Platelet Plug Formation: Aggregation

Activation causes dramatic changes in platelets that lead to their aggregation. Structural changes in a surface receptor (GPIIb/IIIa) expose binding sites for fibrinogen. Bound FI molecules link activated platelets to one another (Fig. 1), with a single FI able to bind two platelets. FI is converted to FIa by FIIa and then covalently cross-linked by FXIIIa coming from both the blood and the platelets. [Note: The exposure of PS on the surface of activated platelets allows formation of the Xase complex (VIIIa, IXa, X, and Ca²⁺) with subsequent formation of FXa and generation of FIIa.] Fibrin formation (secondary hemostasis) strengthens the platelet plug. [Note: Rare defects in the platelet receptor for FI result in Glanzmann thrombasthenia (decreased platelet function), whereas autoantibodies to this receptor are a cause of immune thrombocytopenia (decreased platelet number).]

Figure 1: Linking of platelets by fibrinogen via the glycoprotein (GP) IIb/IIIa receptor. [Note: The shapes in the fibrinogen molecule represent the two D and one E domains.] GPIb = glycoprotein Ib receptor; VWF = von Willebrand factor.

Unnecessary activation of platelets is prevented because 1) an intact vascular wall is separated from the blood by a monolayer of endothelial cells, preventing the contact of platelets with collagen; 2) endothelial cells synthesize prostaglandin I2 (PGI2, or prostacyclin) and nitric oxide, each of which causes vasodilation; and 3) endothelial cells have a cell surface ADPase that converts ADP to adenosine monophosphate.