Primordial germ cell formation in birds.

At the early somite stage, chicken embryo primordial germ cells comprise a population of about 200 dispersed, morphologically distinct cells, located in the extraembryonic germinal crescent. Tracing the origin of primordial germ cells at earlier stages is at present not possible by either cytological or immunohistochemical techniques. To resolve this difficulty we have microdissected embryos at early embryonic stages, isolated fragments from various positions and incubated them to the somite stage, when the primordial germ cells can be visualized accurately. From these studies a distinct developmental pattern has emerged. (1) At the uterine stage, when the area pellucida starts to thin out, there is already a defined area, confined to the most central part of the blastodisc, which constitutes the sole source of the future primordial germ cells. (2) As development proceeds, the area that gives rise to the primordial germ cells expands peripherally. (3) Primordial germ cells thereafter migrate as isolated cells, descending to the lower layer, the hypoblast. (4) This gradual process starts at the onset of blastulation and terminates towards the end of gastrulation. (5) The hypoblast, destined to become the yolk sac endoderm, is pushed anteriorly during gastrulation by morphogenetic movements to be replaced by the endoderm proper; the primordial germ cells are carried forwards with it and occupy an extraembryonic location.