ABSTRACT
The highly coordinated process by which very large numbers of spermatozoa are produced on a daily basis from puberty onward has been well described for a variety of species (1,2). The kinetics of spermatogenesis reveals that, once established, the process does not accelerate or decelerate (1). New generations of spermatozoa are continuously initiated in the seminiferous epithelium of the adult; hence, germ cells leaving the testis on an ongoing basis have been dividing and differentiating for a relatively short period of time, in the range of 35 to 64 days, depending on the mammalian species. As a consequence, a common conclusion has been that, unlike female germ cells, male germ cells are always freshly made and hence do not age. Yet, the cells from which male germ cells arise, the pale spermatogonia, act as stem cells and are present throughout the lifespan of the male.
