ABSTRACT
The mitotic activity, destruction and volume of nuclei of thymocytes were investigated on histological preparations of the thymus of rats flown onboard Cosmos-782 and Cosmos-936. The cytological data showed that immediately after touchdown large quantities of thymocytes of the cortical matter perished. The mitotic activity of the remaining undamaged thymocytes declined and returned to normal 9 hours after recovery. The rats that were centrifuged inflight (at 1 g) did not display a lower mitotic activity or noticeable destruction of thymocytes. Karyometric measurements demonstrated that the population of thymic lymphocytes was heterogeneous in the nuclear volume: it consisted of three peak classes of nuclear volumes. The variance curves of nuclear volumes of lymphocytes of the medullary matter were drastically shifted toward large numbers. An increase in the nuclei of thymocytes of the cortical matter in the flight rats was stable and persisted till R+25.