Table 1.

The Most Important Identification Criteria That Are Attributable to BM-Purified Murine VSELs and Human UCB-Isolated VSELs

Source of VSELs	Characteristics
Murine VSELsa	Enriched in population of cells that are slightly smaller than red blood cells.
	Primitive morphology, large nuclei contain euchromatin, high nuclear/cytoplasmic ratio.
	Quiescent cells due to epigenetic changes in expression of some genes regulated by paternal imprinting (eg, Igf2-H19, RasGrf1, IGF2R, and p57Kip2). It also explains that they neither form teratomas nor complement blastocyst development.
	Purified VSELs are enriched for cells that express some markers characteristic for pluripotent stem cells (Oct-4, Nanog, SSEA-1, and presence of bivalent domains), epiblast and germ line cell markers.
	Number of VSELs decreases with the age and correlates with life span in experimental animals.
	In appropriate experimental models murine VSELs differentiate into cells from all three germ layers, including germ line.
	Highly resistant to stress, irradiation and cytostatics.
Human VSELsb	Enriched in a population of cells that are slightly smaller than red blood cells.
	Primitive morphology, large nuclei contain euchromatin, high nuclear/cytoplasmic ratio.
	Purified human UCB-VSELs are enriched for cells that express some markers characteristic for pluripotent stem cells (eg, Oct-4, Nanog, and SSEA-4) and some epiblast and primordial germ cell markers.
	Do not form teratomas in experimental animals
	Quiescent cells due to epigenetic changes in expression of some genes regulated by paternal imprinting (eg, Igf2-H19).
	In appropriate experimental models, human VSELs differentiate into hematopoietic cells, mesenchymal stem cells, and neural cells. Moreover, VSELs isolated from human gonads also differentiate into germ line cells.

^aThe best characterized so far at molecular level are murine VSELs purified from BM. More work is needed to compare VSELs isolated from other tissues with BM-purified ones.

^bRecent data reported on VSELs-like cells isolated from human ovaries indicate their molecular similarity to murine BM-purified VSELs [14,15,18,68,117,118].

VSEL, very small embryonic-like stem cells; UCB, umbilical cord blood; BM, bone marrow.