Megakaryocytopoiesis in Splenectomized and ‘Hypersplenic’ Rats

S ummary . Megakaryocytopoiesis was studied in splenectomized, in normal and in ‘hypersplenic’ rats in order to evaluate the role of the spleen on platelet kinetics. Bone marrow autoradiography after injection of tritiated thymidine was utilized in this investigation. The splenectomized rats were studied 6 weeks after the operation when their platelet count was stable at a level of 23% higher than before splenectomy. Splenomegaly was produced by repeated intraperitoneal injections of methylcellulose and was associated with a 52% lowering of the platelet count. The results revealed identical patterns of megakaryocytopoiesis in splenectomized and normal rats indicating that post‐splenectomy thrombocy tosis was not the consequence of an increased rate of megakaryocyte maturation. On the contrary, megakaryocytopoiesis in the ‘hypersplenic’ rats was stimulated, although slightly so. This demonstrated that thrombocytopenia in the rats with splenomegaly could not be ascribed to the functional inhibition of megakaryocyte maturation. These findings support the view that in normal conditions the role of the spleen lies only in the distribution of the circulating platelet mass with a higher concentration of platelets in the spleen, and are consistent with the suggestion that in ‘hyper‐splenism’ the thrombocytopenia occurs because of excessive pooling of the platelets in the enlarged spleen, while the bone marrow provides a larger than normal number of platelets.