BACKGROUND
it is now well established that the cns contains a pool of neural stem and progenitor cells that are either located in specific locations called niches or dispersed in the parenchyma, notably in the white matter.
these cells can be isolated using a cell sorting strategy based on the expression of markers such as cd <dig> and a2b <dig>  they typically grow on non adherent substrates when they generate clonal expansions called neurospheres  <cit> , which on differentiation, generate glial cells and neurons.
this endogenous pool of immature cells is activated and recruited in several cns pathologies such as stroke or degenerative diseases in an attempt to compensate for the cellular loss.
much of our knowledge on these cells is derived from studies performed in rodents, but much less is known on these stem cells in human.
adult neural stem and progenitor cells have been isolated from different parts of the human brain, namely the olfactory bulb, the subventricular zone, the hippocampus, the cortex, using the neurosphere assay  <cit> .
more recently, using adherent conditions and a specific media, adult human neural precursors have been isolated from the human temporal cortex and the hippocampus  <cit> .
these cells express glial markers and are able to generate both glial and mature neuronal cell types in vivo and in vitro.
they could represent a potential endogenous cellular source that could be used to alleviate neuronal loss which occurs in many cns diseases.
however, much more work is needed to isolate and characterize these cells from different human cns locations and, in particular, to analyse the signalling mechanisms leading to their differentiation.
besides neural stem cells, the adult cns might also contain a pool of mesenchymal stem cells  which appear to differentiate into mesenchymal lineage cells  and possibly neural lineage cells.
however this has mainly been observed in mice  <cit>  and the actual differentiation capacity of these cells remains to be validated in detail.
compared to brain, little is known about neural precursors in the adult spinal cord  <cit> .
we recently isolated multipotential cells in the human adult spinal cord using the neurosphere method  <cit> .
in this article, we set out to explore the possibility of isolating adult neural stem/progenitor cells from the adult human spinal cord using adherent conditions previously described  <cit> .
we found that these conditions mainly result in the propagation of mesenchymal cells expressing snai <dig> and twist <dig> and with the hallmarks of smooth muscle muscular cells.
these cells do not appear to be msc, nor neural stem cells, but they can readily mineralize in vitro when placed in appropriate conditions.
additional characterization of these cells allowed us to identify the nkx <dig>  homeodomain transcription factor as a marker for these cells.
RESULTS
isolation of nkx <dig> + vascular muscular cells from adult human spinal cord
to isolate human neural stem cells using adherent condition as used in  <cit> , human spinal cord was rapidly processed as previously described  <cit>  and seeded with neural stem cell medium containing serum and egf/fgf.
under these conditions, cells with a mesenchymal morphology  grew rapidly and could be propagated for approximately  <dig> months  before undergoing proliferation decrease, probably as a consequence of cellular senescence .
the cells were characterized at passage  <dig> by qpcr and immunofluorescence.
we first measured the expression of genes typically expressed by cells of the neural lineage, i.e astrocytes , oligodendrocytes , oligodendrocyte progenitor cells , neurons  and stem cells .
surprisingly, although these cells were derived from the cns, we found that the expression of these genes was either low or undetectable  and only nestin, cnp and pdgfrα/β could be detected with less than  <dig> cycles of qpcr.
we confirmed by immunofluorescence the absence of detectable expression of sox <dig>  gfap and olig <dig> which are well documented markers for neural stem cells and glial cells .
we then questioned the presence of markers frequently expressed in mesenchymal cells , bearing in mind that these can also be found expressed in cells of the neural lineage, and mesenchymal transcription factors .
in sharp contrast with the neural lineage genes, the expression of these genes was readily detected .
we confirmed by immunofluorescence and western blot that snai <dig> and twist <dig> were expressed at the protein level .
these two transcription factors were detected in the nuclei of most cells  .
the possibility existed that the isolated cells were derived from the in vitro differentiation of endogenous spinal neural precursor cells which would adopt a mesenchymal phenotype when cultured in serum-containing medium.
to explore this hypothesis, we cultured human fetal brain neural stem cells in this medium for three passages and then explored the phenotype of these cells by qpcr and immunofluorescence .
in contrast with cells isolated from the adult spinal cord, our immunofluorescence analysis revealed that these cells expressed a high level of several typical neural markers such as gfap, sox <dig>  olig <dig> whereas snai <dig> and twist <dig> were not detectable .
qpcr confirmed these results and further indicated the maintenance of several typical markers for neural cells  even when cultured in serum-containing media .
the high expression of transcripts for smooth muscle actin  in spinal cord cells suggested that the culture contained a majority of vascular muscular cells, an assumption consistent with the detection of nestin and pdgfrα/β , two markers also expressed by vascular cells  <cit> .
in order to confirm this possibility, we explored by immunofluorescence the expression of acta <dig> and nestin, together with calponin, caldesmon and ng <dig>  three other markers of vascular muscular cells.
the vast majority of the cells  were positive for these five proteins, confirming that these cultures mostly contained vascular muscular cells .
one unexpected transcription factor detected in the culture by qpcr was nkx <dig>  .
this homeodomain protein is implicated in the ontogenesis of oligodendrocytes and of pancreatic cells  <cit>  but its expression in vascular cells has not been reported in adults.
the presence of nkx <dig>  protein was confirmed by immunofluorescence and western blot in these cultures .
the vast majority of the cells  showed a clear nuclear staining and these cells coexpress calponin, caldmesmon and ng <dig> .
in contrast, nkx <dig>  was not detected in human fetal neural stem cells cultured in the same conditions for three passages .
the expression of nkx <dig>  by vascular cells was then confirmed in vivo on adult human spinal cord sections.
nkx <dig> + cells were detected around large vessels in the meninges and in the parenchyma .
as previously reported  <cit> , nkx <dig>  was also expressed by ependymal cell of the central canal .
we concluded from these data that adherent cultures derived from adult human spinal cord led to the isolation of a cellular population mainly composed of vascular muscular cells that express the homeodomain protein nkx <dig>  and the mesenchymal transcription factors snai <dig> and twist <dig>
nkx <dig> + cells are not multipotential stem cells but mineralize in vitro
it has recently been reported that adult stem cells with mesenchymal features located in various tissues could behave as multipotential stem cells  <cit> .
these cells could give rise to cells of several lineages including neurons and myelinating cells.
we thus explored the multipotentiality of nkx <dig> + cells by placing them in different media and culture conditions.
first, by culturing them in non adherent conditions in defined neural stem cell medium, these cells were not able to form neurospheres, a hallmark of neural stem cells.
second, we further examined the possibility of a neural differentiation by labelling the cells with a gfp-expressing lentivirus, then by coculturing them for  <dig> weeks with human young neurons, obtained by differentiation of human fetal brain neural stem cells.
in these conditions, we could not detect staining of gfp-labelled cells  with neuronal , astrocytic  or oligodendrocytic  markers, strongly indicating the absence of neural differentiation of these cells .
third, we treated the cells with bmp <dig>  bmp <dig>  bmp <dig>  cntf, cntf/bmp <dig>  lif, lif/bmp <dig> or t <dig> to promote an astrocytic  <cit>  or an oligodendrocytic fate  <cit>  respectively, but again no differentiation was observed .
considering the mesenchymal features of these cells and as vascular muscular cells have been reported to be endowed with stem cell properties  <cit>  we next examined the capacity of nkx <dig> + cells to generate chondrocytes, adipocytes and osteoblasts, three lineages commonly produced by mesenchymal stem cells.
the cells were placed in adequate media, and the differentiation was monitored by staining with oil red o , alizarine red s  and alcian blue  and by qpcr for markers for adipocytes , chondrocytes  and osteoblasts .
as a control, the differentiation of human mesenchymal stem cells isolated from bone marrow was monitored in parallel.
under these conditions, no differentiation towards chondrocytes or adipocytes was observed .
only deposition of calcium phosphate precipitates, as observed with alizarine red s staining , could convincingly be detected.
this was accompanied by a sharp increase of alkaline phosphatase mrna  and activity which was detected with naphthol asbi phosphate and fast red trisodium staining .
no increase of the other osteoblast markers compared to that obtained with bone marrow mesenchymal stem cells could be demonstrated  suggesting absence of complete differentiation towards osteoblasts.
equally, we could not detect the expression of nkx <dig>  by western blot  and immunofluorescence  in bone marrow mesenchymal stem cells, suggesting that cns nkx <dig> + vascular cells are weakly related to these cells.
we also examined the capacity of human fetal neural stem cells cultured in the presence of serum to mineralize in the same conditions.
figure 6a, c shows however that these cells were unable to do so.
finally, to ascertain that mineralization was produced by nkx <dig> + cells and not by other contaminating cell types, we performed single cell cloning in  <dig> wells to isolate nkx <dig> + cell clones which were then placed in mineralizating conditions.
among  <dig> isolated nkx <dig> + clones, 95% of them were able to mineralize .
we concluded from these studies that nkx <dig> + vascular muscular cells isolated from the adult cns do not behave as neural or mesenchymal stem cells but can readily mineralize in vitro.
discussion
neural stem cells are generally isolated and cultured using non adherent conditions and defined medium to generate free-floating aggregates, the so-called neurospheres.
adherent cultures with serum-containing medium have also been successfully used to isolate and culture adult neural stem cells from adult human brain .
we previously reported the isolation of human spinal cord stem/progenitor sox2+ cells using the neurosphere method  <cit> , however we were unable to identify such cells using adherent conditions in the present study.
considering the low number of these cells as compared to other spinal cord cells, it is possible that these cells are rapidly diluted during the culture passages.
alternatively it is conceivable that the presence of serum leads to their premature differentiation.
we report here that, in contrast to neural stem/progenitor cells, these conditions lead to the isolation of a cellular population mainly composed of nkx <dig> + vascular muscular cells.
although these cells express some markers for neural and mesenchymal stem cells, such as nestin or cd <dig>  they do not behave as multipotential stem cells, at least in the conditions we employed.
in fact, only differentiation along the osteogenic lineage, as evidenced by their ability to mineralize in vitro, was observed.
the calcification of brain vessel muscle cells has been observed since  <dig>  <cit>  in pathological situations or as part of the brain aging process  <cit> .
in elderly brains, mild calcification of vessels and parenchyma is a normal and common incidental finding in the basal ganglia, pineal gland, choroid plexus, dura and habenula.
this vasculopathy may account for the sclerosis of the aged hippocampus and the concomitant loss of ca <dig> neurons  <cit> .
there is also now clear evidence for a close association between vasculature and neurogenesis  <cit> , the so-called vascular stem cell niche, and the observation of an age-related neurogenesis decline  <cit>  may be related to some modifications of vascular cell properties.
in addition, recent work by göritz c, et al  <cit>  demonstrated a critical role of vascular cells in the formation of the scar following spinal cord injury.
the extensive growth of nkx <dig> + human vascular muscular cells under our conditions will allow further studies on their interactions with adult neural stem cells, their role in the aging process and their implication in the scar formation.
nkx <dig>  is a homeodomain protein that plays a major role in cns development, notably in spinal cord  <cit> .
being a target of the ventralizing shh morphogen, nkx <dig>  is required for motoneuron generation and correct oligodendrogenesis.
in agreement with the detection of nkx <dig>  in adult spinal cord meninges , nkx <dig>  is also expressed in ventral meninges during spinal cord development  <cit> .
outside the cns, nkx <dig>  is required for pancreatic β-cell development and its disruption results in 90% decrease of these cells  <cit> .
nkx <dig>  is also detected in esophageal and tracheal smooth muscle cells during development  <cit> .
consistent with this observation, we report here that in the adult human spinal cord, vascular smooth muscle cells express this transcription factor in vitro and in vivo.
its role in these cells remains to be explored.
as nkx <dig>  is implicated in the positive regulation of β-cell proliferation  <cit> , notably by direct interaction with the cyclin a <dig> and b <dig> promoters, a similar function could be hypothesized for cns vascular muscular cells proliferation.
CONCLUSIONS
in this study we have isolated vascular cells from the human adult spinal cord and showed that these cells are neither endowed with mesenchymal nor neural stem cell properties.
we demonstrated that these cells express the developmental homedomain protein nkx <dig> , two mesenchymal transcription factors  and that they can readily mineralize when placed in osteogenic conditions.
these cells constitute a useful model for studying cns vessel calcification in vitro.

 it is now well established that the cns contains a pool of neural stem and progenitor cells that are either located in specific locations called niches or dispersed in the parenchyma, notably in the white matter. these cells can be isolated using a cell sorting strategy based on the expression of markers such as cd and a2b they typically grow on non adherent substrates when they generate clonal expansions called neurospheres, which on differentiation, generate glial cells and neurons. this endogenous pool of immature cells is activated and recruited in several cns pathologies such as stroke or degenerative diseases in an attempt to compensate for the cellular loss. much of our knowledge on these cells is derived from studies performed in rodents, but much less is known on these stem cells in human. adult neural stem and progenitor cells have been isolated from different parts of the human brain, namely the olfactory bulb, the subventricular zone, the hippocampus, the cortex, using the neurosphere assay. more recently, using adherent conditions and a specific media, adult human neural precursors have been isolated from the human temporal cortex and the hippocampus. these cells express glial markers and are able to generate both glial and mature neuronal cell types in vivo and in vitro. they could represent a potential endogenous cellular source that could be used to alleviate neuronal loss which occurs in many cns diseases. however, much more work is needed to isolate and characterize these cells from different human cns locations and, in particular, to analyse the signalling mechanisms leading to their differentiation. besides neural stem cells, the adult cns might also contain a pool of mesenchymal stem cells which appear to differentiate into mesenchymal lineage cells and possibly neural lineage cells. however this has mainly been observed in mice and the actual differentiation capacity of these cells remains to be validated in detail. compared to brain, little is known about neural precursors in the adult spinal cord. we recently isolated multipotential cells in the human adult spinal cord using the neurosphere method. in this article, we set out to explore the possibility of isolating adult neural stem/progenitor cells from the adult human spinal cord using adherent conditions previously described. we found that these conditions mainly result in the propagation of mesenchymal cells expressing snai and twist and with the hallmarks of smooth muscle muscular cells. these cells do not appear to be msc, nor neural stem cells, but they can readily mineralize in vitro when placed in appropriate conditions. additional characterization of these cells allowed us to identify the nkx homeodomain transcription factor as a marker for these cells.  isolation of nkx + vascular muscular cells from adult human spinal cord to isolate human neural stem cells using adherent condition as used in, human spinal cord was rapidly processed as previously described and seeded with neural stem cell medium containing serum and egf/fgf. under these conditions, cells with a mesenchymal morphology grew rapidly and could be propagated for approximately months before undergoing proliferation decrease, probably as a consequence of cellular senescence. the cells were characterized at passage by qpcr and immunofluorescence. we first measured the expression of genes typically expressed by cells of the neural lineage, i.e astrocytes, oligodendrocytes, oligodendrocyte progenitor cells, neurons and stem cells. surprisingly, although these cells were derived from the cns, we found that the expression of these genes was either low or undetectable and only nestin, cnp and pdgfrα/β could be detected with less than cycles of qpcr. we confirmed by immunofluorescence the absence of detectable expression of sox gfap and olig which are well documented markers for neural stem cells and glial cells. we then questioned the presence of markers frequently expressed in mesenchymal cells, bearing in mind that these can also be found expressed in cells of the neural lineage, and mesenchymal transcription factors. in sharp contrast with the neural lineage genes, the expression of these genes was readily detected. we confirmed by immunofluorescence and western blot that snai and twist were expressed at the protein level. these two transcription factors were detected in the nuclei of most cells. the possibility existed that the isolated cells were derived from the in vitro differentiation of endogenous spinal neural precursor cells which would adopt a mesenchymal phenotype when cultured in serum-containing medium. to explore this hypothesis, we cultured human fetal brain neural stem cells in this medium for three passages and then explored the phenotype of these cells by qpcr and immunofluorescence. in contrast with cells isolated from the adult spinal cord, our immunofluorescence analysis revealed that these cells expressed a high level of several typical neural markers such as gfap, sox olig whereas snai and twist were not detectable. qpcr confirmed these results and further indicated the maintenance of several typical markers for neural cells even when cultured in serum-containing media. the high expression of transcripts for smooth muscle actin in spinal cord cells suggested that the culture contained a majority of vascular muscular cells, an assumption consistent with the detection of nestin and pdgfrα/β, two markers also expressed by vascular cells. in order to confirm this possibility, we explored by immunofluorescence the expression of acta and nestin, together with calponin, caldesmon and ng three other markers of vascular muscular cells. the vast majority of the cells were positive for these five proteins, confirming that these cultures mostly contained vascular muscular cells. one unexpected transcription factor detected in the culture by qpcr was nkx. this homeodomain protein is implicated in the ontogenesis of oligodendrocytes and of pancreatic cells but its expression in vascular cells has not been reported in adults. the presence of nkx protein was confirmed by immunofluorescence and western blot in these cultures. the vast majority of the cells showed a clear nuclear staining and these cells coexpress calponin, caldmesmon and ng. in contrast, nkx was not detected in human fetal neural stem cells cultured in the same conditions for three passages. the expression of nkx by vascular cells was then confirmed in vivo on adult human spinal cord sections. nkx + cells were detected around large vessels in the meninges and in the parenchyma. as previously reported, nkx was also expressed by ependymal cell of the central canal. we concluded from these data that adherent cultures derived from adult human spinal cord led to the isolation of a cellular population mainly composed of vascular muscular cells that express the homeodomain protein nkx and the mesenchymal transcription factors snai and twist nkx + cells are not multipotential stem cells but mineralize in vitro it has recently been reported that adult stem cells with mesenchymal features located in various tissues could behave as multipotential stem cells. these cells could give rise to cells of several lineages including neurons and myelinating cells. we thus explored the multipotentiality of nkx + cells by placing them in different media and culture conditions. first, by culturing them in non adherent conditions in defined neural stem cell medium, these cells were not able to form neurospheres, a hallmark of neural stem cells. second, we further examined the possibility of a neural differentiation by labelling the cells with a gfp-expressing lentivirus, then by coculturing them for weeks with human young neurons, obtained by differentiation of human fetal brain neural stem cells. in these conditions, we could not detect staining of gfp-labelled cells with neuronal, astrocytic or oligodendrocytic markers, strongly indicating the absence of neural differentiation of these cells. third, we treated the cells with bmp bmp bmp cntf, cntf/bmp lif, lif/bmp or t to promote an astrocytic or an oligodendrocytic fate respectively, but again no differentiation was observed. considering the mesenchymal features of these cells and as vascular muscular cells have been reported to be endowed with stem cell properties we next examined the capacity of nkx + cells to generate chondrocytes, adipocytes and osteoblasts, three lineages commonly produced by mesenchymal stem cells. the cells were placed in adequate media, and the differentiation was monitored by staining with oil red o, alizarine red s and alcian blue and by qpcr for markers for adipocytes, chondrocytes and osteoblasts. as a control, the differentiation of human mesenchymal stem cells isolated from bone marrow was monitored in parallel. under these conditions, no differentiation towards chondrocytes or adipocytes was observed. only deposition of calcium phosphate precipitates, as observed with alizarine red s staining, could convincingly be detected. this was accompanied by a sharp increase of alkaline phosphatase mrna and activity which was detected with naphthol asbi phosphate and fast red trisodium staining. no increase of the other osteoblast markers compared to that obtained with bone marrow mesenchymal stem cells could be demonstrated suggesting absence of complete differentiation towards osteoblasts. equally, we could not detect the expression of nkx by western blot and immunofluorescence in bone marrow mesenchymal stem cells, suggesting that cns nkx + vascular cells are weakly related to these cells. we also examined the capacity of human fetal neural stem cells cultured in the presence of serum to mineralize in the same conditions. figure 6a, c shows however that these cells were unable to do so. finally, to ascertain that mineralization was produced by nkx + cells and not by other contaminating cell types, we performed single cell cloning in wells to isolate nkx + cell clones which were then placed in mineralizating conditions. among isolated nkx + clones, 95% of them were able to mineralize. we concluded from these studies that nkx + vascular muscular cells isolated from the adult cns do not behave as neural or mesenchymal stem cells but can readily mineralize in vitro. discussion neural stem cells are generally isolated and cultured using non adherent conditions and defined medium to generate free-floating aggregates, the so-called neurospheres. adherent cultures with serum-containing medium have also been successfully used to isolate and culture adult neural stem cells from adult human brain. we previously reported the isolation of human spinal cord stem/progenitor sox2+ cells using the neurosphere method, however we were unable to identify such cells using adherent conditions in the present study. considering the low number of these cells as compared to other spinal cord cells, it is possible that these cells are rapidly diluted during the culture passages. alternatively it is conceivable that the presence of serum leads to their premature differentiation. we report here that, in contrast to neural stem/progenitor cells, these conditions lead to the isolation of a cellular population mainly composed of nkx + vascular muscular cells. although these cells express some markers for neural and mesenchymal stem cells, such as nestin or cd they do not behave as multipotential stem cells, at least in the conditions we employed. in fact, only differentiation along the osteogenic lineage, as evidenced by their ability to mineralize in vitro, was observed. the calcification of brain vessel muscle cells has been observed since in pathological situations or as part of the brain aging process. in elderly brains, mild calcification of vessels and parenchyma is a normal and common incidental finding in the basal ganglia, pineal gland, choroid plexus, dura and habenula. this vasculopathy may account for the sclerosis of the aged hippocampus and the concomitant loss of ca neurons. there is also now clear evidence for a close association between vasculature and neurogenesis, the so-called vascular stem cell niche, and the observation of an age-related neurogenesis decline may be related to some modifications of vascular cell properties. in addition, recent work by göritz c, et al demonstrated a critical role of vascular cells in the formation of the scar following spinal cord injury. the extensive growth of nkx + human vascular muscular cells under our conditions will allow further studies on their interactions with adult neural stem cells, their role in the aging process and their implication in the scar formation. nkx is a homeodomain protein that plays a major role in cns development, notably in spinal cord. being a target of the ventralizing shh morphogen, nkx is required for motoneuron generation and correct oligodendrogenesis. in agreement with the detection of nkx in adult spinal cord meninges, nkx is also expressed in ventral meninges during spinal cord development. outside the cns, nkx is required for pancreatic β-cell development and its disruption results in 90% decrease of these cells. nkx is also detected in esophageal and tracheal smooth muscle cells during development. consistent with this observation, we report here that in the adult human spinal cord, vascular smooth muscle cells express this transcription factor in vitro and in vivo. its role in these cells remains to be explored. as nkx is implicated in the positive regulation of β-cell proliferation, notably by direct interaction with the cyclin a and b promoters, a similar function could be hypothesized for cns vascular muscular cells proliferation.  in this study we have isolated vascular cells from the human adult spinal cord and showed that these cells are neither endowed with mesenchymal nor neural stem cell properties. we demonstrated that these cells express the developmental homedomain protein nkx, two mesenchymal transcription factors and that they can readily mineralize when placed in osteogenic conditions. these cells constitute a useful model for studying cns vessel calcification in vitro. 
these cells could not differentiate into chondrocytes, adipocytes, neuronal and glial cells, however they readily mineralized when placed in osteogenic conditions. nkx was expressed in vivo by cns vascular muscular cells located in the parenchyma and the meninges. smooth muscle cells expressing nestin and nkx is the main cell population derived from culturing human spinal cord cells in adherent conditions with serum. we previously isolated nestin+ sox2+ neural multipotential cells from the adult human spinal cord using the neurosphere method. in addition, long term propagation of these cells will allow the study of their interaction with other cns cells and their implication in scar formation during spinal cord injury. mineralization of these cells in vitro could represent a valuable model for studying calcifications of cns vessels which are observed in pathological situations or as part of the normal aging. the adult central nervous system contains different populations of immature cells that could possibly be used to repair brain and spinal cord lesions.
 the adult central nervous system contains different populations of immature cells that could possibly be used to repair brain and spinal cord lesions. the diversity and the properties of these cells in the human adult cns remain to be fully explored. we previously isolated nestin+ sox2+ neural multipotential cells from the adult human spinal cord using the neurosphere method.  here we report the isolation and long term propagation of another population of nestin+ cells from this tissue using adherent culture conditions and serum. qpcr and immunofluorescence indicated that these cells had mesenchymal features as evidenced by the expression of snai and twist and lack of expression of neural markers such as sox olig or gfap. indeed, these cells expressed markers typical of smooth muscle vascular cells such as calponin, caldesmone and acta. these cells could not differentiate into chondrocytes, adipocytes, neuronal and glial cells, however they readily mineralized when placed in osteogenic conditions. further characterization allowed us to identify the nkx transcription factor as a marker for these cells. nkx was expressed in vivo by cns vascular muscular cells located in the parenchyma and the meninges.  smooth muscle cells expressing nestin and nkx is the main cell population derived from culturing human spinal cord cells in adherent conditions with serum. mineralization of these cells in vitro could represent a valuable model for studying calcifications of cns vessels which are observed in pathological situations or as part of the normal aging. in addition, long term propagation of these cells will allow the study of their interaction with other cns cells and their implication in scar formation during spinal cord injury. human central nervous systemspinal cordstem cellsvascular muscle cellsosteogenesisnkx calcification 