Cytological and histological changes in the spleen of continuously irradiated rats.

The study evaluates quantitative cytological and histological changes in spleen white and red pulp of rats continuously irradiated for up to 140 days with daily dose rates ranging from 0.5R--82.5R. Significant reduction of lymphocyte incidence begins at the rate of 5R/day and the decrease deepens with the rising dose rate until a steady state around 5 per cent is reached at 82.5R/day. Small lymphocytes are relatively more affected. Irradiated animals show a wave-like rising of ectopic erythropoiesis in spleen red pulp from day 30 on even with the lowest dose rates of 0.5R and 1R/day. The ratio or relative incidences of haemocytoblasts to 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. Beginning with the 40th day of exposure, the incidence of myeloid elements also rises, the increase being more apparent at medium-range dose rates for eosinophils than neutrophils. Both the red and white spleen pulps of exposed animals display an increased incidence of plasma cells.