The allocation and differentiation of mouse primordial germ cells

Analysis of the lineage potency of epiblast cells of the early-streak stage mouse embryo reveals that the developmental fate of th e cells is determin ed by their position in the germ layer. Epiblast cells that are fated to be come neuroectoderm can give rise to primordial germ cells (PGCs) a nd other types of somatic cells when they were transplanted to th e proxima I reg i on of the epiblast. On the contrary, proximal epiblast cells transplanted t o the distal region of the embryo do not form PGCs. Therefore, the germ lin e in the mouse is unlikely to be derived from a predetermined progenitor po pulation, but may be specified as a result of tissue interactions that take place in the proximal epiblast of the mouse gastrula. The initial phase of the establishment of the PGC population requires, in addition to BMP activ ity emanating from the extraembryonic ectoderm, norm at Lim1 and Hnf3 beta activity in the germ layers. The entire PGC population is derived from a fi nite number of progenitor cells and there is no further cellular recruitmen t to the germ line after gastrulation. The XX PGCs undergo X-inactivation a t the onset of migration from the gut endoderm and re-activate the silenced X-chromosome when they enter the urogenital ridge. Germ cells that are loc alised ectopically in extragonadal sites do not re-activate the X-chromosom e, even when nearly all germ cells in the fetal ovary have restored full ac tivity of both X-chromosomes. XXSxr germ cells can re-activate the X-chromo some in the sex-reversed testis, suggesting that the regulation of X-chromo some activity is independent of ovarian morphogenesis.