A brief introduction on platelet secretion (see Fig. 43.1)

The essential role played by platelets in hemostasis, thrombosis and vascular remodelling is, to a large extent, mediated by effector molecules that they secrete at sites of vascular injury. These molecules are contained in the platelet dense (or δ-) and α-granules. Dense granules are small organelles (200–300 nm in diameter) that are dense to electrons in osmium-stained platelets. They mostly contain small molecules, such as nucleotides, serotonin, catecholamines, Ca2+, Mg2+ and pyrophosphate. More recent studies have demonstrated the presence of Pselectin, granulophysin, glycoprotein (GP) Ib and GPIIb/IIIa bound to their membrane. Alpha granules, at variance with -granules, are moderately electron-dense and have a diameter of about 200–500 nm. The most numerous of the platelet storage organelles, they contain membrane-bound proteins (GPIb/IX/V, GPIIb/IIIa, Pselectin, GPIV, osteonectin, PECAM1, GMP33, CD9) and are the storage site for proteins which are synthesized in the megakaryocytes (von Willebrand factor, thrombospondin, fibronectin, coagulation factor V, platelet-derived growth factor and other growth factors), or acquired from plasma either by receptor-mediated endocytosis (e.g., fibrinogen) or by fluid-phase pynocytosis (e.g., albumin and immunoglobulin G).

While δ-granules function primarily for the amplification of platelet activation through the secretion of platelet agonists such as serotonin and adenosine diphosphate (ADP), α- granules enhance the adhesive process, promote cell–cell interactions and stimulate vascular repair.