Molecular mechanisms involved in the differentiation of spermatogenic stemcells

In male mammals, spermatogenesis proceeds for the reproductive lifetime of the animal. The continuation of this process depends upon a pool of spermat ogenic stem cells within the testes that undergo asymmetric division to bot h maintain the stem cell population and give rise to progenitors that will proceed through spermatogenesis to generate mature spermatozoa. Thus, the d evelopment of functional spermatozoa may be divided into two distinct stage s. The second, the process of spermatogenesis, is dependent upon the first, the successful formation of spermatogenic stem cells. Although spermatogen esis is characterized by marked cellular differentiation, the initial stage s 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 refractor y to the differentiation signals affecting the soma, they undergo a number of phenotypic shifts before and after colonizing the genital ridge. Upon co lonization of the genital ridge, the somatic tissue of the male genital rid ge directs the final differentiation events that result in the formation of spermatogenic stem cells. It is this cell population that provides the bas is for the maintenance of spermatogenesis in the adult.