Ans. a (Platelets). (Ref. Robbin, Pathology, 6th ed., 70, 71; Harper's biochemistry 27th ed. Table 50-3)# Platelets contain the enzyme thromboxane synthetase and hence TXA2, is major product in these cells.Thromboxane A2# TxA2, a potent platelet-aggregating agent and vasoconstrictor, is itself unstable and rapidly converted to its inactive form TxB2.# TXA, acts in cyclooxygenase pathway of inflammation.Inflammatory actions of eicosanoids: MetaboliteActions1TXA2 LTC4, D4, E4Vasoconstriction2PGI2 PGE1 PGE2 PGD2Vasodilatation3LTC4 D4, and E4Increased vascular permeability4LTB4 HETE, lipoxinChemotaxis, leukocyte adhesion5Chemokine eotaxinRecruitment of EosinophilsMolecules Synthesized by Endothelial Cells - Role in the Regulation of Thrombosis and Fibrinolysis.MoleculeActionADPase (an ectoenzyme)Degrades ADP (an aggregating agent of platelets) to AMP + PiNitric oxide (NO)Inhibits platelet adhesion and aggregation by elevating levels of cGMPHeparan sulfate (a glycosaminoglycan)Anticoagulant; combines with antithrombin to inhibit thrombinProstacyclin (PGI2 a prostaglandin)Inhibits platelet aggregation by increasing levels of cAMP MoleculeActionThrombomodulin (a glycoprotein)Binds protein C, which is then cleaved by thrombin to yield activated protein C; in combination with protein S degrades factors Va and Villa, limiting their actions.Tissue plasminogen activator(t-PA, a protease)Activates plasminogen to plasmin, which digests fibrin; the action of t-PA is opposed by plasminogen activator inhibitor-1 (PA 1 -1)