Platelet demand modulates the type of intravascular protrusion of megakaryocytes in bone marrow

Megakaryocytes (MKs) generate platelets via intravascular protrusions termed proplatelets, which are tandem arrays of platelet-sized swellings with a beaded appearance. However, it remains unclear whether all intravascular protrusions in fact become proplatelets, and whether MKs generate platelets without forming proplatelets. Here, we visualised the sequential phases of intravascular MK protrusions and fragments in living mouse bone marrow (BM), using intravital microscopy, and examined their ultrastructure. The formation of intravascular protrusions was observed to be a highly dynamic process, in which the size and shape of the protrusions changed sequentially prior to the release of platelet progenitors. Among these intravascular protrusions, immature thick protrusions were distinguished from proplatelets by their size and the dynamic morphogenesis seen by time-lapse observation. In ultrastructural analyses, the thick protrusions and their fragments were characterised by a peripheral zone, abundant endoplasmic reticulum and demarcation membrane system, and random microtubule arrays. Proplatelets were predominant among BM sinusoids in the physiological state; however, during an acute thrombocytopenic period, thick protrusions increased markedly in the sinusoids. These results strongly suggested that BM MKs form and release two types of platelet progenitors via distinct intravascular protrusions, and that platelet demand modulates the type of intravascular protrusion that is formed in vivo.