How are platelets formed from megakaryocytes?

Platelets form when mature megakaryocytes extend cytoplasmic projections through sinusoid walls in red bone marrow; these projections rupture into the bloodstream, and the plasma membrane around each fragment seals, producing the small, disc-shaped platelets.

In red bone marrow, megakaryocytes arise from megakaryoblasts. The megakaryoblast undergoes repeated mitoses, but cytokinesis does not occur, so the cell becomes a large mature megakaryocyte with a huge, multilobed nucleus. The mature megakaryocyte presses against a sinusoid, a specialized capillary of red marrow, and sends cytoplasmic extensions through the sinusoid wall into the bloodstream. Those extensions then rupture, releasing platelet fragments into the blood, much like leaves blowing off a tree. The plasma membrane around each fragment quickly seals, forming the grainy, roughly disc-shaped platelets that are about 2 to 4 micrometers in diameter. Thus, platelets are not true cells; they are cytoplasmic fragments of megakaryocytes, and their formation is regulated by the hormone thrombopoietin.