Molecular mechanisms involved in the differentiation of spermatogenic stem cells.

In male mammals, spermatogenesis proceeds for the reproductive lifetime of the animal. The continuation of this process depends upon a pool of spermatogenic stem cells within the testes that undergo asymmetric division to both maintain the stem cell population and give rise to progenitors that will proceed through spermatogenesis to generate mature spermatozoa. Thus, the development of functional spermatozoa may be divided into two distinct stages. The second, the process of spermatogenesis, is dependent upon the first, the successful formation of spermatogenic stem cells. Although spermatogenesis is characterized by marked cellular differentiation, the initial stages of germ line differentiation involve an avoidance of the differentiation signals acting during embryo development. The germ line is set aside early in embryo development and, while the primordial germ cells remain refractory to the differentiation signals affecting the soma, they undergo a number of phenotypic shifts before and after colonizing the genital ridge. Upon colonization of the genital ridge, the somatic tissue of the male genital ridge directs the final differentiation events that result in the formation of spermatogenic stem cells. It is this cell population that provides the basis for the maintenance of spermatogenesis in the adult.