Eosinopoiesis in the spleen of normal rats. Autoradiographic studies.

Two groups of normal inbred female rats, aged 3 and 5–6 months, and three groups of germ-free female rats, aged 3, 5–6, and 10–11 months, received a single injection of tritiated thymidine. After varying intervals the rats were killed. The spleens were homogenized and suspended in physiological saline. The suspensions were studied for eosinophil count and the percentage of radioactively labelled eosinophils. The corresponding values for the spleens were calculated. At the same time the eosinophil cells and the labelling index in the peripheral blood were studied. The germ-free rats showed considerably lower counts in the blood and spleen than did the others. In all groups there was parallelism between eosinophils in the blood and in the spleen. All spleens exhibited eosinopoiesis and some parallelism between this activity and the circulating eosinophils. Splenectomy in control rats did not give rise to measurable changes in the number of circulating eosinophils. It is concluded that apart from being formed by local eosinopoiesis, the eosinophilic cells of the spleen have been transported by the blood and stored in the spleen. The function of the spleen as a depot of eosinophilic cells probably plays no major role. Splenic eosinopoiesis is not, and cannot be expected to be, of importance to the number of circulating eosinophils.