Stem cells of the germline: the specialized facets of their differentiation program

The differentiation program of male germ cells in mammals is based on a sequence of cytological events that results in the formation of haploid spermatozoa from precursor stem cells. This process begins by mitotic division of germ-cell spermatogonia to give rise to diploid spermatocytes, which themselves replicate their DNA content, before undergoing the two successive meiotic divisions, which results in the production of haploid round spermatids. The latter germ cells fully mature in the process of spermiogenesis, during which cells are sculptured by an extensive biochemical and morphological restructuring. The whole process occurs in a remarkably coordinated manner and involves a series of highly specialized mechanisms. Indeed, differentiation of male germ cells is achieved by unique chromatin remodeling, a series of specialized transcription complexes and a specific program of cell division. Defects in these processes have drastic effects, resulting in infertility and cancer.