Many species have special factors which commit certain embryonic cells to the germline. Flies, nematode worm, frogs: germ cells inherit cytoplasmic determinants in the fertilized egg. Cytoplasmic determinants include RNA and protein. These factors, as well as evidence supporting their role, are described below.
|P Granules||Nematode||P pranules, aka germ plasm, were found via antibody staining to be present in a specific lineage that gives rise to only germ cells.||Adding p granules to other embryonic cells led to more germ cells.|
|Polar Plasm||Insects||Germ cells arise at the posterior of the insect egg, during the blastoderm stage; a unique cytoplasm containing polar granules is localized to the oocyte posterior and is incorporated into the germ cells. Destruction of the polar plasm prior to cellularization -- via a hot needle pick, manual cytoplasm removal or UV irradiation -- eliminates germ cells.||Transplantation experiments reveal that the pole plasm can cause functional germ cells to form ectopically. Normal germ cells will arise in the third embryo if pole plasm from the posterior of one embryo is transplanted to the anterior of another embryo, then the resultant cells are transplanted to the posterior of a third embryo.|
|Germ Plasm||Frogs||Frog germ plasm is morphologically similar to insect polar plasm, with germ plasm localized to the vegetal pole. During cleavage, germ plasm becomes localized to the blastocoele base, which will eventually give rise to the endoderm.||Eggs whose germ plasm has been destroyed by UV irradiation can be restored by injection of cytoplasm from an unirradiated egg.|