Table 2. In vitro and in vivo criteria for pluripotent stem cells.

	VSELs
In vitro criteria for PSCs
Undifferentiated morphology, euchromatin and high nucleus/cytoplasm ratio	Yes
PSC markers (for example, Oct4, Nanog, SSEA), open chromatin at the Oct4 promoter, bivalent domains and reactivation of the X chromosome in female PSCs	Yes
Broad multilineage differentiation into cells from all three germ layers (meso-, ecto- and endoderm)	Yes
In vivo criteria for PSCs
Complement blastocyst development	No
Teratoma formation after inoculation into immunodeficient mice	No

Abbreviations: PSC, pluripotent stem cell; VSEL, very small embryonic-like stem cell.