Relation between X-Ray Sensitivity and Stages of the Cell Cycle in Spermatogonia of the Mouse

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Abstract
Male mice were injected intraperitoneally with 15 [mu]c of H3-thymidine per mouse, X-irradiated 1 hour later with 20, 50, 100, 200, or 300 r, and killed at various time intervals afterward and autoradiograms of 5-[mu] sections of the testes were prepared. Damaged spermatogonia die as they reach a definite, critical stage in the cell cycle, regardless of the stage at which they were irradiated. Cell death occurs in late interphase or very early prophase for type A and intermediate spermatogonia and in anaphase or telophase for type B spermatogonia. Type B spermatogonia are remarkably resistant to cell killing for a period of about 5 to 7 hours after completion of DNA synthesis. This period corresponds approximately to the duration of prophase. Interphase is the sensitive stage. Since type A and intermediate spermatogonia die before division, the fate of the cells irradiated while in mitosis escapes detection. The much longer duration of the X-ray-sensitive period (interphase) and the much shorter duration of the X-ray-resistant period (prophase) of the cell cycle in intermediate and type B spermatogonia in comparison to type A spermatogonia may account for the greater X-ray sensitivity of intermediate and type B cells. Accordingly, the existence in the type A population of four cell generations each having different durations of the X-ray-sensitive and of the X-ray-resistant periods of the cell cycle offers a possible explanation for the heterogeneity of the radiation survival curve of these cells.