In most of the irradiated monkeys at the time they were killed, full spermatogenesis was present in at least some tubules, indicating that these animals were potentially fertile. However, with the lowest dose given (4 Gy), in none of the monkeys was the repopulation complete. Apparently, with doses of 4 Gy and higher, all stem cells were always killed in one or more seminiferous tubules, preventing complete repopulation. Complete sterility was seen in two of the five animals that received doses of 8 Gy or higher as well as in both monkeys that received two fractions of 6 Gy each separated by 24 h. The latter indicates that, in the monkey as in the mouse, fractionation of the irradiation does not have a sparing effect on spermatogonial stem cells. Importantly, when tubules had become repopulated, their diameter was similar to that in the control testes, indicating that the surviving stem cells were able to give rise to a normal seminiferous epithelium.
In the dose-response group, only a poor correlation was found between the percentage of repopulated tubule crosssections, RI, and dose of irradiation. Although a better correlation was found between testicular weight and dose of irradiation, neither the RI nor the testicular weight could be used to calculate a meaningful D0 value for spermatogonial stem cell killing by x-rays in the young monkey. The explanation for this lack of correlation became apparent from the correlation between RI and testicular weight. Clearly, testicular weight depended highly on the amount of seminiferous epithelium that was present. Interestingly, extrapolation of the graph to a fully recovered seminiferous epithelium (RI = 100%) indicated that, on average, the fully recovered testis would weigh approximately 13 g. This was considerably less than the testicular weight of approximately 23 g in adult control monkeys.