Platelet biogenesis. In vivo, megakaryocytes sit at the bone marrow sinus wall (top left), where they extend proplatelets into the vascular sinus. The membrane required for extension of the proplatelets is derived from the megakaryocyte invaginated membrane system. Proplatelets (shown in yellow) are formed by a microtubule mediated extension but also require reorganization of the actin-spectrin cytoskeleton for their formation. Platelet biogenesis occurs at the terminals of the proplatelets and is associated with the formation of platelet microtubule coils that may be linked to the actin-spectrin membrane skeleton. Electron micrograph by Fern Tablin. Professional illustration by Paulette Dennis.