A quantitative study of the number of type A spermatogonia surviving 4 days after a single exposure to the testes of 800 R, 1000 R, 1200 R, 1400 R, or 1600 R was made on albino Sprague-Dawley descendant rats. The mean percentage survival of type A spermatogonia surviving was 13.72, 7.75, 4.62, 2.78, and 1.63, respectively, for the above exposures. Nine nonirradiated rats served as experimental controls. The percentage of type A spermatogonia surviving each exposure was calculated and a survival curve plotted with a D0 estimated as 380 R with a 95% confidence interval of 365 to 395 R and an apparent extrapolation number of 1.12 with a 95% confidence interval of 1.08 to 1.15 R. The low extrapolation number is probably due to the heterogeneity of type A spermatogonia since the extrapolation number obtained is equal to the percentage of resistant type A spermatogonia times the true extrapolation number of the resistant type A spermatogonia. However, the estimated D0 of 380 R is for the resistant type A spermatogonia.

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