STEM-CELLS IN THE TESTIS

The origin and development of the spermatogenic cell lineage is review ed, as well as spermatogonial kinetics in adult nonprimate mammals in relation to the cycle of the seminiferous epithelium, the emphasis bei ng on spermatogonial stem cells. A hypothesis is presented for the tra nsition from foetal germ cells, gonocytes, to adult type spermatogonia at the start of spermatogenesis. An overview is given of the present knowledge on the proliferation and differentiation of undifferentiated spermatogonia (spermatogonial stem cells and their direct descendants ) and the regulation of these processes. It is concluded that the diff erentiation of the undifferentiated into differentiating type spermato gonia is a rather vulnerable moment during spermatogenesis and the mod els for studying this are described. Research into the molecular basis of the regulation of spermatogonial proliferation, differentiation an d apoptosis is at its infancy and the first results are reviewed. An e xciting new research tool is the spermatogonial stem cell transplantat ion technique which is described. Finally, reviewing the nature of hum an germ cell tumours it is concluded that at present there are no anim al or in vitro models to study these tumours experimentally.