The estimation of damage to testicular cell lineages.

Assuming that spermatogonial multiplication in primates will not be fundamentally different from that in non-primates it can be deduced that Ap and Ad spermatogonia in primates are analogous to the undifferentiated spermatogonia in non-primates. Ap and Ad spermatogonia consist of single, pairs and chains of cells, among which the single cells likely are the stem cells. The single Ap are the active stem cells while the single Ad only become active after cell loss. After cell loss, Ad spermatogonia without division transform into Ap spermatogonia. The human testis is a very sensitive indicator of radiation. However, the full effect on stem cells of radiation or other insults will only become visible in the ejaculate after 3 to 5 months. When no cell loss is inflicted, damaged to Ad stem cells may remain hidden for years. Damage to spermatocytes and spermatids will become visible much earlier. However, these cells heavily depend on a normal Sertoli cell function. Hence, effects found on spermatocytes and spermatids may be caused by a direct action on these cells but also via a deteriorated Sertoli cell or Leydig cell function. The number of Sertoli cells per testis can only be affected by treatment before puberty. Leydig cell numbers can also be affected in the adult, but this cell population is more vulnerable when treatment takes place before adulthood. However, the functional capacity of the Leydig cell population as a whole only decreases after severe cell loss.