Reflections on the biology of embryonic stem (ES) cells.

Remarkably little is known about mammalian embryonic stem (ES) cells despite their very widespread use in studies on gene disruption and transgenesis. As yet, it is only in the mouse that lines of ES cells which retain the ability to form gametes following reintroduction into the early conceptus have been obtained. Even in this species, most strains have so far proved refractory to the derivation of such cell lines. Apart from persisting ignorance as to how the various procedures that have been claimed to improve success actually do so, even the tissue of origin of ES remains uncertain. Furthermore, it is doubtful whether retention of pluripotency or expression of so-called "stem cell" marker molecules provide an adequate basis for classifying cells as genuine ES cells. This is because epiblast cells, their presumed precursors, lose the capacity to colonize the preimplantation conceptus well before they become restricted in the types of cell they can form or cease to express such marker molecules. In addition, it has yet to be established whether heterogeneity of cells within individual ES cell lines arises entirely during culture or is at least partly attributable to lack of homogeneity among their precursors. Finally, it has yet to be explained why ES chimeras evidently differ from those obtained by combining cells from different conceptuses in showing greater variation between tissues in the level of chimerism.