Spermatogonial stem cells in the albino rat.

The existence of two classes of spermatogonial stem cells in the rat testis, i.e., reserve type A0 spermatogonia and renewing, types A1-A4 spermatogonia, postulated by Clermont and Bustos-Obregon ('68), was reexamined in a quantitative analysis of type A spermatogonia in both whole mounts of tubules and in radioautographed sections of testes from animals killed at various times, up to 26 days, after one or multiple injections of 3-H-thymidine. The cell counts obtained from whole mounts of tubules revealed that the number of isolated type A0 cells per unit area of limiting membrane remained constant throughout the cycle of the seminiferous epithelium. Paired type A0 spermatogonia also remained unchanged in number per unit area of basement membrane from stage I to stage VIII of the cycle. The low mitotic index of type A0 spermatogonia (0.1%) indicated that these cells were not actively involved in the production of spermatogonia or spermatocytes during each cycle of the seminiferous epithelium and thus were considered as reserve stem cells. The type A1 spermatogonia, which are formed during stage I of the cycle, remained resting until stage IX, when they undertook a series of four successive divisions resulting in the production of new type A1 and Intermediate-type spermatogonia. An analysis of the labeling indices of type A spermatogonia obtained from cell counts in radioautographed testicular sections after a single or multiple 3-H-thymidine injections indicated that the percentages of labeled type A cells corresponded to the percentages of type A1-A4 at each stage, whereas the percentages of unlabeled type A cells corresponded to the percentages of type A0 spermatogonia obtained from counts of cells in whole mounts. This confirmed that type A0 cells were generally non-proliferative throughout the cycle of the seminiferous epithelium while the type A1-A4 spermatogonia underwent complete renewal during each cycle. The present results thus support the concept of the existence of two classes of spermatogonial stem cells in rats.