The contractile system of blood platelets and its function.

The various manifestations of platelet activation are derived from a reorganization of components of the contractile and microtubular systems. The controversial initial stages of excitation-contraction coupling in platelets lead to the release of calcium from the dense tubular system, the morphological counterpart of the muscle sarcotubular closed vesicles. Calcium triggers the actin-myosin interaction and the developing force, possibly together with a local increase of the cation concentration, may cause the collapse of the microtubular ring and its reappearance in the forming long pseudopodia. Actin-myosin interaction is modulated by several factors among which tropomyosin-troponin, responsible for the calcium-sensitivity of contractile processes, and phosphorylation of one of the myosin light chains. Platelet actin is anchored to the membrane and its sliding towards the short myosin filaments may form the basis for platelet shape change. Platelet alpha-actinin and actin-binding protein are able to aggregate actin into an impressive gel. Therefore, the contractile proteins seem to have a double role in controlling the consistency of the cytoplasmic gel on the one hand, and the contractile manifestations related to motility on the other hand. One of the most important features of the 'contracted' platelet is the rigidity of the pseudopodia brought about by the 'gelification' of actin filaments and the presence of microtubules. A new model for clot contraction is proposed, based on the rigidity of the long spiky pseudopodia and on the motile properties of platelets. While migrating towards each other, the interlocking pseudopodia from different platelets adhere to the polymerizing fibrin, compressing the fibrin nets in their pathway. Since the anchoring of contractile fibers to membranes is crucial for the platelet contractile manifestations, the integrity of the membrane structure should be considered in the study of pathological aspects of platelet function.