Electron microscopical observations on actinoid and myosinoid filaments in blood platelets

Actinoid and myosinoid filaments of the contractile apparatus of human platelets were studied in normal platelets, in osmotically shocked platelets, and in glycerol extracted platelets, both in negatively stained preparations and in sectioned material. Negatively stained preparations of ruptured platelets contained unbranched, beaded actinoid filaments 60 Å wide and 0.1–2 μ long. When actinoid filaments on electron microscope grids were exposed to heavy meromyosin (HMM), prepared from striated muscle myosin, arrowhead complex formation between the actinoid filaments and HMM resulted. The arrowheads were similar to those seen in test preparations of HMM and actin filaments from striated muscle. Arrowhead formation was inhibited by ATP. Negatively stained preparations also contained myosinoid filaments. These were 0.2–0.5 μ long, had a smooth central zone and terminated in bulbous thickenings at either end, thus showing obvious resemblance to synthetic filaments of myosin from striated and smooth muscle. The central zone exhibited a definite substructure consisting of parallel, regularly spaced, fine filaments 15–20 Å thick. Myosinoid filaments sometimes occurred singly, but were frequently encountered in groups, arranged either side-to-side or as chains or nets in which filaments were interconnected by the bulbous terminal thickenings. HMM did not react with myosinoid filaments, and there was no detectable binding of HMM to microtubules or membrane fragments. The number and dimensions of the myosinoid filaments varied with the method of preparation: few were found in normal platelets; they were abundant both in osmotically shocked and in glycerol extracted (80–120 Å) platelets. Actinoid and myosinoid filaments were identified in sections of glycerinated platelets. Only the actinoid filaments formed arrowhead complexes with HMM. In sections of untreated platelets, few filaments could be detected, and those present measured 50–60 Å. Platelets in retracted clots contained an abundance of filaments which varied in thickness from 40 to 120 Å, but 60 Å filaments were dominating. No ordered spatial relationship between filaments of various thicknesses was evident. Our results provide suggestive evidence that actin and myosin mainly occur in the circulating platelet in monomeric form, and that physiological activation (aggregation and clotting) and unphysiologic treatments as osmotic shock and glycerination cause the formation of filamentous polymers. It is conclud