Radiation Survival and Regeneration Characteristics of Spermatogenic Stem Cells of Mouse Testis

Mouse testes were studied histologically 35 or 42 days after irradiation. When doses greater than about 800 rads of γ-rays are given, regeneration of spermatogenic epithelium occurs in discrete foci. It is assumed that a single surviving spermatogenic stem cell can regenerate a spermatogenic focus, and that the absence of spermatogenesis in a tubule signifies the absence of a surviving stem cell in that region of the tubule. The fraction of tubules containing spermatogenic colonies was used as an assay of stem-cell survival. The mean D₀ value of the single-dose survival curve was 180 rads. Survivors of a first dose of 600 rads repair sublethal damage during the first few hours after irradiation. The amount of recovery measured in split-dose studies using intervals of 24 hr to 14 days is less than that measured 4 hr after exposure. This is interpreted as evidence for the existence of phases in the division cycle in which the stem cells are sensitive to radiation and into which a proportion of the first-dose survivors progress. Although surviving stem cells progress through the division cycle, and eventually form a focus of spermatogenesis, regeneration of stem cell numbers was not found in experiments continuing for 14 days after irradiation. The relevance of these characteristics to the development of fractionated-dose radiotherapy is discussed.