Patent Publication Number: US-2021163918-A1

Title: Cell-Containing Hydrogel Body and Method for Producing Same

Description:
TECHNICAL FIELD 
     The present invention relates to a cell-containing hydrogel body and a method of producing the same. 
     BACKGROUND ART 
     In Patent Literature 1, there is a description that regenerated hair follicle primordia were prepared by adding a cell mixture suspension of epithelial cells and mesenchymal cells collected from a mouse embryo to a microwell plate having regularly arranged microwell portions, and culturing a mixture of the cells while supplying oxygen thereto. 
     In Patent Literature 2, there is a description of a method of producing a cell mass, involving adding a Wnt signal activator to a mixed cell culture solution, which contains hair papilla cells and keratinocytes, and culturing the culture solution containing the Wnt signal activator by a hanging drop method. 
     In Patent Literature 3, there is a description of a method involving forming a cell lump of hair follicle mesenchymal cells in a culture solution, and then allowing the cell lump to coexist with epithelial cells in the culture solution to cause the epithelial cells to undergo cell adhesion to the periphery of the cell lump of the hair follicle mesenchymal cells through a cell sorting phenomenon, to thereby produce an artificial hair bulb in which the epithelial cells are in cell adhesion to the outside of the cell lump of the hair follicle mesenchymal cells. In Patent Literature 3, there is also a description of a method involving allowing hair follicle mesenchymal cells and epithelial cells to coexist in a culture solution to form a cell lump of the hair follicle mesenchymal cells, and cause the epithelial cells to undergo cell adhesion to the periphery of the cell lump, through a cell sorting phenomenon, to thereby produce an artificial hair bulb in which the epithelial cells are in cell adhesion to the outside of the cell lump of the hair follicle mesenchymal cells. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] WO 2017/073625 A1 
     [PTL 2] JP 2008-029331 A 
     [PTL 3] JP 2003-070466 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, hitherto, in the case of using two or more kinds of cells, it has not been easy to control the size of a boundary surface at which the cells of one kind interact with the cells of another kind. 
     The present invention has been made in view of the above-mentioned problem, and one of the objects of the present invention is to provide a cell-containing hydrogel body and a method of producing the same, which enable simple and effective control of the size of a boundary surface for an interaction between cells. 
     Solution to Problem 
     In order to achieve the above-mentioned object, according to one embodiment of the present invention, there is provided a method of producing a cell-containing hydrogel body, including: forming, under a gas phase, a first hydrogel droplet on a surface of a substrate, the first hydrogel droplet containing first cells being dispersed therein and a first hydrogel polymer; forming, under a gas phase, a second hydrogel droplet on the surface, the second hydrogel droplet containing second cells being dispersed therein and a second hydrogel polymer, the second hydrogel droplet being combined with the first hydrogel droplet; and forming, under a gas phase, a cell-containing hydrogel body on the surface by gelling a hydrogel droplet-combined body including a first droplet portion derived from the first hydrogel droplet and a second droplet portion derived from the second hydrogel droplet. According to the one embodiment of the present invention, a method of producing a cell-containing hydrogel body, which enables simple and effective control of the size of a boundary surface for an interaction between cells, is provided. 
     The method may further include culturing the first cells and the second cells in the cell-containing hydrogel body. In this case, the first cells and the second cells in the cell-containing hydrogel body may be cultured after the cell-containing hydrogel body is removed from the surface. Further, in this case, the first cells and the second cells may be cultured in the cell-containing hydrogel body in a floating state after the cell-containing hydrogel body is removed from the surface. 
     In addition, in the method, the first cells and the second cells in the cell-containing hydrogel body may be cultured to provide a cell-containing hydrogel body containing a first cell aggregate formed through aggregation of the first cells and/or a second cell aggregate formed through aggregation of the second cells. 
     In addition, in the method, the first cells and the second cells in the cell-containing hydrogel body may be cultured to provide a cell-containing hydrogel body having the following property (a) and/or property (b): (a) the cell-containing hydrogel body contains the first cell aggregate and a first hydrogel covering portion covering the first cell aggregate, and a density of the first hydrogel polymer inside the first cell aggregate is higher than that in the first hydrogel covering portion; (b) the cell-containing hydrogel body contains the second cell aggregate and a second hydrogel covering portion covering the second cell aggregate, and a density of the second hydrogel polymer inside the second cell aggregate is higher than that in the second hydrogel covering portion. 
     In addition, in the method, the first hydrogel polymer may have a cell binding property with respect to the first cells, and/or the second hydrogel polymer may have a cell binding property with respect to the second cells. In addition, in the method, a combination of the first cells and the second cells may be a combination of epithelial cells and mesenchymal cells. 
     In order to achieve the above-mentioned object, according to one embodiment of the present invention, there is provided a cell-containing hydrogel body, including: a first hydrogel portion, which is a gelled body of a first hydrogel droplet containing first cells and a first hydrogel polymer; and a second hydrogel portion, which is a gelled body of a second hydrogel droplet containing second cells and a second hydrogel polymer, and which is combined with the first hydrogel portion, wherein the first hydrogel portion contains a first cell aggregate that is an aggregated body of the first cells, and/or wherein the second hydrogel portion contains a second cell aggregate that is an aggregated body of the second cells. According to the one embodiment of the present invention, a cell-containing hydrogel body, in which the size of a boundary surface for an interaction between cells is simply and effectively controlled, is provided. 
     The cell-containing hydrogel body may have the following property (a) and/or property (b): (a) the cell-containing hydrogel body contains the first cell aggregate and a first hydrogel covering portion covering the first cell aggregate, and a density of the first hydrogel polymer inside the first cell aggregate is higher than that in the first hydrogel covering portion; (b) the cell-containing hydrogel body contains the second cell aggregate and a second hydrogel covering portion covering the second cell aggregate, and a density of the second hydrogel polymer inside the second cell aggregate is higher than that in the second hydrogel covering portion. 
     In addition, in the cell-containing hydrogel body, the first hydrogel polymer may have a cell binding property with respect to the first cells, and/or the second hydrogel polymer may have a cell binding property with respect to the second cells. In addition, in the cell-containing hydrogel body, a combination of the first cells and the second cells may be a combination of epithelial cells and mesenchymal cells. In addition, the cell-containing hydrogel body may be in a floating state in a solution. 
     Advantageous Effects of Invention 
     According to the present invention, a cell-containing hydrogel body and a method of producing the same, which enable simple and effective control of the size of a boundary surface for an interaction between cells, are provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is an explanatory diagram for schematically illustrating some operations included in an example of a method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 1B  is an explanatory diagram for schematically illustrating other operations included in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 1C  is an explanatory diagram for schematically illustrating still other operations included in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 1D  is an explanatory diagram for schematically illustrating still other operations included in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 1E  is an explanatory diagram for schematically illustrating still other operations included in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 2  is an explanatory diagram for schematically illustrating an automatic dispensing system to be used in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 3A  is an explanatory diagram for schematically illustrating some operations, which involve using the automatic dispensing system, included in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 3B  is an explanatory diagram for schematically illustrating other operations, which involve using the automatic dispensing system, included in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 3C  is an explanatory diagram for schematically illustrating still other operations, which involve using the automatic dispensing system, included in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 4  is an explanatory diagram showing a photograph of hydrogel droplet-combined bodies formed using an automatic dispensing apparatus in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 5A  is an explanatory diagram for schematically illustrating some of culture operations included in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 5B  is an explanatory diagram for schematically illustrating other culture operations included in an example of the method of producing a cell-containing hydrogel body according to one embodiment of the present invention. 
         FIG. 6A  is an explanatory diagram showing a fluorescence micrograph of a cell-containing hydrogel body on the initial day of culture in Example 1 according to one embodiment of the present invention. 
         FIG. 6B  is an explanatory diagram showing a fluorescence micrograph of cell-containing hydrogel bodies on day 1 of culture in Example 1 according to one embodiment of the present invention. 
         FIG. 6C  is an explanatory diagram showing a fluorescence micrograph of cell-containing hydrogel bodies on day 3 of culture in Example 1 according to one embodiment of the present invention. 
         FIG. 7  is an explanatory diagram showing the results of evaluation of the projected areas of cell-containing hydrogel bodies under a phase-contrast microscope in Example 1 according to one embodiment of the present invention. 
         FIG. 8A  is an explanatory diagram showing a photograph of cell-containing hydrogel bodies, from which hair has grown, on day 11 after transplantation in Example 2 according to one embodiment of the present invention. 
         FIG. 8B  is an explanatory diagram showing a photograph of cell-containing hydrogel bodies, from which hair has grown, on day 19 after transplantation in Example 2 according to one embodiment of the present invention. 
         FIG. 8C  is an explanatory diagram showing a photograph of cell-containing hydrogel bodies, from which hair has grown, on day 24 after transplantation in Example 2 according to one embodiment of the present invention. 
         FIG. 9A  is an explanatory diagram showing a fluorescence micrograph of a spheroid-fused tissue on day 1 of culture in Example 3 according to one embodiment of the present invention. 
         FIG. 9B  is an explanatory diagram showing a fluorescence micrograph of a spheroid-fused tissue on day 3 of culture in Example 3 according to one embodiment of the present invention. 
         FIG. 9C  is an explanatory diagram showing a fluorescence micrograph of a spheroid-fused tissue on day 6 of culture in Example 3 according to one embodiment of the present invention. 
         FIG. 10A  is an explanatory diagram showing a fluorescence micrograph of a cell-containing hydrogel body on day 1 of culture in Example 3 according to one embodiment of the present invention. 
         FIG. 10B  is an explanatory diagram showing a fluorescence micrograph of a cell-containing hydrogel body on day 3 of culture in Example 3 according to one embodiment of the present invention. 
         FIG. 10C  is an explanatory diagram showing a fluorescence micrograph of a cell-containing hydrogel body on day 6 of culture in Example 3 according to one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, embodiments of the present invention will be described. The present invention is not limited to these embodiments. 
     In  FIG. 1A  to  FIG. 1E , operations included in an example of a method of producing a cell-containing hydrogel body according to one embodiment of the present invention (hereinafter referred to as “method of the present invention”) are schematically illustrated. The method of the present invention includes: forming, under a gas phase  100 , a first hydrogel droplet  21  on a surface  111  of a substrate  110 , the first hydrogel droplet  21  containing first cells  11  being dispersed therein and a first hydrogel polymer; forming, under the gas phase  100 , a second hydrogel droplet  22  on the surface  111 , the second hydrogel droplet  22  containing second cells  12  being dispersed therein and a second hydrogel polymer, the second hydrogel droplet  22  being combined with the first hydrogel droplet  21 ; and forming, under the gas phase  100 , a cell-containing hydrogel body  40  on the surface  111  by gelling a hydrogel droplet-combined body  30  including a first droplet portion  31  derived from the first hydrogel droplet  21  and a second droplet portion  32  derived from the second hydrogel droplet  22 . 
     That is, in the method of the present invention, first, as illustrated in  FIG. 1A  and  FIG. 1B , the first hydrogel droplet  21  is formed on the surface  111  of the substrate  110  in the gas phase  100 . The first hydrogel droplet  21  contains the dispersed first cells  11  and the first hydrogel polymer. 
     The first hydrogel droplet  21  is formed by dropping a first hydrogel aqueous solution containing the dispersed first cells  11  and the first hydrogel polymer onto the surface  111  of the substrate  110 . In the example illustrated in  FIG. 1A , the first hydrogel droplet  21  is formed by dropping the first hydrogel aqueous solution onto the surface  111  of the substrate  110  through use of a pipette  200 . 
     The volume of the first hydrogel droplet  21  is not particularly limited, but may be, for example, 0.01 μL or more and 1 mL or less, 0.1 μL or more and 100 μL or less, or 1 μL or more and 10 μL or less. 
     The density of the first cells  11  contained in the first hydrogel droplet  21  is not particularly limited, but may be, for example, 1×10 2  cells/mL or more and 1×10 9  cells/mL or less, 1×10 3  cells/mL or more and 1×10 8  cells/mL or less, 1×10 4  cells/mL or more and 1×10 8  cells/mL or less, 1×10 5  cells/mL or more and 1×10 7  cells/mL or less, or 1×10 6  cells/mL or more and 1×10 7  cells/mL or less. 
     The first hydrogel droplet  21  mainly contains the first cells  11  as cells. That is, the ratio of the number of the first cells  11  to the total number of cells contained in the first hydrogel droplet  21  may be, for example, 50% or more, and is preferably 60% or more, more preferably 70% or more, still more preferably 80% or more, and particularly preferably 90% or more. 
     In the method of the present invention, after the first hydrogel droplet  21  is formed on the surface  111  of the substrate  110 , as illustrated in  FIG. 1C , the second hydrogel droplet  22  combined with the first hydrogel droplet  21  is formed on the surface  111  in the gas phase  100 . The second hydrogel droplet  22  contains the dispersed second cells  12  and the second hydrogel polymer. 
     The second hydrogel droplet  22  is formed by dropping a second hydrogel aqueous solution containing the dispersed second cells  12  and the second hydrogel polymer to a position on the surface  111  of the substrate  110  so that the second hydrogel droplet  22  becomes adjacent to the first hydrogel droplet  21  and the second hydrogel droplet  22  comes into contact with the first hydrogel droplet  21 . As a result, a part of the second hydrogel droplet  22  is brought into contact with the surface  111 , and another part thereof is brought into contact with the first hydrogel droplet  21 . In the example illustrated in  FIG. 1C , the second hydrogel droplet  22  is formed by dropping the second hydrogel aqueous solution onto the surface  111  of the substrate  110  through use of the pipette  200 . 
     The volume of the second hydrogel droplet  22  is not particularly limited, but may be, for example, 0.01 μL or more and 1 mL or less, 0.1 μL or more and 100 μL or less, or 1 μL or more and 10 μL or less. 
     The density of the second cells  12  contained in the second hydrogel droplet  22  is not particularly limited, but may be, for example, 1×10 2  cells/mL or more and 1×10 9  cells/mL or less, 1×10 3  cells/mL or more and 1×10 8  cells/mL or less, 1×10 4  cells/mL or more and 1×10 8  cells/mL or less, 1×10 5  cells/mL or more and 1×10 7  cells/mL or less, or 1×10 6  cells/mL or more and 1×10 7  cells/mL or less. 
     The second hydrogel droplet  22  mainly contains the second cells  12  as cells. That is, the ratio of the number of the second cells  12  to the total number of cells contained in the second hydrogel droplet  22  may be, for example, 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more. 
     Then, in the method of the present invention, as illustrated in  FIG. 1D , the hydrogel droplet-combined body  30  (hereinafter referred to simply as “droplet-combined body  30 ”) including the first droplet portion  31  derived from the first hydrogel droplet  21  and the second droplet portion  32  derived from the second hydrogel droplet  22  is formed on the surface  111  in the gas phase  100 . 
     The first droplet portion  31  of the droplet-combined body  30  contains the dispersed first cells  11  and the first hydrogel polymer. In addition, the second droplet portion  32  of the droplet-combined body  30  contains the dispersed second cells  12  and the second hydrogel polymer. In the droplet-combined body  30 , the first droplet portion  31  and the second droplet portion  32  each have a part thereof brought into contact with the surface  111 , and are combined with each other at other parts thereof. 
     The first hydrogel droplet  21  and the second hydrogel droplet  22  contain the first hydrogel polymer and the second hydrogel polymer, respectively, and hence each have higher viscosity and specific gravity than water. Accordingly, although the droplet-combined body  30  formed on the surface  111  in the gas phase  100  is a liquid and hence has fluidity, as illustrated in  FIG. 1D , the first cells  11  and the second cells  12  are maintained in the state of being dispersed in the first droplet portion  31  and the second droplet portion  32  of the droplet-combined body  30 , respectively. 
     The dispersed cells are cells that are dispersed and floating in a solution, each of which is not bound to other cells or only bound to several cells. For example, when a cell suspension containing dispersed cells is centrifuged in order to separate the cells and a solvent contained in the cell suspension, a pellet of the cells formed after the centrifugation is an aggregate of the cells, and the cells constituting the pellet are not dispersed cells. 
     In the example illustrated in  FIG. 1A  to  FIG. 1E , the droplet-combined body  30  is formed of the two hydrogel droplet portions  31  and  32 , but may be formed by, for example, similarly combining three or more hydrogel droplets. 
     That is, for example, after the droplet-combined body  30  formed of the first hydrogel droplet portion  31  and the second hydrogel droplet portion  32  is formed on the surface  111 , a third hydrogel droplet containing dispersed third cells and a third hydrogel polymer may be further formed on the surface  111  in the gas phase  100  so as to be combined with the droplet-combined body  30 , to thereby form a droplet-combined body  30  that further contains a third hydrogel droplet portion derived from the third hydrogel droplet. 
     The volume of the droplet-combined body  30  which is finally formed on the surface  111  is not particularly limited, but may be, for example, 0.02 μL or more and 100 mL or less, 0.2 μL or more and 10 mL or less, or 2 μL or more and 1 mL or less. 
     The density of the cells contained in the droplet-combined body  30  is not particularly limited, but may be, for example, 1×10 2  cells/mL or more and 1×10 9  cells/mL or less, 1×10 3  cells/mL or more and 1×10 8  cells/mL or less, 1×10 4  cells/mL or more and 1×10 8  cells/mL or less, 1×10 5  cells/mL or more and 1×10 7  cells/mL or less, or 1×10 6  cells/mL or more and 1×10 7  cells/mL or less. 
     For such dropping of the hydrogel droplets  21  and  22  as described above, for example, as illustrated in  FIG. 2 , an automatic dispensing system  300  including a plurality of the pipettes  200  that are regularly arranged is preferably used. The automatic dispensing system  300  illustrated in  FIG. 2  includes a tank  310  configured to accommodate a mixed aqueous solution of an aqueous solution S 1  containing dispersed cells  10  and a hydrogel aqueous solution S 2  containing a hydrogel polymer, and a stage  320  on which the substrate  110  is to be placed, with operation of the dispensing system being controlled by a computer  400 . 
     Through use of the automatic dispensing system  300 , a plurality of regularly arranged hydrogel droplets  20  are efficiently formed on the surface  111  of the substrate  110  by dropping the mixed aqueous solution in the tank  310  from each of the plurality of the pipettes  200  onto the surface  111 . 
     In  FIG. 3A  to  FIG. 3C , examples of operations involving using the automatic dispensing system  300  are schematically illustrated. First, as illustrated in  FIG. 3A , a plurality of the first hydrogel droplets  21  are dropped from the plurality of the pipettes  200  onto the surface  111  in the gas phase  100 . Then, as illustrated in  FIG. 3B , a plurality of the second hydrogel droplets  22  are dropped from the plurality of the pipettes  200  onto the surface  111  so as to each be combined with one of the plurality of the first hydrogel droplets  21 . As a result, as illustrated in  FIG. 3C , a plurality of the droplet-combined bodies  30  are formed, regularly arranged on the surface  111  and each formed of the first droplet portion  31  and the second droplet portion  32 . 
     In  FIG. 4 , a photograph of the droplet-combined bodies  30  actually formed using the automatic dispensing system  300  is shown. In each of the plurality of the droplet-combined bodies  30  regularly arranged on the surface  111  shown in  FIG. 4 , a colored left half is the first hydrogel droplet portion  31  having a dye added thereto in order to enhance visibility, and an uncolored right half is the second hydrogel droplet portion  32 . 
     In the method of the present invention, as illustrated in  FIG. 1E , the cell-containing hydrogel body  40  (hereinafter referred to simply as “hydrogel body  40 ”) is formed on the surface  111  in the gas phase  100  by further gelling the droplet-combined body  30 . 
     Through the gelling, the hydrogel polymers contained in the droplet-combined body  30  form an intermolecular network, and the droplet-combined body  30  loses fluidity, with the result that the hydrogel body  40  is obtained. A method for the gelling is not particularly limited, and for example, the gelling is performed under appropriate conditions depending on the kinds and/or concentrations of the hydrogel polymers contained in the droplet-combined body  30 . Specifically, for example, when the droplet-combined body  30  contains type I collagen as each of the hydrogel polymers, the gelling may be performed by maintaining the droplet-combined body  30  at from 25° C. to 37° C. for from 15 minutes to 60 minutes. 
     The hydrogel body  40  includes a first hydrogel portion  41  which is a gelled body of the first hydrogel droplet  21 , and a second hydrogel portion  42  which is a gelled body of the second hydrogel droplet  22  and is combined with the first hydrogel portion  41 . The first hydrogel portion  41  is also a gelled body of the first droplet portion  31  of the droplet-combined body  30 , and the second hydrogel portion  42  is also a gelled body of the second droplet portion  32  of the droplet-combined body  30 . 
     As described later, in the hydrogel body  40  that has not been cultured (i.e., the hydrogel body  40  before culture or at the start of culture), the first hydrogel portion  41  contains the dispersed first cells  11  and the gelled first hydrogel polymer, and the second hydrogel portion  42  contains the dispersed second cells  12  and the gelled second hydrogel polymer. 
     The hydrogel body  40  does not have fluidity because it is gelled. Accordingly, also in the first hydrogel portion  41  and the second hydrogel portion  42  of the hydrogel body  40  obtained by the gelling, the first cells  11  and the second cells  12  are respectively maintained in the state of being dispersed. In the hydrogel body  40 , the first hydrogel portion  41  and the second hydrogel portion  42  each have a part thereof brought into contact with the surface  111 , and are combined with each other at other parts thereof. 
     The volume of the hydrogel body  40  that has not been cultured is not particularly limited, but may be, for example, 0.02 μL or more and 100 mL or less, 0.2 μL or more and 10 mL or less, or 2 μL or more and 1 mL or less. 
     The density of the cells contained in the hydrogel body  40  that has not been cultured is not particularly limited, but may be, for example, 1×10 2  cells/mL or more and 1×10 9  cells/mL or less, 1×10 3  cells/mL or more and 1×10 8  cells/mL or less, 1×10 4  cells/mL or more and 1×10 8  cells/mL or less, 1×10 5  cells/mL or more and 1×10 7  cells/mL or less, or 1×10 6  cells/mL or more and 1×10 7  cells/mL or less. 
     As described above, in the method of the present invention, first, a plurality of hydrogel droplets each containing dispersed cells are sequentially dropped so as to be combined with each other to form a droplet-combined body on the surface  111  of the substrate  110  in the gas phase  100 , and then the droplet-combined body is gelled to provide the hydrogel body  40 , which contains the dispersed cells, on the surface  111  in the gas phase  100 . 
     Since the plurality of hydrogel droplets are combined with each other in the gas phase  100 , the size of a boundary surface for an interaction between the cells in the finally formed hydrogel body  40  is simply and effectively adjusted by, for example, adjusting the size of each hydrogel droplet and/or the arrangement of the plurality of hydrogel droplets. 
     The first cells  11  and the second cells  12  are not particularly limited as long as the cells are living cells derived from an animal. The animal may be a human or a non-human animal (animal other than a human). The non-human animal is not particularly limited, but is preferably a non-human vertebrate (vertebrate other than a human). The non-human vertebrate is not particularly limited, but is preferably a non-human mammal. The non-human mammal is not particularly limited, but may be, for example, a primate (e.g., a monkey), a rodent (e.g., a mouse, a rat, a hamster, a guinea pig, or a rabbit), a carnivore (e.g., a dog or a cat), or an ungulate (e.g., a pig, a cow, a horse, a goat, or a sheep). 
     The first cells  11  and/or the second cells  12  may be differentiated cells or stem cells (undifferentiated cells). The stem cells may be totipotent stem cells, pluripotent stem cells, or tissue stem cells. Specifically, the stem cells may be, for example, induced pluripotent (iPS) stem cells, embryonic stem (ES) cells, or embryonic germ (EG) cells. The differentiated cells are not particularly limited as long as the cells have a differentiated function, but may be, for example, cells collected from a living body (which may be cells cultured after being collected from a living body) or cells induced from stem cells in vitro. 
     The first cells  11  and/or the second cells  12  may be cells derived from a tissue in a living body. The tissue in a living body is not particularly limited, but may be, for example, a hair follicle tissue, a skin tissue, a liver tissue, a heart tissue, a renal tissue, a nervous tissue, a bone tissue, a cartilage tissue, a bone marrow tissue, a lung tissue, a gland tissue, a periodontal tissue, or blood. 
     The first cells  11  and/or the second cells  12  may be adherent cells or non-adherent cells. The adherent cells are cells that are present in the state of adhering to other cells and/or an extracellular matrix in a living body. The non-adherent cells are cells that are present in a floating state in a living body (e.g., blood cells, such as lymphocytes). 
     The combination of the first cells  11  and the second cells  12  is not particularly limited as long as the combination is a combination of different cells that interact with each other. The combination of different cells is, for example, a combination of cells different in one or more selected from the group consisting of differentiation function, growth potential, and cell surface markers. 
     An interaction between the first cells  11  and the second cells  12  is not particularly limited, but for example, a combination in which a substance secreted by cells of one kind acts on cells of the other kind and/or a combination capable of forming binding between cells is preferred. When a substance secreted by cells of one kind acts on cells of the other kind, a substance secreted by the cells of the other kind may further act on the cells of the one kind. In addition, when a substance secreted by cells of one kind acts on cells of the other kind, the substance may be diffused in the hydrogel body  40  to act on the cells of the other kind, or the substance may be diffused into a solution (e.g., culture solution) containing the hydrogel body  40  and then act on the cells of the other kind in the hydrogel body  40  from within the solution. 
     The combination of the first cells  11  and the second cells  12  is preferably a combination of cells that interact with each other in a living body. In this case, the combination of the first cells  11  and the second cells  12  may be a combination of cells that interact with each other in the same tissue in a living body. The tissue in a living body is not particularly limited, but may be, for example, a hair follicle tissue, a skin tissue, a liver tissue, a heart tissue, a renal tissue, a nervous tissue, a bone tissue, a cartilage tissue, a bone marrow tissue, a lung tissue, a gland tissue, a periodontal tissue, or blood. 
     The combination of the first cells  11  and the second cells  12  may be a combination of cells derived from the same animal or a combination of cells derived from different animals. That is, the first cells  11  and the second cells  12  may both be human cells (cells derived from a human), may both be non-human animal cells (cells derived from an animal other than a human), or may be human cells and non-human animal cells, respectively, or vice versa. 
     The combination of the first cells  11  and the second cells  12  may be, for example, a combination of epithelial cells and mesenchymal cells. The combination of epithelial cells and mesenchymal cells is not particularly limited, but may be, for example, a combination of epithelial cells and mesenchymal cells that interact with each other in a hair follicle tissue. 
     In this case, the epithelial cells and/or the mesenchymal cells may be cells collected from a hair follicle tissue of a living body (which may be cells cultured after being collected from the hair follicle tissue) or cells induced from stem cells in vitro. 
     Specifically, the epithelial cells may be cells of the outermost layer of the outer root sheath in the bulge region of a hair follicle tissue, epithelial cells derived from the hair matrix portion, or hair follicle epithelial cells induced from stem cells (e.g., iPS cells, ES cells, or EG cells). In addition, the epithelial cells may be epithelial stem cells. 
     The mesenchymal cells may be hair papilla cells, dermal root sheath cells, skin mesenchymal cells in a developmental period, or hair follicle mesenchymal cells induced from stem cells (e.g., iPS cells, ES cells, or EG cells). 
     The first hydrogel polymer and the second hydrogel polymer are not particularly limited as long as the first hydrogel polymer and the second hydrogel polymer are each a hydrophilic polymer having a gelling ability. The first hydrogel polymer and/or the second hydrogel polymer may be a naturally occurring polymer or an artificially synthesized polymer, but is preferably a naturally occurring polymer. In addition, the first hydrogel polymer and/or the second hydrogel polymer are preferably a biocompatible polymer. 
     The first hydrogel polymer and/or the second hydrogel polymer are preferably an extracellular matrix. The extracellular matrix is not particularly limited as long as the extracellular matrix exists in a living body. 
     The first hydrogel polymer may have a cell binding property with respect to the first cells  11 , and/or the second hydrogel polymer may have a cell binding property with respect to the second cells  12 . A hydrogel polymer having a cell binding property is a polymer that binds to a cell surface molecule, and has, for example, a particular amino acid sequence and/or sugar chain that specifically or non-specifically binds to the cell surface molecule. 
     Specifically, the hydrogel polymer having a cell binding property may be, for example, one or more selected from the group consisting of collagen (e.g., one or more selected from the group consisting of type I, type II, type III, type IV, type V, and type XI), fibronectin, laminin, elastin, glycosaminoglycans (e.g., hyaluronic acid), proteoglycans, fibrin, and gelatin. 
     In addition, the hydrogel polymer may be one or more selected from the group consisting of gelatin, agarose, sodium alginate, and synthetic polymers (e.g., polyacrylamide, polyvinyl alcohol, methylcellulose, and polyethylene oxide). 
     The first hydrogel polymer and the second hydrogel polymer may be polymers of the same kind or polymers of different kinds. That is, for example, the first hydrogel polymer and the second hydrogel polymer may both be type I collagen, or may be as follows: the first hydrogel polymer is type I collagen, and the second hydrogel polymer is a glycosaminoglycan. 
     The substrate  110  is not particularly limited as long as the hydrogel droplets  21  and  22 , the droplet-combined body  30 , and the hydrogel body  40  can be formed on the surface  111  thereof. The substrate  110  may be, for example, a resin, glass, ceramics, or metal substrate. 
     The surface  111  of the substrate  110  is not particularly limited as long as the hydrogel droplets  21  and  22 , the droplet-combined body  30 , and the hydrogel body  40  can be formed thereon, but is preferably a water-repellent surface. 
     The water contact angle of the water-repellent surface may be, for example, 90° or more, and is preferably 100° or more, more preferably 105° or more, particularly preferably 110° or more. 
     The surface  111  that is water-repellent is achieved by, for example, the use of the substrate  110  that is made of a water-repellent material (e.g., a hydrophobic resin, such as a fluorine-containing polymer), and/or water-repellent treatment (e.g., modification with a hydrophobic functional group, such as a fluorine-containing functional group). In addition, the surface  111  of the substrate  110  is preferably flat. 
     The gas phase  100  is not particularly limited as long as the gas phase  100  is a phase of a gas, but the gas preferably contains oxygen, and air is preferably used. In the formation of the first hydrogel droplet  21 , the formation of the second hydrogel droplet  22  (formation of the droplet-combined body  30 ), and the formation of the hydrogel body  40 , the respective gas compositions of the gas phase  100  may be identical to or different from each other. 
     The method of the present invention may further include culturing the first cells  11  and the second cells  12  in the hydrogel body  40 . In this case, the hydrogel body  40  is immersed in a culture solution, and the first cells  11  and the second cells  12  are cultured in the hydrogel body  40 . 
     The culture solution is not particularly limited as long as the culture solution is an aqueous solution having properties, such as composition, pH, and osmotic pressure, required for maintaining the survival of the first cells  11  and the second cells  12 . As components to be contained in the culture solution, there are given, for example, sugars, amino acids, vitamins, inorganic salts, antibiotics, and growth factors. 
     A culture time is not particularly limited, and may be, for example, 12 hours or more and 10 days or less, or 1 day or more and 7 days or less. 
     A culture temperature is not particularly limited as long as the culture temperature falls within a range in which the survival of the first cells  11  and the second cells  12  can be maintained. The culture temperature may be, for example, 25° C. or more and 40° C. or less, and is preferably 30° C. or more and 39° C. or less. 
     In the method of the present invention, the first cells  11  and the second cells  12  may be cultured in the hydrogel body  40  on the surface  111  of the substrate  110 . In this case, for example, the surface  111  having the hydrogel body  40  formed thereon is immersed in a culture solution, and in the culture solution, the first cells  11  and the second cells  12  are cultured in the hydrogel body  40  fixed onto the surface  111 . 
     In the method of the present invention, the first cells  11  and the second cells  12  may be cultured in the hydrogel body  40  after the hydrogel body  40  is removed from the surface  111 . In this case, first, the hydrogel body  40  is removed from the surface  111  and recovered. A method of removing the hydrogel body  40  from the surface  111  is not particularly limited as long as the method causes the hydrogel body  40  to be removed from the surface  111  while maintaining the survival of the cells contained in the hydrogel body  40 , but for example, a method involving immersing the surface  111  in an aqueous solution, such as a culture solution, or a method involving applying a flow of an aqueous solution, such as a culture solution, to the surface  111  is preferably used. 
     In the method of the present invention, since the hydrogel body  40  that has been removed from the surface  111  and recovered is used as it is for the culture of the cells in the hydrogel body  40 , it is preferred to remove the hydrogel body  40  from the surface  111  without performing enzymatic treatment for decomposing the hydrogel polymers contained in the hydrogel body  40 . 
     After the hydrogel body  40  is removed from the surface  111 , the hydrogel body  40  may be fixed onto a surface of another substrate to culture the first cells  11  and the second cells  12  in the hydrogel body  40  on this surface. 
     In the method of the present invention, after the hydrogel body  40  is removed from the surface  111 , as illustrated in  FIG. 5A , the first cells  11  and the second cells  12  may be cultured in the hydrogel body  40  which is in a floating state. 
     In this case, first, the hydrogel body  40  that has been removed from the surface  111  and recovered is immersed in a culture solution  300  in a culture vessel  120 , and the first cells  11  and the second cells  12  contained in the hydrogel body  40  are cultured in a state in which the hydrogel body  40  is floating in the culture solution  300 . 
     The state in which the hydrogel body  40  is floating in the culture solution  300  is a state in which the hydrogel body  40  is substantially free from adhering to wall surfaces  121  and  122  of the culture vessel  120 . That is, for example, the hydrogel body  40  in a floating state is not only the hydrogel body  40  floating in the culture solution  300  in which there is no flow, but also the hydrogel body  40  in the state of adhering to the wall surfaces  121  and  122  of the culture vessel  120  so weakly as to be easily removed from the wall surfaces  121  and  122  when a flow is generated in the culture solution  300 . 
     In the method of the present invention, as illustrated in  FIG. 5B , the hydrogel body  40  may be cultured to provide a hydrogel body  40  that contains a first cell aggregate  51  formed through aggregation of the first cells  11  and/or a second cell aggregate  52  formed through aggregation of the second cells  12 . 
     The first cell aggregate  51  is formed by the first cells  11  binding to each other and spontaneously aggregating in the hydrogel body  40 . Similarly, the second cell aggregate  52  is formed by the second cells  12  binding to each other and spontaneously aggregating in the hydrogel body  40 . 
     When the first cells  11  and the second cells  12  form binding in the hydrogel body  40 , as illustrated in  FIG. 5B , the hydrogel body  40  that contains the first cell aggregate  51  and the second cell aggregate  52  bound to the first cell aggregate  51  is obtained. 
     A cell-containing hydrogel body according to one embodiment of the present invention is the hydrogel body  40  obtained as described above. The hydrogel body  40  includes: a first hydrogel portion  41 , which is a gelled body of a first hydrogel droplet  21  containing first cells  11  and a first hydrogel polymer; and a second hydrogel portion  42 , which is a gelled body of a second hydrogel droplet  22  containing second cells  12  and a second hydrogel polymer, and that is combined with the first hydrogel portion  41 , wherein the first hydrogel portion  41  contains a first cell aggregate  51  that is an aggregated body of the first cells  11 , and/or wherein the second hydrogel portion  42  contains a second cell aggregate  52  that is an aggregated body of the second cells  12 . 
     The hydrogel body  40  that contains the first cell aggregate and/or the second cell aggregate  52  may be adhered to a surface of a substrate, but may be in a floating state in a solution. That is, the hydrogel body  40  may be floating in a solution such as the above-mentioned culture solution. 
     When, as described above, a plurality of hydrogel droplets are first combined with form the hydrogel body  40  in the gas phase  100 , and then cells are cultured in the hydrogel body  40  to form a plurality of cell aggregates, the size of a boundary surface for an interaction between the plurality of cell aggregates in the hydrogel body  40  to be finally obtained can be simply and effectively controlled by, for example, adjusting the sizes of the plurality of hydrogel droplets and/or the arrangement of the plurality of hydrogel droplets. 
     In the cultured hydrogel body  40 , the first cell aggregate  51  and the second cell aggregate  52  may be formed apart from each other, but as illustrated in  FIG. 5B , the first cell aggregate  51  and the second cell aggregate  52  may be bound to each other. 
     In this case, some of the first cells  11  contained in the first cell aggregate  51  form binding with some of the second cells  12  contained in the second cell aggregate  52 . As a result, as illustrated in  FIG. 5B , a composite cell aggregate  50 , which includes the first cell aggregate  51  and the second cell aggregate  52  bound to each other, is formed in the hydrogel body  40 . 
     Specifically, for example, when the first cells  11  are hair follicle epithelial cells (e.g., epithelial stem cells derived from a hair follicle tissue) and the second cells  12  are hair follicle mesenchymal cells (e.g., hair papilla cells), the composite cell aggregate  50  that is a hair follicle primordium may be formed in the following manner: in the hydrogel body  40 , the first cells  11  form the first cell aggregate  51  and the second cells  12  form the second cell aggregate  52 , and further, the first cell aggregate  51  and the second cell aggregate  52  bind to each other. 
     When, as described above, a plurality of hydrogel droplets are first combined with form the hydrogel body  40  in the gas phase  100 , and then cells are cultured in the hydrogel body  40  to form a plurality of cell aggregates bound to each other, the size of a binding surface between the plurality of cell aggregates in the hydrogel body  40  to be finally obtained can be simply and effectively controlled by, for example, adjusting the sizes of the plurality of hydrogel droplets and/or the arrangement of the plurality of hydrogel droplets. 
     The hydrogel body  40  that contains the first cell aggregate  51  and/or the second cell aggregate  52  is effectively produced by culturing the first cells  11  and the second cells  12  in the floating hydrogel body  40  after removing the hydrogel body  40  from the surface  111 . 
     In the method of the present invention, the hydrogel body  40  ( FIG. 5A ) that contains the dispersed first cells  11  and the dispersed second cells  12  may be cultured to provide a shrunken hydrogel body  40  ( FIG. 5B ) that contains the first cell aggregate  51  and/or the second cell aggregate  52 . 
     That is, when the cells  11  and  12  are bound to each other and gradually aggregate to form the cell aggregates  51  and  52  in the hydrogel body  40 , the hydrogel body  40  shrinks with the progress of culture time. 
     Specifically, the ratio of the volume of the hydrogel body  40  that contains the first cell aggregate  51  and/or the second cell aggregate  52  to the volume of the hydrogel body  40  at the start of culture, which contains the first cells  11  and the second cells  12  that are dispersed, may be, for example, 50% or less, 40% or less, 30% or less, 20% or less, or 10% or less. 
     Such shrinkage of the hydrogel body  40  with the progress of culture becomes remarkable when the first hydrogel polymer has a cell binding property with respect to the first cells  11 , and/or the second hydrogel polymer has a cell binding property with respect to the second cells  12 . 
     The density of cells in the hydrogel body  40  containing the first cell aggregate  51  and/or the second cell aggregate  52  (number of cells contained in a unit volume of the hydrogel body  40 ) may be, for example, 1×10 2  cells/mL or more and 1×10 11  cells/mL or less, 1×10 3  cells/mL or more and 1×10 10  cells/mL or less, 1×10 4  cells/mL or more and 1×10 10  cells/mL or less, 1×10 5  cells/mL or more and 1×10 9  cells/mL or less, or 1×10 6  cells/mL or more and 1×10 9  cells/mL or less. 
     In the method of the present invention, as illustrated in  FIG. 5B , the first cells  11  and the second cells  12  may be cultured in the hydrogel body  40  to provide a hydrogel body  40  having the following property (a) and/or property (b): (a) the hydrogel body contains the first cell aggregate  51  and a first hydrogel covering portion  41   a  covering the first cell aggregate  51 , and a density of the first hydrogel polymer inside the first cell aggregate  51  is higher than that in the first hydrogel covering portion  41   a ; (b) the hydrogel body contains the second cell aggregate  52  and a second hydrogel covering portion  42   a  covering the second cell aggregate  52 , and a density of the second hydrogel polymer inside the second cell aggregate  52  is higher than that in the second hydrogel covering portion  42   a.    
     That is, as illustrated in  FIG. 5A  and  FIG. 5B , when the first cells  11  aggregate to form the first cell aggregate  51  and/or the second cells  12  aggregate to form the second cell aggregate  52  in the hydrogel body  40  with the progress of culture time, a hydrogel part between the first cells  11  and/or between the second cells  12  shrinks remarkably compared to a hydrogel part covering the first cell aggregate  51  and/or the second cell aggregate  52 . 
     As a result, in the hydrogel body  40  containing the first cell aggregate  51  and/or the second cell aggregate  52 , the density of the first hydrogel polymer inside the first cell aggregate  51  becomes higher than that in the first hydrogel covering portion  41   a , and/or the density of the second hydrogel polymer inside the second cell aggregate  52  becomes higher than that in the second hydrogel covering portion  42   a.    
     Such local concentrating of the first hydrogel polymer and/or the second hydrogel polymer becomes remarkable particularly when the first hydrogel polymer has a cell binding property with respect to the first cells  11 , and/or the second hydrogel polymer has a cell binding property with respect to the second cells  12 . 
     That is, in this case, since the first cells  11  and/or the second cells  12  aggregate while pulling the hydrogel polymer to which cells are adhering toward themselves, a hydrogel polymer in the vicinity of the first cells  11  and/or the second cells  12  is remarkably concentrated compared to a hydrogel polymer distant from the first cells  11  and/or the second cells  12 . 
     The distribution of the density of a hydrogel polymer in the hydrogel body  40  may be identified by, for example, using a staining method specific to the hydrogel polymer. Specifically, for example, the density of the first hydrogel polymer in the first cell aggregate  51  and the density of the first hydrogel polymer in the first hydrogel covering portion  41   a  may be quantitatively compared by: staining the first hydrogel polymer contained in the hydrogel body  40  with a fluorescence-labeled antibody; observing the hydrogel body  40  after the staining under a fluorescence microscope; and comparing a fluorescence intensity in the first cell aggregate  51  and a fluorescence intensity in the first hydrogel covering portion  41   a.    
     Specifically, in the hydrogel body  40  that contains the first cell aggregate  51 , the density of the first hydrogel polymer inside the first cell aggregate  51  may be, for example, 2 or more times, 5 or more times, or 10 or more times the density of the first hydrogel polymer in the first hydrogel covering portion  41   a.    
     Such uneven distribution of a hydrogel polymer in the hydrogel body  40  results from the characteristic operations of the method of the present invention of first forming the hydrogel body  40  containing dispersed cells in the gas phase  100  and then culturing the cells in the hydrogel body  40  to form cell aggregates. 
     In the hydrogel body  40  to be produced by the method of the present invention, the size of a boundary surface for an interaction between the first cells  11  and the second cells  12 , and/or the size of a boundary surface for an interaction between the first cell aggregate  51  and the second cell aggregate  52  (their binding surface when the first cell aggregate  51  and the second cell aggregate  52  are bound to each other) can be arbitrarily set, and hence the hydrogel body  40  is useful as, for example, a research tool for an interaction between cells, or a tissue body for transplantation in which a desired interaction between cells is achieved. 
     Specifically, for example, the hydrogel body  40  that contains a hair follicle primordium formed through binding of a cell aggregated body of hair follicle epithelial cells and a cell aggregated body of hair follicle mesenchymal cells as described above also enables the number of hairs grown from the hair follicle primordium after transplantation to be adjusted by adjusting the size of the binding surface of the two cell aggregated bodies. 
     Accordingly, the present invention is useful, for example, for the treatment of a patient having damaged hair follicles due to a disease, an accident, or the like, or as a research tool for the treatment. Examples of the disease to which the present invention is applicable include androgenetic alopecia (AGA), female androgenetic alopecia (FAGA), postpartum alopecia, diffuse alopecia, seborrheic alopecia, alopecia pityroides, traction alopecia, alopecia caused by metabolic disorders, pressure alopecia, alopecia areata, neurotic alopecia, hair-pulling disorder, alopecia universalis, and symptomatic alopecia. 
     In addition, the hydrogel body  40  to be produced by the method of the present invention is not limited to a hair follicle tissue, and is also useful, for example, as a tissue body reconstructing in vitro a tissue such as a hair follicle tissue, a skin tissue, a liver tissue, a heart tissue, a renal tissue, a nervous tissue, a bone tissue, a cartilage tissue, a bone marrow tissue, a lung tissue, a gland tissue, a periodontal tissue, or blood, for the treatment of a disease (e.g., utilization as a transplantation tissue), or as a research tool for, for example, searching for a substance that may be used for the treatment or prevention of a disease associated with hair loss and/or searching for a substance involved in the mechanism of the disease. 
     Next, specific Examples according to the embodiments of the present invention will be described. 
     EXAMPLE 1 
     [Collection of Epithelial Cells and Mesenchymal Cells] 
     A dorsal skin tissue was collected from a C57BL/6 mouse embryo at embryonic day 18, and was subjected to dispase treatment using a partially modified version of a method reported by Nakao et al. (Koh-ei Toyoshima et al. Nature Communications, 3, 784, 2012) at 4° C. under the shaking condition of 30 rpm for 1 hour to separate the epithelial layer and mesenchymal layer of the skin tissue. After that, the epithelial layer was treated with 100 U/mL collagenase for 1 hour and 20 minutes and further treated with trypsin for 10 minutes to isolate epithelial cells. In addition, the mesenchymal layer was treated with 100 U/mL collagenase for 1 hour and 20 minutes to isolate mesenchymal cells. 
     [Formation of Cell-containing Hydrogel Bodies] 
     The mesenchymal cells collected as described above were suspended in a type I collagen solution (collagen Type 1-A, manufactured by Nitta Gelatin Inc.) to prepare a first hydrogel cell suspension containing the dispersed mesenchymal cells at a density of 1×10 4  cells/2 μL. Subsequently, in the atmosphere, about 2 μL of the first hydrogel cell suspension was dropped onto a water-repellent surface of a polystyrene substrate to form a first hydrogel droplet containing dispersed mesenchymal cells. 
     Meanwhile, the epithelial cells collected as described above were suspended in a type I collagen solution (collagen Type 1-A, manufactured by Nitta Gelatin Inc.) to prepare a second hydrogel cell suspension containing the dispersed epithelial cells at a density of 1×10 4  cells/2 μL. Subsequently, in the atmosphere, about 2 μL of the second hydrogel cell suspension was dropped to a position on the water-repellent surface adjacent to the first hydrogel droplet to form a second hydrogel droplet containing dispersed epithelial cells and being combined with the first hydrogel droplet. Thus, hydrogel droplet-combined bodies each formed by combining the first hydrogel droplet and the second hydrogel droplet were obtained on the water-repellent surface in the atmosphere. 
     After that, in the atmosphere, the cell-containing hydrogel bodies were obtained by incubating the hydrogel droplet-combined bodies on the water-repellent surface at 37° C. for 30 minutes for gelling the collagen. 
     [Culture of Cell-Containing Hydrogel Bodies] 
     A culture solution was poured on the cell-containing hydrogel bodies, which had been formed as described above, with a pipette, and thus the cell-containing hydrogel bodies were removed from the water-repellent surface and recovered, and were dispersed in the culture solution. A mixed medium of DMEM medium and KG2 medium (containing 10% fetal bovine serum and 1% penicillin) was used as the culture solution. 
     Subsequently, with use of a 96-well plate for suspension culture (Primesurface (trademark) 96U plate) as a culture vessel, 100 μL aliquots of the culture solution containing the cell-containing hydrogel bodies were added so that each well of the plate contained one of the cell-containing hydrogel bodies. After that, the cell-containing hydrogel bodies were cultured in a floating state in the wells for 3 days. 
     [Results] 
     In  FIG. 6A ,  FIG. 6B , and  FIG. 6C , fluorescence micrographs of cell-containing hydrogel bodies on the initial day (D 0 ) of culture, day 1 (D 1 ) of culture, and day 3 (D 3 ) of culture are shown, respectively. Parts shown in a whitish color in  FIG. 6A  to  FIG. 6C  are mesenchymal cells having their cell membranes stained in advance with a fluorescent dye (Vybrant (trademark) DiI Cell-Labeling Solution). In addition, parts surrounded by white dashed lines in  FIG. 6A  to  FIG. 6C  are cell-containing hydrogel bodies. The scale bar in each of  FIG. 6A  to  FIG. 6C  represents 200 μm. 
     In  FIG. 7 , the results of evaluation of the projected areas (mm 2 ) of the cell-containing hydrogel bodies on the initial day (D 0 ) of culture, day 1 (D 1 ) of culture, day 2 (D 2 ) of culture, and day 3 (D 3 ) of culture under a phase-contrast microscope are shown (n=10). 
     As shown in  FIG. 6A  to  FIG. 6C  and  FIG. 7 , as the culture time progressed, in each of the cell-containing hydrogel bodies, the mesenchymal cells spontaneously aggregated with each other and the epithelial cells spontaneously aggregated with each other to respectively form a mesenchymal cell aggregate and an epithelial cell aggregate, and besides, the cell-containing hydrogel body shrank. In addition, in each of the cell-containing hydrogel bodies, the mesenchymal cell aggregate and the epithelial cell aggregate had formed binding. 
     Calculation based on the results of  FIG. 7  found that the volume of the cell-containing hydrogel bodies on day 1 of culture was about 5% of that of the cell-containing hydrogel bodies on the initial day of culture, and the volume of the cell-containing hydrogel bodies on day 3 of culture was about 1% of the volume of the cell-containing hydrogel bodies on the initial day of culture. 
     Such shrinkage of the cell-containing hydrogel bodies with the progress of culture was presumed to have occurred because, for example, the cells contained in the cell-containing hydrogel bodies adhered to fibers of collagen gel, and further, the cells aggregated while pulling the collagen gel toward themselves. 
     EXAMPLE 2 
     [Collection of Epithelial Cells and Mesenchymal Cells] 
     Epithelial cells and mesenchymal cells were each isolated from a skin tissue of a C57BL/6 mouse embryo at embryonic day 18 in the same manner as in Example 1 described above. 
     [Formation of Cell-Containing Hydrogel Bodies] 
     In the same manner as in Example 1 described above, in the atmosphere, on a water-repellent surface, first hydrogel droplets each containing dispersed mesenchymal cells and second hydrogel droplets each containing dispersed epithelial cells were combined with form hydrogel droplet-combined bodies, and further, the hydrogel droplet-combined bodies were gelled to form cell-containing hydrogel bodies. 
     [Culture of Cell-Containing Hydrogel Bodies] 
     In the same manner as in Example 1 described above, the cell-containing hydrogel bodies were removed from the water-repellent surface and recovered, and were cultured in a floating state in a mixed medium for 3 days. 
     [Transplantation to Mouse] 
     The cell-containing hydrogel bodies cultured for 3 days as described above (hair follicle primordia produced in vitro) were recovered and intradermally transplanted to a nude mouse. That is, the nude mouse was anesthetized by inhalation of isoflurane, and the dorsal part thereof was disinfected with Isodine. Subsequently, a V-lance micro-scalpel (Alcon Japan Ltd.) was used to form incisions for transplantation ranging from the epidermal layer of the skin to a lower part of the dermal layer. Then, the incisions for transplantation were each injected with one of the cell-containing hydrogel bodies. The care of the nude mouse and the transplantation experiment were performed in conformity with the guidelines of the animal experimental committee at Yokohama National University. 
     [Results] 
     In  FIG. 8A ,  FIG. 8B , and  FIG. 8C , photographs of hairs grown from the cell-containing hydrogel bodies transplanted to the nude mouse on day 11 after transplantation, day 19 after transplantation, and day 24 after transplantation are shown, respectively. 
     As shown in  FIG. 8A  to  FIG. 8C , hair growth from the transplanted cell-containing hydrogel bodies was recognized on day 11 after transplantation. Then it was recognized on day 19 after transplantation and day 24 after transplantation that hair was further lengthened. That is, the cell-containing hydrogel bodies produced as described above were shown to be useful for treatment or research concerning hair regeneration. 
     EXAMPLE 3 
     [Collection of Epithelial Cells and Mesenchymal Cells] 
     Epithelial cells and mesenchymal cells were each isolated from a skin tissue of a C57BL/6 mouse embryo at embryonic day 18 in the same manner as in Example 1 described above. 
     [Formation of Cell-containing Hydrogel Bodies] 
     In the same manner as in Example 1 described above, in the atmosphere, on a water-repellent surface, first hydrogel droplets each containing dispersed mesenchymal cells and second hydrogel droplets each containing dispersed epithelial cells were combined with form hydrogel droplet-combined bodies, and further, the hydrogel droplet-combined bodies were gelled to form cell-containing hydrogel bodies. 
     [Culture of Cell-Containing Hydrogel Body] 
     The cell-containing hydrogel bodies were subjected to suspension culture in a mixed medium for 3 days in the same manner as in Example 1 described above. 
     [Formation of Spheroid-Fused Tissue] 
     Meanwhile, as a comparative control, a spheroid-fused tissue was produced by fusing an epithelial cell spheroid and a mesenchymal cell spheroid to each other, each of which having been formed in advance. That is, first, epithelial cells and mesenchymal cells were each isolated from a skin tissue of a C57BL/6 mouse embryo at embryonic day 18 in the same manner as in Example 1 described above. 
     Subsequently, a cell suspension containing dispersed epithelial cells was inoculated into each well of a 96-well spheroid culture plate (Prime surface 96U, manufactured by Sumitomo Bakelite Co., Ltd.) at 1×10 4  cells/100 μL. 
     Then, the epithelial cells were subjected to suspension culture in each well for 1 day to allow the epithelial cells to spontaneously aggregate, to thereby form one epithelial cell spheroid as an aggregate of the epithelial cells in each well. 
     Similarly, the mesenchymal cells were subjected to suspension culture in each well for 1 day to allow the mesenchymal cells to spontaneously aggregate, to thereby form one mesenchymal cell spheroid as an aggregate of the mesenchymal cells in each well. 
     Further, one epithelial cell spheroid and one mesenchymal cell spheroid were placed in the same well, and subjected to suspension culture for 6 days. A mixed medium of DMEM medium and KG2 medium (containing 10% fetal bovine serum and 1% penicillin) was used as a culture solution. In the culture of the spheroid mixture, one epithelial cell spheroid and one mesenchymal cell spheroid fused to each other with the progress of culture time to form one spheroid-fused tissue. 
     [Results] 
     In  FIG. 9A ,  FIG. 9B , and  FIG. 9C , fluorescence micrographs of the spheroid-fused tissue on day 1, day 3, and day 6 of culture are shown, respectively. In  FIG. 10A ,  FIG. 10B , and  FIG. 10C , fluorescence micrographs of the cell-containing hydrogel body on day 1, day 3, and day 6 of culture are shown, respectively. In each of the figures, the part surrounded by a white dashed line is the spheroid-fused tissue or the cell-containing hydrogel body. In addition, the part shown in a whitish color in the spheroid-fused tissue or the cell-containing hydrogel body of each figure indicates mesenchymal cells having their cell membranes stained in advance with a fluorescent dye (Vybrant (trademark) DiI Cell-Labeling Solution). The scale bar in each figure represents 200 μm. 
     As shown in  FIG. 9A  to  FIG. 9C , with the progress of culture time, the shape of the spheroid-fused tissue changed significantly and the cells therein migrated, and on day 6 of culture, the epithelial cells and the mesenchymal cells were present as a mixture throughout the entire tissue. 
     On the other hand, as shown in  FIG. 10A  to  FIG. 10C , in the cell-containing hydrogel body, on day 1 of culture, the epithelial cells had aggregated to form an epithelial cell aggregate, and the mesenchymal cells had aggregated to form a mesenchymal cell aggregated body. Then, after day 1 of culture, the shape of the cell-containing hydrogel body hardly changed, and a portion containing the epithelial cell aggregated body and a portion containing the mesenchymal cell aggregated body were stably maintained. 
     That is, in the cell-containing hydrogel body, a boundary between the epithelial cell aggregated body and the mesenchymal cell aggregated body was stably maintained throughout the culture period. Black spots observed on the mesenchymal cell aggregate in the cell-containing hydrogel body shown in  FIG. 10C  were presumed to indicate the presence of a melanin pigment.