Morphological Distinction Unravels Mechanisms Of Platelet Biogenesis From Bone Marrow Megakaryocytes

Blood platelets are generated in the bone marrow (BM) from their precursors, megakaryocytes (MK). Although we know that MKs produce platelets throughout life, precisely how platelets are produced in vivo remains uncertain, largely because of the rarity of MKs in the BM and the lack an adequate visualization technique. In the present study, we were able to visualize MK dynamics leading to platelet release in living animals at high resolution.

To clearly understand the nature of thrombopoiesis in BM MKs, we optimized an in vivo imaging technique based on two-photon microscopy that enabled us to visualize living BM in CAG- enhanced green fluorescent protein (eGFP) mice. By visualizing living bone marrow in vivo, we observed that two modes (fragmentation and proplatelet formation) can be ongoing simultaneously in the same mouse. We observed that these two modes detectable by different morphological behavior can be ongoing simultaneously in the same BM of mouse, and are regulated by specific cytokines. Short proplatelets from megakaryocytes predominated at steady state, and more elongated proplatelets were accelerated by thrombopoietin (TPO) with responding to chronic platelet needs including recovery form BM transplantations. In contrast, acute platelet needs by blood loss, 5-FU administration or pritoneal acute inflammation increased cytoplasmic fragmentation following rapid ‘rupture’. Observed two modes are both dependent on tubulin reorganization on platelet biogenesis. In addition, platelet increase at acute phase is independent of proliferation by MK progenitors and this factor might exert apoptosis machinery on already reserved mature type of MKs.

This humoral factor was identified by combination of in vitro screening systems and in vivo MK visualization analysis. Factor serum levels were reduced independently of the thrombopoietin level in human subjects with low platelet counts. It thus appears the cytokine balance dynamically regulates the mode of thrombopoiesis and the cellular programming of MKs. Thus, these novel factor may be a novel therapeutic target in thrombocytopenic situations, especially when associated with acute loss of platelets or when platelet transfusion is limited or unsuccessful.