Very small embryonic-like (VSEL) cells have been described as putatively pluripotent stem cells present in murine bone\r\nmarrow and human umbilical cord blood (hUCB) and as such are of high potential interest for regenerative medicine.\r\nHowever, there remain some questions concerning the precise identity and properties of VSEL cells, particularly those\r\nderived from hUCB. For this reason, we have carried out an extensive characterisation of purified populations of VSEL cells\r\nfrom a large number of UCB samples. Consistent with a previous report, we find that VSEL cells are CXCR4+, have a high\r\ndensity, are indeed significantly smaller than HSC and have an extremely high nuclear/cytoplasmic ratio. Their nucleoplasm\r\nis unstructured and stains strongly with Hoechst 33342. A comprehensive FACS screen for surface markers characteristic of\r\nembryonic, mesenchymal, neuronal or hematopoietic stem cells revealed negligible expression on VSEL cells. These cells\r\nfailed to expand in vitro under a wide range of culture conditions known to support embryonic or adult stem cell types and\r\na microarray analysis revealed the transcriptional profile of VSEL cells to be clearly distinct both from well-defined\r\npopulations of pluripotent and adult stem cells and from the mature hematopoietic lineages. Finally, we detected an\r\naneuploid karyotype in the majority of purified VSEL cells by fluorescence in situ hybridisation. These data support neither\r\nan embryonic nor an adult stem cell like phenotype, suggesting rather that hUCB VSEL cells are an aberrant and inactive\r\npopulation that is not comparable to murine VSEL cells.
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