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Testes of mature male mice were irradiated with a single dose of 1000 rads. Enumeration of the Sertoli cells at different intervals showed an increase in their number per mm3 of tubule, but after the application of correction factors there was an immediate fall in the total Sertoli cells per testis which later stabilized at about half the preirradiation level. There was a reduction in the mean number of chromocentres per Sertoli cell, owing to a relative increase in the number of nuclei with one chromocentre, 14 days after irradiation. It was further seen that labelled spermatocytes were transformed to restitution cells at the time that unirradiated spermatocytes would have produced spermatids. Seven days later the label appeared in spermatozoa. This implied that the restitution cells were not eliminated but were transformed to spermatozoa and the timetable of events was similar to that in normal spermiogenesis. No label appeared in Sertoli cells. The number of grains present in spermatocytes and restitution cells and spermatozoa was nearly the same, indicating no intervening nuclear divisions. Enumeration of the number of surviving labelled cells confirmed preleptotene as the most sensitive stage of spermatocytes.

Summary. The testes of mature male A2G mice were exposed to a single dose of 1000 r of X-rays, and examined at varying intervals up to 146 days after irradiation. Lipids, phospholipids and cholesterol were studied in the control and irradiated mice using histochemical techniques, and estimated quantitatively by the paper weight technique.The relative proportion of lipids per unit area of testis decreased 2 and 6 days after exposure, but increased dramatically from 20 to 72 days; by 146 days, it had nearly returned to the normal level.The relative proportion per unit area of tissue containing phospholipid decreased 2 and 10 days after exposure, steadily rose at 12 days and after until 103 days, and returned to normal at 146 days. The relative proportion of cholesterol increased markedly after 20 days but at 103 and 146 days, it fell to the control level. The significance of these findings is discussed.

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