INITIAL CHROMOSOMAL LESIONS INDUCED BY X-IRRADIATING PRIMARY SPERMATOCYTES OF MICE

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Abstract
Ten adult Swiss Webster male mice were X-irradiated at 500 R and their testes examined according to an air-dry technique for chromosomal aberrations at 2, 4, 8, 24 and 48 hr after irradiation. Chromosomal aberrations were found in 85 to 92% of cells examined from 4 to 48 hr after exposure compared with 1% of damaged cells observed in control testes and 68% in testes examined 2 hr after irradiation. The proportions of cells with chromosomal lesions but without rearrangements decreased significantly from 51% at 2 hr to 19% at 8 hr. The combined means per cell of unrejoined breaks and breaks accounted for by rearrangements increased linearly from 0 to 8 hr after irradiation, whereas the mean numbers of unrejoined breaks increased linearly only up to 4 hr after irradiation. It seems that initial radiation damage to the chromosomes of primary spermatocytes of mice (late pachytene to metaphase I) consisted predominantly of unrejoined lesions at 2 hr after exposure, and that by 8 hr a high proportion of lesions had combined to form rearrangement figures involving two, three or four bivalents. These results emphasize the importance of the time of examination for comparative studies of irradiation-induced chromosomal anomalies.