The ligand-binding domains include the amino-terminal segment and the extracellular segments of the transmembrane chain. These receptors are coupled to G-protein adapter molecules, and agonist binding triggers intracellular signalling pathways

leading to platelet activation, including enhanced affinity of integrin αIIbβ3 for its ligands. Rare mutations have been identified in the ADP receptor P2Y12, and in the TxA2 receptor that cause mild clinical bleeding. Because ADP and TxA2 are important in augmenting platelet responses to other agonists, these defects are NVP-AUY922 mouse typically associated with decreased in vitro platelet aggregation responses to multiple agonists [11]. Glanzmann thrombasthenia is characterized by marked impairment of platelet aggregation caused by quantitative or qualitative

defects in the platelet mTOR inhibitor integrin αIIbβ3 that mediates fibrinogen and other adhesive ligand binding. GT is discussed in detail in the section on Glanzmann thrombasthenia. There are three primary types of platelet granules: lysosomes and two types of platelet specific storage granules (electron-dense δ-granules and α-granules). There are three to eight δ-granules per platelet. They contain serotonin, a non-metabolic pool of adenine nucleotides and Ca2+, which is responsible for the electron density of these granules in platelets examined by the whole mount technique [12]. α-Granules are the predominant granule type, with approximately 50 per platelet. They contain a broad range of proteins including adhesive proteins, coagulation factors, anticoagulant factors, chemokines and growth factors. Many of the storage granule disorders are linked to defects in intracellular trafficking mechanisms that regulate the movement

of newly synthesized proteins from the endoplasmic reticulum and the Golgi complex to intracellular organelles and to the plasma membrane. Delta granule disorders.  3-oxoacyl-(acyl-carrier-protein) reductase Delta granule disorders [also called storage pool deficiencies (SPDs)] cause a mild-to-moderate bleeding diathesis, and are associated with impaired secondary aggregation responses to some agonists. Decreased or absent δ-granules may occur in isolation, or more rarely, as part of a syndrome associated with defects in other organelles such as melanosomes and lysosomes, or α-granules (αδ-SPD) [13]. Several δ-SPD syndromes are associated with varying degrees of oculocutaneous albinism, recurrent infections and defects in vesicular trafficking [14]. Chediak–Higashi syndrome is the result of mutations in the lysosomal trafficking regulatory gene, LYST. Hermansky–Pudlak syndrome is the result of mutations in at least eight different genes (Table 2), each involved with organelle biogenesis or cargo protein trafficking [15].