Developmental fates of the mouse germ cell line.

The specification of mouse germ cell lineage takes place after a population of pluripotential cells is established, and cell communication among the pluripotential cells may be important for this process. Primordial germ cells (PGCs) first appear around the allantois at 7 dpc which are distinct from pluripotential cells in the early embryo because they can not colonize blastocysts. However, a portion of PGCs are transformed into pluripotential cells in the ectopic environment or in culture, suggesting that the developmental fate of PGCs may still be somewhat plastic. PGCs may be destined only for gametes after they enter into the mitotic arrest phase or the meiotic prophase in embryonic gonads, which may be regulated by intrinsic and/or environmental molecules. After fetal germ cells are mitotically arrested, a large number of germ cells undergo programmed cell death. Bcl-2 and its related molecules are involved in the determination of death or survival of fetal germ cells, as well as of spermatogonia in adult testis. The cell death of spermatogonia may be necessary either for eliminating impaired germ cells or for arranging optimal interactions between germ cells and their supporting cells. Although maturating germ cells seem to differentiate only to sperm cells, oocytes that complete the first meiotic division can give rise to pluripotential cells, suggesting that maternal molecules accumulated in oocyte may play a role in the restoration of pluripotency.