Mechanisms of platelet production.

The precise mechanism by which platelets are formed from megakaryocytes (MK) remains unclear, despite numerous studies which have been performed during this century. Models have been proposed that attempt to account for platelet formation from disruption of elongated processes of MK cytoplasm, designated proplatelets, or by fragmentation of MK cytoplasm. MK demarcation membranes are hypothesized by some investigators to delineate platelet territories in the MK cytoplasm, and by others to act as a membrane reservoir for MK process formation. Platelet production has been variously speculated to occur primarily in the bone marrow or lung. Each theory or model has attempted to elucidate the phenomenon of size heterogeneity of circulating platelets and the changes that occur under conditions of altered thrombopoiesis. In this article, we have analyzed and compared the characteristics of previously proposed models for platelet production and suggested additional techniques for future studies of thrombopoiesis.