Germ Cell Differentiation in Culture

The generation of animals cloned by means of the nuclear transfer technique indicates that somatic cell nuc le i can be rep rogrammed when t hey a re transplanted into the egg cytoplasm—in other words, the transplanted somatic nuclei can be induced to return to a pluripotent state similar to that of the nuclei of fert il ized eggs. Therefore, the potential for both reprogramming and pluripotency is retained in germ cells throughout their development. For example, embryonic germ (EG) cells that are generated from primordial germ cells (PGCs) show similar pluripotency to the undifferentiated embryonic stem (ES) cells that originate in the inner cell mass of blastocyst embryos in mice [1 ] . And some te ra tomas tha t genera te differentiated cells of various types—from all three pr imary germ layers—or ig inate in PGCs in the embryonic gonads [2]. Therefore, germ cells must have the ab i l i t y to be reprogrammed, as we l l as to differentiate to form gametes. So how do germ cells acquire these character ist ics, and how are they maintained during development? The discovery of the molecular and cellular mechanisms that are involved in these processes will provide important insights not only for basic research into reproductive biology but also for the development of new techniques for regenerative medical treatments. There is an urgent need to develop i n v i t r o s ys tems t ha t can mode l t he en t i r e developmental process—from pluripotent stem cells to func t i ona l spe rm and eggs—to f u r t he r ou r understanding of the mechanisms of germ-cel l differentiation.