Platelet Birth.

How and where human platelets get born are two questions still harboring great mystery. In vivo and in vitro approaches have contributed to a better understanding of the phenomenon. The recent study of a family with congenital autosomal-dominant thrombocytopenia showed ectopic platelet release in the bone marrow of the affected individuals. Further, molecular studies identified a new mutation in the gene encoding cytochrome c, which yields a cytochrome c with enhanced apoptotic activity. It is known that platelet birth occurs when megakaryocytes (MKs) undergo compartmentalized apoptotic activity. In this family case, enhanced apoptosis in MKs led to the premature release of platelets in the bone marrow, preventing their natural birth into the circulation after MK migration and causing thrombocytopenia. A similar finding, i.e., platelet birth in the marrow, was made in Wiskott Aldrich syndrome, providing a pathophysiological explanation for the thrombocytopenia, related to impaired migration capacities of MKs upon SDF chemotaxis. Indeed, direct platelet delivery in the human bone marrow space is not currently observed. However, maturing MKs are located close to the marrow sinusoids whose barrier they are able to cross, and entire MKs have been seen in the circulating blood where they become exposed to circulatory shear forces. Since the rate of platelet shedding in culture systems is remarkably low, we hypothesised that a missing element for efficient platelet production in static culture systems would be shear stress. Thus, we exposed human MKs, cultured in vitro until full maturation, to a substrate of matrix protein and high shear rates equivalent to those encountered in capillaries and small arteries. Cells were observed by real-time videomicroscopy, immunofluorescence, and electron microscopy. Shear forces specifically induced a sequence of morphological changes, converting mature MKs into proplatelets, and platelets, which consistently detached from the mother cells into the flow,, at a rate 20 times higher than static culture conditions. Using specific antibody inhibition, we showed the major involvement of GPIb in platelet formation, since its blockade inhibited MK adhesion and subsequent platelet formation. In addition, aIIbb3 was essential for firm MK anchorage, which was required for subsequent platelet formation. These experiments show that MK exposure to high shear rates promotes platelet production via GPIb (and secondarily aIIbb3) interactions with vascular matrix proteins. In conclusion, the above studies converge to present evidence that platelet birth takes place in circulating blood, being regulated by the migratory and apoptotic capacities of MKs and dependent on GPIb and hemodynamic forces.