Platelet Storage Granule Defects

Platelets contain two major types of granules, dense and α-granules. Granule contents include numerous factors essential to the normal formation and stabilization of a hemostatic plug. Either a decrease in the number of platelet granules or a defect in their release can result in a bleeding diathesis. Several clinical syndromes are associated with storage granule defects. Bleeding manifestations in patients with storage granule defects are typically mild.

Characteristics of the δ-granules and α-granules are as follows:

• Platelet-dense granules : contain calcium, serotonin, adenosine diphosphate (ADP) and adenosine triphosphate (ATP), and have a dark, electron-dense appearance in electron microscopy. Dense granule contents contribute to the activation of neighboring platelets.

• Platelet α-granules : contain proteins with a number of different functions. Generally, these contents promote the adhesion of platelets and the stimulation of inflammatory and vascular cells. Adhesion proteins, procoagulant molecules, pro- and antiangiogenic proteins, inflammatory cytokines, platelet receptors, and bactericidal proteins have been identified within the α-granule. Release of different α-granule components may be differentially regulated depending on the agonist used.

Either a decrease in number of platelet granules or a defect in their release can result in a bleeding diathesis. Several clinical syndromes, such as Hermansky–Pudlak, Chediak–Higashi, gray platelet, arthrogryposis–renal dysfunction–cholestasis (ARC) and others, can be associated with platelet granule deficiency in addition to defects in other cells and organs. Bleeding manifestations in patients with storage granule defects are typically mild.