Extramedullary megakaryocytes and contribution to physiological platelet production. Previous studies have reported that bone marrow and lung megakaryocytes equally contribute to platelet production in mice. The current model, based on the relative megakaryocyte abundance, phenotype, and ex vivo capacity of proplatelet formation, suggests that bone marrow is the primary site of thrombopoiesis, whereas extramedullary megakaryocytes in the lung and spleen are marginally involved in the maintenance of circulating platelet mass. Created with BioRender.com.