Aspects of platelet formation and release.

The surface architecture and location of megakaryocytes in the extravascular compartment of mouse bone marrow are considered. The finger-like cytoplasmic processes that extend into sinuses pass through apertures, approximately 3.0 millimicron in diameter, occurring singly or in groups. The cytoplasmic processes enlarge on first entering a sinus and form villi that seem to anchor them to endothelium. The villi consist largely of microfilaments. The cytoplasmic processes extend, attenuate, and undergo irregular constriction along their lengths. The considerations demarcate modicums of cytoplasm of platelet size one from another that are released when they break. Extravascular platelet release may also occur, and evidence is presented that points to the likelihood of the platelet's being engulfed and phagocytosed by macrophages before reaching the microcirculation.