The Submembranous Fibrils of Human Blood Platelets

Among several functions attributed to blood platelets is a contractile one which is believed to play a significant role in hemostasis. In addition, platelets are known to take up a large variety of substances which are transported interiorly by means of surface-connected channels (1) . Since propulsion of substances through this canalicular system is known to be energy dependent (13, 14), it has also been suggested that contractile mechanisms are operative here. Moreover, during clot formation or in vitro aggregation following treatment of platelets with thrombin or adenosine diphosphate (ADP), the canaliculi move toward the center of the cell where they appear to fuse (18, 20). Lastly, an actomyosin-like protein, thrombosthenin, has been isolated from human (2, 3) and porcine (6) platelets, and examination of partially purified thrombosthenin by electron microscopy has disclosed an abundance of microfibrils (21) . This has raised the question whether the microfibrils seen in the cytoplasm of platelets represent the contractile protein . In intact platelets, microfibrils run in parallel bundles within pseudopods, and under certain conditions the entire cytoplasm can be shown replete with these organelles (20) . However, even the assumption that microfibrils consist of thrombosthenin would not explain how these randomly oriented structures could be instrumental in displacing the plasma membrane or in moving the canaliculi through their various formations . Therefore, specimens used in previous studies (20) were reviewed and additional experiments were conducted to determine whether microfibrils are also attached to platelet membranes. The observation that