Splenic megakaryocytes and circulating platelets in pregnant rats

A comparative study of the hemopoietic tissues and blood of non‐pregnant and pregnant rats of graded duration of gestation was undertaken and the results were compared with those obtained on postsplenectomized rats at similar stages of pregnancy. The number of megakaryocytes was determined in representative fields of the liver, spleen and bone marrow. Platelet counts were performed on blood collected from the venae cavae of these rats using a modified Rees‐Ecker technique. The liver of normal adult animals, pregnant or non‐pregnant, does not reveal megakaryocyte production. Although bone marrow is the most active site of megakaryocytopoiesis in the adult animal there is only a slight numerical increase in megakaryocytes during pregnancy. The most dramatic change occurs in the spleen of the pregnant rats and is reflected by increased splenic weight and increases in red pulp and in the number of platelets circulating in venous blood. Splenectomy of the rat produces an abrupt increase in the number of circualting platelets which is not sustained into the later postoperative period when platelet counts below those of control animals are found. When pregnancy occurs in the later postoperative period, the splenectomized rat develops a trend toward increased platelt counts but these never reach the level obtained in nonsplenectomized pregnant rats.