Origin and Differentiation of Primordial Germ Cells in Vertebrates, Especially Fishes

Abstract

1. Predetermination of primordial germ cells (PGCs) in vertebrates is an exception, as it occurs only in anuran amphibians. There is some doubt whether there is really predetermination in that group. In urodeles, birds and mammals, PGCs are derived from the epiblast at the late blastula 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 primordial germ cells arise by induction in the epiblast like the PGCs in urodeles, birds and mammals. In these fishes the PGCs seem to be passively 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 injecting cultured embryonic cells into host blastocysts (mouse) or by injecting early blastula cells into host embryos of the same developmental stage (chick, fish). In mouse it appeared possible to culture isolated inner-cell-mass cells (ES-cells) and recently also 8.5 dpc PGCs (EG-cells), without loss of their ability to produce germline chimeras. Moreover, it was demonstrated in the mouse that PGCs, shortly after arrival 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 developmental processes but also for medical prospects, such as research on cancer. The repair of genetically faulty blastocysts by the injection of cells from normal ones might be possible too.