Abstract
Measurements of megakaryocyte number, volume, nuclear number, and cytoplasmic granulation were compared in animals with induced thrombocytosis, normal platelet counts, and induced thrombocytopenia for 4- and 10-day periods. Thrombopoiesis, as measured by megakaryocytic mass and (35)S incorporation into platelets, appeared to be regulated by the demand for circulating platelets. In addition, two mechanisms were operative in the alteration of thrombopoiesis. The first mechanism involved the regulation of endomitosis in the immature megakaryocytes which in turn determined the subsequent volume of platelet-producing cytoplasm. The second mechanism regulated the number of megakaryocytes formed from the precursor, "stem cell," compartment.