Germ and somatic cell lineages in the developing gonad.

The germ cell lineage in the mouse becomes lineage-restricted about 7.2 days post coitum. Its progenitors have migrated from the proximal region of the epiblast, where they were subject to a predisposing signal from the adjacent extra-embryonic ectoderm. It appears that this and other signals determine the emergence of germ cells: unlike in some other organisms, this event is not pre-determined. After about 24 h in their initial extraembryonic location, the primordial germ cells migrate back into the embryo and make their way into the region of the developing gonad. Less is known about the origin of the various somatic cell lineages in the gonad, but some are known to derive from cells that migrate in from the mesonephros and others from the coelomic epithelium. Within the developing gonad, numerous interactions occur between the germ and somatic cell lineages. These are particularly important for the establishment of the spermatogenic lineage in the testis and for the differentiation of somatic tissue in the ovary. This paper will describe first the development of the germ cell lineage, up until about the time of birth, then the various somatic components of the gonad and finally the interactions that are known to occur between lineages. Unless otherwise stated, all the information refers to the mouse.