Phenotypic Changes in the Megakaryocyte-Platelet Lineage

Morphologic and phenotypic changes occur during the maturation of megakaryocytes (MK) from the pluripotent stem cell to platelets. As the MK acquires organelles, it also acquires membrane glycoproteins and granule contents. Platelet membrane GP IIb/IIIa and platelet peroxidase are present from early stages of maturation to the final product of the megakaryocyte, the platelet, while Ia-like antigen appears to be expressed only during early stages of maturation. The MK synthesizes increasing amounts of lysosomal enzymes, GP Ib, and alpha granule proteins as it matures from the megakaryoblast stage to the mature cell. The platelet contains only vestiges of a protein synthetic apparatus; it therefore has acquired most of its contents, except for serotonin, during the maturation of the MK

Redistribution of alpha-granules and their contents in thrombin-stimulated platelet

The redistribution of beta-thromboglobulin (beta TG), platelet Factor 4 (PF4), and fibrinogen from the alpha granules of the platelet after stimulation with thrombin was studied by morphologic and immunocytochemical techniques. The use of tannic acid stain and quick-freeze techniques revealed several thrombin-induced morphologic changes. First, the normally discoid platelet became rounder in form, with filopodia, and the granules clustered in its center. The granules then fused with one another and with elements of the surface-connected canalicular system (SCCS) to form large vacuoles in the center of the cell and near the periphery. Neither these vacuoles nor the alpha granules appeared to fuse with the plasma membrane, but the vacuoles were connected to the extracellular space by wide necks, presumably formed by enlargement of the narrow necks connecting the SCCS to the surface of the unstimulated cell. The presence of fibrinogen, beta TG, and PF4 in corresponding large intracellular vacuoles and along the platelet plasma membrane after thrombin stimulation was demonstrated by immunocytochemical techniques in saponin-permeabilized and nonpermeabilized platelets. Immunocytochemical labeling of the three proteins on frozen thin sections of thrombin-stimulated platelets confirmed these findings and showed that all three proteins reached the plasma membrane by the same pathway. We conclude that thrombin stimulation of platelets causes at least some of the fibrinogen, beta TG, and PF4 stored in their alpha granules to be redistributed to their plasma membranes by way of surface-connected vacuoles formed by fusion of the alpha granules with elements of the SCCS