THE SPERMATOGONIAL STEM CELL POPULATION IN ADULT RATS

Radioautographs of histological sections and whole mounted seminiferous tubules from adult rat testes were used to investigate the kinetic properties of the undifferentiated type A spermatogonial population (1) immediately following multiple injections of 3 H‐TdR and (2) at long intervals after single or repeated injections. Most of the undifferentiated type A cells became labeled by five or six successive injections of 3 H‐TdR given at 12 hr intervals. Labeling frequency was greater than 90% in stages X‐I, and was maximal in stage I (96‐97%) where only a small proportion (15%) of the isolated undifferentiated spermatogonia (the A s stem cells) failed to incorporate the label. Since this unlabeled cohort represented only 1‐3% of the total undifferentiated type A population in stages I‐VII, the existence of a large non‐cycling A 0 or reserve stem cell population (which formed 21% of the total A population in stages II‐VII according to Clermont & Bustos‐Obregon, 1968) could not be confirmed. the non‐labeling of a few isolated stem cells was consistent with the previous finding of a prolonged G 2 in some of these spermatogonia (Huckins, 1971c). It was therefore concluded that all undifferentiated A spermatogonia, including the A s stem cells, were in active cell cycle. Some well‐labeled isolated stem cells persisted for long periods and were still common 13 days (one cycle of the seminiferous epithelium) after 3 H‐TdR injection. These appeared to be slow cycling elements which did not divide at all or possibly just once during this interval, but thereafter became mitotically active, giving rise to labeled pairs which in further divisions formed lightly labeled chains of aligned undifferentiated cells. There was a gradual disappearance of the labeled cells, so that by 26 days after 3 H‐TdR injection no labeled A spermatogonia remained in the seminiferous epithelium.