The submembrane filaments of blood platelets.

Famous rnmyurs concentrated in the hyaloplasm of blood platelets are involved in platelet contractiocnY Following exposure to a variety of aggregating agents, microtubules and microfilaments are shifted into a ring that encirle platelet organefles moved to the cell center dug itnal transformation. The centripetal movement of organelles and fibers is an esstial prelude to the release reaction.4 A dichotomy exists, however, between the events at the center and those at the platelet surface. The surface membrane becomes irregular, and is extruded focally into spiky pseudopods. Membrane alterations can be independent of those caused inside platelets by aggregating agents, since chilling 5 and EDTA produce only the membrane nges. Irregularities in the platelet membrane have been attributed to altered permeability caused by aggregating agents,7 increased plasticity due to chelation, and to destruction or internal shift of the circumferential bundle of microtubules." IThese factors are unquestionably important, but do not explain how long& narrow pseudopods containing parallel microfi ts, microtubules, or both, are projected from the platelet surface."1The present report will describe a fibrous element closely associated with the platelet surface which may be integral to pseudopod formation and to the contraction of surface projections during viscous metamorphosis.