Cytological and histological changes in the spleen of continuously irradiated rats

The study evaluates quantitative cytological and histological changes in splenic white and red pulp of rats continuously irradiated for up to 140 days with daily doses ranging from 0.5 R to 82.5 R. A significant reduction of lymphocyte incidence begins at 5 R/day and it continues to drop with every further increase in the dose rate until a steady state of about 5 per cent is reached at 82.5 R/day. Small lymphocytes are relatively more affected. Irradiated animals show a wave-like increase in ectopic erythropoiesis in spleen red pulp from day 30 on even with the lowest dose rates of 0.5 and 1 R/day. The ratio or relative incidences of haemocytoblasts and erythroblasts increases approximately twofold, indicating that a substantial reduction of stem cells is compensated to a large extent already at an early stage of erythropoiesis. The incidence of myeloid elements rises, from the 40th day of exposure, the increase being more apparent at medium dose rates for eosinophils than for neutrophils. Both the red and white splenic pulps of exposed animals display an increased incidence of plasma cells. (author)