Spermatogonial stem‐cell renewal in the mouse

Abstract Type A spermatogonia in the mouse can be separated into five successive classes on the basis of nuclear morphology and stage of the cycle in which they occur. Enumeration of all types throughout the cycle of the seminiferous epithelium reveals that the A s spermatogonia are the stem cells. They divide throughout the cycle and, especially at stages IX to I, form chains of cells which then give rise to spermatogonia A 1 at stages II–VIII. The A 1 cells divide in IX to form the A 2 , which divide in XI to form A 3 , and the A 3 cells divide in I to form the class A 4 spermatogonia. Spermatogonia A 4 give rise only to the In type; there is no evidence for the formation of either A s or A 1 from A 4 spermatogonia. Repeated injections of 3 H‐thymidine and tracing the history of labeled cells to 15 days after labeling supported the conclusion obtained from morphological and numerical data that A s spermatogonia are the stem cells of the testis.