The platelet: form and function.

Platelets are small subcellular fragments that are released from megakaryocytes. They are composed of a concentrate of megakaryocyte membrane, cytoplasm, granules, and organelles, and circulate throughout blood vessels and survey the integrity of the vascular system. They circulate as discs, a form specified by their internal microtubule and actin cytoskeleton. When encountering vascular damage, platelets rapidly convert into their active forms, which function to seal off the injury and prevent fluid loss. To assume the active shape, the internal cytoskeleton of each platelet is rapidly disassembled and replaced. New actin filament assembly provides the force that spreads platelets across damaged surfaces and allows the formation of filopodia, which are used to interconnect platelets in solution. In this review, we discuss the role of the cytoskeleton in defining the resting and active forms of the platelet.