells and fewer S34-MSC cluster cells. 
     In addition, cells of Sca1 + CD34 + , Sca1 + CD34 − , and Sca1 − CD34 +  were included in the S34-MSC cluster. Pα cells of vertebra were then separated by FACS using Sca1 and CD34 expressions as indicators, and a CFU assay was performed. As the result, CFU activity was high in the order of Sca1 + CD34 +  cells&gt;Sca1 + CD34 −  cells and Sca1 − CD34 +  cells&gt;Sca1 − CD34 −  cells ( FIG. 20 ). 
     Example 8 
     Example 8 
     Correspondence Between Pα Cells in Peripheral Blood and Pα Cells of Vertebra 
     Clustering analysis was performed with Pα cells of vertebra and femur, based on transcriptome analysis data of iCFPα cells in peripheral blood. As the result, iCFPα cells in peripheral blood were located near the S34-MSC cluster ( FIG. 21 ). Furthermore, the peripheral blood iCFPα cell was CD34 + Sca1 − . Considering this result in conjunction with lineage markers, it is suggested that peripheral blood iCFPα cells correspond to CD34 + Sca1 −  cells included in the S34-MSC cluster of vertebra. 
     Example 9 
     Example 9 
     Sca1 + CD34 +  Cells in Bone Marrow 
     Sca1 + CD34 +  cells contained in the S34-MSC cluster of bone marrow specifically expressed Procr ( FIG. 22 ). Thus, Procr can be a marker for Sca1 + CD34 +  cells that are considered to be the most proliferative and the most hierarchical cell among bone marrow Pα cells. Furthermore, an examination of the amount of Sca1 + CD34 +  cells present in cervical vertebra, thoracic vertebra, lumbar vertebra and femur showed that amount was high in the order of cervical vertebra&gt;thoracic vertebra&gt;lumbar vertebra&gt;femur ( FIG. 23 ). 
     Example 10 
     Example 10 
     Contribution of Circulating Cells in Blood Induced by HMGB1 Administration to Tissue-Regeneration 
     The present inventors have previously identified a peptide (HA1-44 peptide) consisting of the amino acid sequence of positions 1-44 (SEQ ID NO: 1) at the N-terminus of HMGB1 protein as a domain having the activity of mobilizing bone marrow-derived Pα-positive mesenchymal stem cells into peripheral blood. Now, the present inventors have obtained the following experimental results concerning cells in peripheral blood induced by administration of the HA1-44 peptide.
     (1) Culturing peripheral blood of the lineage tracing mice administered the HA1-44 peptides resulted in more colonies than peripheral blood in the control group (saline administration). All of the colonies were Pα-positive, and the majority of which were Prx1 lineage-negative ( FIG. 24 ).   (2) A parabiosis model of Pα-Cre::Rosa26-EYFP mouse and wild-type (WT) mouse was created, and a skin of epidermolysis bullosa mouse was grafted into the wild-type mouse, then HA1-44 peptide was administered to Pα-Cre::Rosa26-EYFP mouse. As the result, the presence of Pα lin+  cells expressing type 7 collagen was confirmed in a regenerated epithelial tissue in the skin graft ( FIG. 25 ).   (3) A parabiosis model of Pα-H2B-GFP::Prx1-Cre::Rosa26-tdTomato mouse and a wild-type mouse was created, and a skin of a wild-type neonate mouse was grafted on the back of the wild-type mouse, then the HA1-44 peptide was administered to the Pα-H2B-GFP::Prx1-Cre::Rosa26-tdTomato mouse. As the result, the presence of Pα+ and Prx1 lin−  cells in the skin graft was confirmed ( FIG. 26 ).   (4) A parabiosis model of P0-Cre::Rosa26-tdTomato mouse and a wild-type mouse was created, and a cartilage injury was created to the knee joint of the wild-type mouse, then the HA1-44 peptide was administered to the P0-Cre::Rosa26-tdTomato mouse. As the result, the accumulation of P0 lin+  cells was confirmed at the cartilage injury site in the HA1-44 peptide administration group, whereas no accumulation of P0 lin+  cells was seen in the control group (saline administration) ( FIG. 27 ). Furthermore, a cartilage injury was created to the knee joint of a wild-type mouse alone that was not a parabiosis model, then the HA1-44 peptide or saline was administered. As the result, hyaline cartilage was regenerated at the cartilage injury site in the HA1-44 peptide administration group, whereas only fibrous cartilage was seen at the cartilage injury site in the saline administration group ( FIG. 28 ).   

     From the above results, it is believed that Pα cells (Pα + P0 lin+ Prx1 lin− cells) in peripheral blood induced by the HA1-44 peptide are the same as iCFPα cells induced by a necrotic tissue injury, or include at least iCFPα cells, which serve to repair injury of tissues such as epidermis and cartilage. 
     Example 11 
     Example 11 
     Change in Cells Induced by HMGB1 Administration 
     (1) Peripheral blood was collected from the mouse administered with the HA1-44 peptides and from the mouse administered with saline, and cultured on a plastic plate to obtain a colony of adhesive cells. Transcriptome analysis was performed on the cells on a colony basis, and clustering was performed with an ICGS algorithm based on the obtained data. The obtained results are shown in  FIG. 29 . In addition,  FIG. 30  is a simplified representation of the results of the clustering. In the screening method of the present application, a substance having a result similar to the HA1-44 peptide, for example, a substance to have a result in which a cluster characterized by predicted cell types (or expression of a gene set corresponding to predicted cell types) similar to  FIG. 30  is formed, and the number of colonies belonging to each cluster is “cluster 1: saline group test substance group, cluster 2: saline group&lt;test substance group, cluster 3: saline group&gt;test substance group, cluster 4: saline group&gt;test substance group” can be evaluated as a candidate for a substance having multipotent stem cell-inducing activity. 
     (2) Transcriptome analysis was performed on Pα cells of vertebra collected from the mouse administered with HA1-44 peptide and the mouse administered with saline, and pathway analysis was performed with IPA based on the obtained data. As the result, pathways associated with EIF2 signaling, regulation of eIF4 and p70S6K signaling, and mTOR signaling were activated in vertebral Pα cells in the HA1-44 peptide administration group compared to those in the control group ( FIG. 31 ). Furthermore, expression of cell death-related genes was suppressed in vertebral Pα cells in the HA1-44 peptide administration group compared to those in the control group ( FIG. 32 ). 
     Example 12 
     Example 12 
     Activity of HMGB1 Peptide in Human 
     In Phase I clinical studies, intravenous administration of the HA1-44 peptide showed an increase of CD45-negative, TER-119-negative, and PDGFRβ-positive cells in circulating blood ( FIG. 33 ). Since PDGFRβ is a marker of human mesenchymal stem cells, it is believed that a marker (including a combination of multiple markers) defining iCFPα cells and vertebral CFPα cells in peripheral blood described herein, in which the term “PDGFRα” is replaced with the term “PDGFRβ”, will also be a marker (including a combination of multiple markers) defining EMSC (colony-forming PDGFR-positive cells in peripheral blood or in vertebra) in humans. 
     INDUSTRIAL APPLICABILITY 
     An ectomesenchymal stem cell in peripheral blood according to the present invention has superior proliferative ability and multi-lineage differentiation potency than a bone marrow-derived mesenchymal stem cell conventionally used in regenerative medicine, and can be used in a cell transplantation therapy or the like as a therapeutic cell obtainable by peripheral blood collection method that is less invasive than bone marrow aspiration. Furthermore, the characteristics (markers, or the like) of ectomesenchymal stem cells in peripheral blood that contribute to the regeneration of injured tissues have been revealed. Thus, a substance having an activity to induce multipotent stem cells in vivo can be efficiently screened using the cell as an indicator.