Development of primordial germ cells in the mouse*

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Abstract
Primordial germ cells in the mouse are known to be derived from the epiblast. They can be identified histochemically, by their high alkaline phosphatase activity. At 8 d post coitum they have been observed within the embryonic part of the egg cylinder, at the posterior end of the primitive streak. Earlier, at 7.25 days post coitum, we have observed them embedded in the extra-embryonic mesoderm, as a tight clump. Germ cell counts over the 7-8 d period of gastrulation have been made. They are consistent with either of two models: (1) derivation of the germ cell lineage from a very small stem cell pool, followed by a constant rate of proliferation, and (2) derivation from a larger initial stem cell pool, followed by a period when germ cells are differentiating but not dividing. From their initial extra-embryonic location, germ cells spread into the mesoderm of the primitive streak, and the endoderm of the yolk sac and hind gut. Active locomotion is probably required for their passage up the dorsal mesentery and into the genital ridges. Mutant alleles at two loci, W (White-spotting) and Sl (Steel), drastically reduce the number of germ cells reaching the ridges. Since those that succeed in reaching the ridges suffer little if any delay, the defect is unlikely to be due to reduced powers of locomotion, but rather to a failure of proliferation or survival. W acts cell-autonomously: its gene product is the c-kit polypeptide, a transmembrane tyrosine kinase receptor.(ABSTRACT TRUNCATED AT 250 WORDS)