Blood Cells - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 85

Tabl e 3. 8. Platelet released substances (Sources: [ 159 , 160 ]; 5HT: 5-hydroxytryptamine, or
serotonin; HETE: hydroxyeicosatrienoic acid; PAF: platelet-activating factor; VWF: von Wille-
brand factor). Peptidase nexin-2, the secreted form of -amyloid precursor protein, is an abundant

platelet protein that, once secreted, is a potent inhibitor of activated coagulation factor XI.
(Peptida se nexin-1, or serpin-E2, is a potent inhibitor of the serine peptidase thrombin).

β

Location Substance Effect

α β

granule IGF, PDGF, TGF Chemotaxis, ECM deposition

CXCL4, β -thromboglobulin GF activation

Fibronectin, fibrinogen, TC aggregation

thrombospondin
Albumin
Nexin-2 Peptidase inhibition

Tetraspanin-30

ADP, ATP, Ca 2 +

Dense granule TC aggregation

( δ granule) 5HT, catecholamines Vasoconstriction

Cytosol vWF TC adhesion

Fibronectin Coagulation

PAF TC activation

TxA2, 12HETE Vasoconstriction

- Granule membrane contains growth factors, 57 lophysin and P-selectin. α clotting

factors, 58 adhesion molecules, 59 and chemotactic compounds.
Granules fuse by exocytosis with the plasma membrane or membrane of the
tubular system for content release. Vessel wall collagen and various molecules,
such as platelet-activating factor, serotonin, 60 and products of arachidonic acid
metabolism, secreted by platelets and other cells, activate platelets. Activated
platelets become spherical (contraction of actin filaments and microtubules), form
pseudopodia for adhesion, secrete substances, and aggregate.
Activation of glycoprotein-6, a platelet-specific receptor for collagen, causes
platelet-derived vesicle release and shedding in response to platelet activation [ 161 ].
These pro-inflammatory microparticles can transport molecules throughout the
body, possibly in association with transmigrating leukocytes during inflammation,
as they adhere to activated polymorphonuclear leukocytes and monocytes in blood
circulation.

57 Platelet growth factors target fibroblasts and smooth myocytes for wound healing (Chap. 11 ).
58 Fibrinogen and von Willebrand factor (vWF) can be used as

-granule markers.
59 Tetraspanin-29 (a.k.a. CD9, P24, cell growth-inhibiting gene product GIG2, leukocyte antigen
MIC3 or 5H9, and Motility-related protein MRP1) and PECAM1 (or CD31) are glycoproteins
(GP) of the platelet membrane implicated in platelet adhesion and aggregation. Tetraspanin-29 and
PECAM1 also line the membrane of

α

α granule, thus having an intracellular distribution identical

to α 2B β 3 -integrin, a major aggregation platelet receptor [ 158 ].

60 Dense granules contain serotonin (5-hydroxytryptamine) after transport from blood. Serotonin
causes vasoconstriction in blood vessels with damaged endothelium and activates platelets.