Renewal of type A spermatogonia in adult rats.

The origin of type A spermatogonia in the rat, formed at each cycle of the seminiferous epithelium, has been investigated in segments of tubules dissected from the testes of animals killed at 1 and 44 hr after a single injection of [3H]thymidine. After fixation and staining these tubular segments were radioautographed according to the technique of Huckins & Kopriwa (1969). Labelled and unlabelled type A0, A4, A1 and intermediate spermatogonia were mapped and scored in tubular segments at stages I-IV of the cycle of the seminiferous epithelium, i.e. before, during and after the stage I peak of spermatogonial mitosis. In stage I of the cycle, the relatively rare type A0 spermatogonia (0.7 per frame) were infrequently labelled at the two time intervals, while the large majority of numerous type A4 spermatogonia (8.7 per frame) were labelled. In stages II-IV of the cycle, of the numerous aligned type A1 spermatogonia, only a few were labelled at 1 hr, whereas many were labelled at 44 hr after [3H]thymidine injection. At the latter time interval, the intermediate spermatogonia were also well labelled. Thus, considering cell numbers and labelling indices, it appears that the type A4 spermatogonia may serve as precursors for both type A1 and intermediate spermatogonia.