(thrombocyte). A proteinaceous cellular structure occurring in blood in the amount of 150–500 × 103 units/mm3. Platelets range from 2 to 4 µm in diameter and contain no nuclei. They are rich in amine compounds, which constrict the blood vessels at the site of an injury, to which the platelets adhere; on dissolution they release thromboplastin, which initiates the coagulation mechanism.See Blood; Fibrinogen; Thrombin.