| Abstract: | Freeze-fracture and thin-section electron microscopy indicate that a sequence of fusion-fission leads to reorganization of membranes and the demarcation of platelets within the cytoplasm of megakaryocyte. Invagination of the megakaryocyte plasma membrane leads to the formation of tubular structures within the cytoplasm of megakaryocytes. Fusion of these tubular membranes in the plane of their long axes is followed by fission in the perpendicular plane. This results in the formation of two flat membranes, forming plasma membranes of two adjacent platelets. A similar fusion-fission reorganization of membranes could mediate a wide variety of other biologic phenomena. These observations also indicate that megakaryocytes are located in the subendothelial compartment of the marrow with their projections penetrating the endothelium and reaching the lumen. This direct contact with the circulation may serve as a means of receiving information as to the requirements of the body for platelet production. |
