The termination of the TBI experiments 3-8 yr after irradiation rendered unique material to study the long-term effects of such treatment on various organs, including the testis. Despite the material gathered from different experiments being directed to study quite different consequences of irradiation, important and long-lasting effects of TBI before adulthood within the monkey testis could be established.
The age at which the monkeys were irradiated varied between 17 and 53 mo. The onset of puberty in the rhesus monkey has been studied by Plant. Both FSH and LH levels were found to increase from approximately 30 mo of age onward, whereas testosterone levels started to increase at 36 mo, reaching adult levels at approximately 42 mo of age.
However, how the changes in hormone levels relate to the start of spermatogenesis and Sertoli cell proliferation and differentiation is not clear. In rhesus monkeys, Sertoli cell numbers were found to increase sixfold from 15-17 to 60 mo of age. In cynomolgus monkeys, which start puberty at approximately the same time as rhesus monkeys, very few spermatocytes were present at 42 mo of age, and at this age, Sertoli cell proliferation still took place. From this, it can be concluded that in most, if not all, of our monkeys, the testis was still developing at the time of irradiation.