Germ line development

The cells that give rise to the gametes are often set aside during embryonic cleavage. During development, these cells will differentiate into primordial germ cells, migrate to the location of the gonad, and form the germline of the animal. The cells that give rise to the gametes are often set aside during embryonic cleavage. During development, these cells will differentiate into primordial germ cells, migrate to the location of the gonad, and form the germline of the animal. Cleavage in most animals segregates cells containing germ plasm from other cells. The germ plasm effectively turns off gene expression to render the genome of the cell inert. Cells expressing germ plasm become primordial germ cells (PGCs) which will then give rise to the gametes. The germ line development in mammals, on the other hand, occurs by induction and not by an endogenous germ plasm. Germ plasm has been studied in detail in Drosophila. The posterior pole of the embryo contains necessary materials for the fertility of the fly. This cytoplasm, pole plasm, contains specialized materials called polar granules and the pole cells are the precursors to primordial germ cells. Pole plasm is organized by and contains the proteins and mRNA of the posterior group genes (such as oskar, nanos gene, Tudor, vasa, and Valois). These genes play a role in germ line development to localize nanos mRNA to the posterior and localize germ cell determinants. Drosophila progeny with mutations in these genes fail to produce pole cells and are thus sterile, giving these mutations the name 'grandchildless'. The genes Oskar, nanos and germ cell-less (gcl) have important roles. Oskar is sufficient to recruit the other genes to form functional germ plasm. Nanos is required to prevent mitosis and somatic differentiation and for the pole cells to migrate to function as PGCs (see next section). Gcl is necessary (but not sufficient) for pole cell formation. In addition to these genes, Pgc polar granule component blocks phosphorylation and consequently activation of RNA polymerase II and shuts down transcription. Similar germ plasm has been identified in Amphibians in the polar cytoplasm at the vegetal pole. This cytoplasm moves to the bottom of the blastocoel and eventually ends up as its own subset of endodermal cells. These cells eventually become PGCs. The presence of homologs of nanos and vasa also implicate this germ plasm as germ-determining. The first phase of migration in Drosophila occurs when the pole cells move passively and infold into the midgut invagination. Active migration occurs through repellents and attractants. The expression of wunen in the endoderm repels the PGCs out. The expression of columbus and hedgehog attracts the PGCs to the mesodermal precursors of the gonad. Nanos is required during migration. Regardless of PGC injection site, PGCs are able to correctly migrate to their target sites.