Previous descriptions of the radioresponse of rodent spermatogonia have relied on schemes of spermatogonial proliferation and differentiation that were either incomplete or inaccurate. This study was undertaken, therefore, to interpret the radioresponse of rat spermatogonia after a currently accepted pattern for their renewal and evolution. Rats were irradiated with60 Co γ radiation at doses varying from 12 to 600 rad at 30 rad/min. Tubules were isolated from Zenker-fixed testes and evaluated according to the method of Huckins (Anat. Rec. 169, 533 [1971]). Stem spermatogonial number was little affected by doses up to 200 rad, and the dose-response curve for this criterion yielded a D0 of 373 ± 37 rad and an extrapolation number (n) of 2.1 ± 0.3. Stem-cell mitosis, as reflected by counts of type A1 spermatogonia, was, however, substantially affected by a dose of 12 rad and was characterized by a D0 of 148 ± 8 rad and an n of 0.5 ± 0.1. The dose required to produce a measurable effect increased as the spermatogonium attained higher states of differentiation. D0 and n varied from 139 ± 24 rad and 1.2 ± 0.9, respectively, for type A1 to 215 ± 14 rad and 2 ± 0.4 for type A4. Due to the extreme lability of its mitosis, the stem spermatogonium appeared to be the weakest link in the chain leading to the spermatozoon.

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