Role of spermatogonia in the repair of the seminiferous epithelium following X‐irradiation of the rat testis

In the normal adult rat testis, type A 0 spermatogonia do not appear to participate to a significant extent in the production of spermatocytes, while type A 1 spermatogonia periodically initiate a series of divisions resulting in the production of spermatocytes and new type A 1 spermatogonia. The behavior of type A 0 and A 1 spermatogonia was investigated following administration of a single dose of x‐rays (300 r) to the testis. Using whole mounts of seminiferous tubules, the type A 0 and A 1 cells were counted at various intervals after irradiation. At 8 and 13 days after irradiation, type A 1 spermatogonia reached lowest values, i.e., 6% and 3% of non‐irradiated control, while type A 0 reached the lowest value, i.e., 62% of control at eight days. Thereafter the numbers of type A 0 and A 1 progressively increased to return to normal at 39 days. It was thus concluded that the type A 0 were comparatively more resistant to x‐irradiation than type A 1 spermatogonia. To verify if the surviving type A 0 proliferated in the irradiated testis, animals were injected with 3 H‐thymidine three hours before they were sacrificed at various times after x‐irradiation. In irradiated testes the labeling indices of the surviving type A (A 0 , A 1 –A 4 ) were the same as in the non‐irradiated testes except in stages V‐VI of the cycle of the seminiferous epithelium. While in the controls only 2% of type A cells were labeled at these two stages of the cycle, after irradiation the labeling index of type A reached a maximum of 31% at 13 days to return to control values by 39 days. Since at 13 days after irradiation type A 0 spermatogonia were the predominant component of the spermatogonial population, it was concluded that these cells must have incorporated 3 H‐thymidine and thereby contributed to the reconstruction of the spermatogonial population partially destroyed by irradiation.