Effects of acute γ-irradiation on the development of the thymus in embryos and fry of Oryzias latipes

Quantitative treatment of radiation effects on the thymus of fish embryos and fry is reported, through the measurement of thymus volume and numbers of lymphocytes in histological sections of the organ in Oryzias latipes. Irradiation was carried out with a γ-ray source of about 4kCi 137Cs at 250C. Exposure rate was about 360 R/min in air, doses of 1,2, or 4 KR being administered. The thymic development proved a radiosensitive process in embryos and fry, depending on several factors, such as dose, the stage of embryos at the time of irradiation, and the stage of fry when its thymus is examined. Even irradiation prior to the organogenesis caused an effect on the later development of the thymus, although no external morphological abnormalities of the fry themselves were observed. It was also commonly noticed that, after a temporary reduction in volume, repopulation of lymphocytes and recovery of the growth pattern of the organ eventually occurred in the subsequent development of fry. (U.K.)