ORIGIN AND DIFFERENTIATION OF PRIMORDIAL GERM-CELLS IN VERTEBRATES, ESPECIALLY FISHES

1. Predetermination of primordial germ cells (PGCs) in vertebrates is an exception, as it occurs only in anuran amphibians. There is some do ubt whether there is really predetermination in that group. In urodele s, birds and mammals, PGCs are derived from the epiblast at the late b lastula or early gastrula stage, but they differ with respect to their subsequent location and to their migration pathways. With respect to the cyprinids carp and rosy barb it remains to be elucidated whether p rimordial germ cells arise by induction in the epiblast like the PGCs in urodeles, birds and mammals. In these fishes the PGCs seem to be pa ssively translocated towards the gonadal ridges contrary to the active migratory movements in higher vertebrates. 2. Convincing evidence has been presented that germline chimeras can be produced by either injec ting cultured embryonic cells into host blastocysts (mouse) or by inje cting early blastula cells into host embryos of the same developmental stage (chick, fish). In mouse it appeared possible to culture isolate d inner-cell-mass cells (ES-cells) and recently also 8.5 dpc PGCs (EG- cells), without loss of their ability to produce germline chimeras. Mo reover, it was demonstrated in the mouse that PGCs, shortly after arri val in the gonadal ridges (12.5 dpc) still have the potency to produce germline chimeras, but that potency is lost at 15.5 dpc. The results obtained offer reliable methods for gene transfer in these vertebrate groups. The obtained chimeras will be valuable for the analysis of dev elopmental processes but also for medical prospects, such as research on cancer. The repair of genetically faulty blastocysts by the injecti on of cells from normal ones might be possible too.