[The thrombocyte membrane]

The thrombocyte membrane is characterized by several pecularities, first by its morphology which includes deep invaginations into the interior of the cell and, second, by its capacity to become stimulated by a wide variety of seemingly unrelated external agents which extend from proteases to collagen, certain immune complexes and small molecular weight substances such as ADP, adrenaline, serotonin and vasopressin. The response of the membrane to stimulation consists in a drastic rearrangement of its constituents, as exemplified by the appearance on the outer surface of components which are not accessible in the resting platelet. Stimulation may either lead to morphological changes and to aggregation or to more far-reaching alterations linked to aggregation, namely the release of substances from storage organelles and manifestations of gross contractile activity. The generation of these sequential reactions involves the production, by the exited membrane of a hitherto ill-defined signal to the interior of the cell. One of the most important consequences of this signal consists in the release, from internal sources, of calcium ions. Calcium ions are directly involved in the rapid shape change of stimulated platelets, due to their depolymerizing effect on the microtubules, they furthermore trigger the release reaction, in which the prostaglandin in system seems also to be involved and, finally, they are essential for the activation of the contractile system. Simultaneous with the release reaction, the platelet plasma membrane acquires calcium permeability; hence, in a later phase, cytoplasmic calcium originates not only from internal sources, but also from the surrounding medium. It is particularly noteworthy that all these alterations of the plasma membrane are reversible, which means that not only the essential structural rearrangements which occur upon stimulation in the membrane are reversed but also that the cell is capable of removing the Ca2+-ions which have entered the cytoplasm during the activation phase.