The results of the analysis of spermatocytes for translocation induction in spermatogonial stem cells further confirm previous, preliminary data suggesting an insensitivity of the rhesus monkey to this type of genetic change. The earlier data obtained on prepubertally irradiated monkeys were rather limited (i.e., seven individuals in total), but the present, more extensive results point in the same direction: that no clear differences in chromosomal radiosensitivity exist between monkeys irradiated before or after the onset of puberty. Similar observations have been made for the mouse. As discussed before, the low recovery of radiation-induced reciprocal translocations in rhesus monkey stem cell spermatogonia itself is mainly due to postirradiation proliferation-differentiation patterns of surviving spermatogonia. That no induction was found at all in the present study is probably due to a combination of two other factors.
The first is the relatively high doses of irradiation used, which kill all stem cells sensitive to translocation induction and leave the resistant ones. The existence of sensitive and resistant stem cells leads to a so-called humped dose-response relationship, which has been observed in all mammals studied so far, and, in the rhesus monkey, to no induction at all in the higher dose range. A second factor might be the long recovery periods after irradiation in the present study, causing the elimination of stem cell clones carrying aberrations.
In conclusion, TBI in young monkeys has long-lasting effects within the testis, including incomplete germ cell repopulation, permanently lowered numbers of Sertoli cells, changes in Sertoli cell behavior, and damage to tubule structure. However, complete sterility only occurs after high or fractionated doses. These effects on the testis further complete the discovery of problems that may arise at long intervals after TBI. In the same group of monkeys, effects on body fat and thyroid gland as well as gastrointestinal function, ophthalmological complications, and cardiovascular damage have already been described.