Morphological and quantitative analysis of spermatogonia in mouse testes using whole mounted seminiferous tubules. II. The irradiated testes

In adult male mice exposed to 300 R X-irradiation, the sper-matogonial population was selectively killed except for the radioresistant type A_s stem cells. Type A spermatogonia were minimal two days after irradiation, when only 20% of the control population was present in stages 5-6; these were predominately single and paired undifferentiated cells. When multiple injections of ³HTdR were given between 2 and 3.5 days post-irradiation, 90–95% of these survivors in stages 4-6 became labeled. Enhanced proliferation of these stem cells, and at times when they were normally quiescent, led to restoration of all classes of spermatogonia by 11 days after irradiation. Several autoradiographic studies were undertaken to better characterize the radioresistant cells. In mice given single or multiple injections of ³HTdR prior to irradiation, there was appreciable retention of label by those type A_s sper-matogonia that had originally incorporated ³HTdR in stages 2-4. This labeling pattern was identical to that of the long-cycling A_s stem cells in nonirradiated testes. Since the long-cycling A_s stem cells are thought to be characterized by a prolonged G₁ or “A-phase” which is known to be a highly radioresistant portion of the cell cycle, it was clear why these cells could preferentially survive irradiation doses that killed other spermatogonial types. It was proposed that following germ cell depletion, as after irradiation injury, the long-cycling A_s survivors could be prematurely triggered from A-phase into DNA synthesis, thereby, initiating restoration of the germ cell population.