Megakaryocytes generate thousands of platelets over their lifespan. More recently, roles in infection and inflammation have been reported. These findings have driven a study of extra-medullary thrombopoiesis, and megakaryocytes have been increasingly reported within the spleen and lung. However, the relative abundance of megakaryocytes in these organs compared to the bone marrow and the scale of their contribution to the platelet pool in steady state remains controversial. We investigated …