thromboplastin (tissue factor; Factor III) A membrane glycoprotein expressed on the surface of damaged tissue cells that initiates the cascade of reactions leading to formation of a blood clot. It forms a complex with Factor VIIa in the presence of phospholipid and calcium ions; this complex converts Factor X to Xa, which in turn converts prothrombin to thrombin. See blood clotting.

thromboplastin One of a group of lipoprotein compounds apparently released by blood platelets at the site of an injury. In the presence of calcium ions and other factors, it catalyses the conversion of prothrombin into thrombin during the clotting of blood.

thromboplastin (thrombokinase) (throm-boh-plast-in) n. a substance formed during the earlier stages of blood coagulation. It acts as an enzyme, converting the inactive substance prothrombin to the enzyme thrombin.