The Formation of Fusiform Proplatelets and their Transformation to Discoid Platelets.

Blood platelets are produced in the circulation by fragmentation of long, slender processes of cytoplasm formed from the megakaryocyte, the parent cell of platelets. Fragmentation occurs at local constrictions, forming 6 to 15 µm long, fusiform fragments (elongated proplatelets). The fusiforms transform into the circular, disc-shaped mature platelet by curving into a ring, which closes by fusion at the tips. The hole in the ring is finally filled in by a centripetal flow of membrane from the periphery. It is presumed that the curving of the fusiform is mediated by curving of its contained bundle of microtubules, which becomes the marginal bundle of the disc-shaped platelet. When curving begins in the fusiform, microtubules are closely associated with a membraneous tubule that becomes the submarginal tubule of the dense tubular system.