The bone marrow is the primary site of thrombopoiesis

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 the relative abundance of megakaryocytes in the adult murine bone marrow, spleen, and lung using whole-mount light sheet and quantitative histological imaging, flow cytometry, intravital imaging, and an assessment of scRNA-seq repositories. Flow cytometry revealed significantly higher numbers of hematopoietic stem and progenitor cells and megakaryocytes in the murine bone marrow compared to spleens or perfused lungs. Two-photon intravital and light-sheet microscopy, as well as cryosections confirmed these findings. Moreover, ex-vivo cultured megakaryocytes from the bone marrow subject to static or microfluidic platelet production assays had a higher capacity for proplatelet-formation than MKs from other organs. Analysis of previously published murine and human scRNA-seq datasets revealed that only a marginal fraction of megakaryocyte-like cells can be found within the lung and most likely only marginally contribute to platelet production in the steady state.