Platelet Structure and Function

Abstract

1. George A. Fritsma, MS, MLS[⇑][1] 1. The Fritsma Factor, Your Interactive Hemostasis Resource, Fritsma & Fritsma LLC, Birmingham, AL 1. Address for Correspondence: George A. Fritsma, MS, MLS, The Fritsma Factor, Your Interactive Hemostasis Resource, Fritsma & Fritsma LLC, 153 Redwood Drive, Birmingham, AL 35173, George{at}fritsmafactor.com 1. Diagram platelet structure, including glycocalyx, plasma membrane, filaments, microtubules, and granules. 2. Illustrate platelet adhesion, including the role of von Willebrand factor 3. Illustrate platelet aggregation, including the role of fibrinogen 4. List the secretions of platelet dense bodies and α-granules 5. Demonstrate the relationship of platelets and the plasma coagulation mechanism. Platelets are blood cells that are released from bone marrow megakaryocytes and circulate for approximately 10 days. They possess granular cytoplasm with no nucleus and their diameter when seen in a Wright-stained peripheral blood film averages 2.5 um with a subpopulation of larger cells, 4–5 um. Mean platelet volume (MPV), as measured in a buffered isotonic suspension flowing through the impedance-based detector cell of a clinical profiling instrument, is 8–10 fL. Circulating, resting platelets are biconvex, although in EDTA blood they tend to “round up.” On a blood film, platelets appear circular to irregular, lavender, and granular, although their diminutive size makes them hard to examine for internal structure.1 In the blood, their surface is even, and they flow smoothly through veins, arteries, and capillaries. The normal peripheral blood platelet count is 150–400,000/μL. This count represents only two thirds of available platelets because the spleen sequesters the remainder. In hypersplenism or splenomegaly, increased sequestration may cause a relative thrombocytopenia. Under conditions of hemostatic need, platelets move from the spleen to the peripheral blood and answer cellular and humoral stimuli by becoming irregular and sticky, extending pseudopods, and adhering to neighboring structures or aggregating with one another. Platelet Plasma Membrane The platelet plasma membrane is a standard bilayer composed of proteins and lipids (Figure 1). The predominant lipids are phospholipids, which form the basic structure, and cholesterol, which distributes asymmetrically throughout the phospholipids. The phospholipids form a bilayer with their polar heads oriented toward aqueous environments—toward the plasma externally… ABBRVIATIONS: ADP-adenosine diphosphate; ATP-adenosine triphosphate; CAM-cell adhesion molecule; cAMP-cyclic adenosine monophosphate; DAG-diacylglycerol; DTS-dense tubular system; ECM-extracellular matrix; EGF-endothelial growth factor; GMP-guanidine monophosphate; GP-glycoprotein; HMWK-high-molecular-weight kininogen; Ig-immunoglobulin; IP3-inositol triphosphate; IP-PGI2 receptor; MPV-mean platelet volume; P2Y1 and P2Y12-ADP receptors; PAI-1-plasminogen activator inhibitor-1; PAR-protease-activated receptor; PF4-platelet factor 4; PGG2-prostaglandin G2; PGH2-prostaglandin H2; PDCI-platelet-derived collagenase inhibitor; PDGF-platelet-derived growth factor; PECAM-1-platelet–endothelial cell adhesion molecule-1; PGI2-prostaglandin I2 (prostacyclin); RGD-arginine-glycine-aspartic acid receptor target; SCCS-surface-connected canalicular system; STR-seven-transmembrane repeat receptor; TGF-β-transforming growth factor-β; TPα and TPβ-thromboxane receptors; TXA2-thromboxane A2; VEGF/VPF-vascular endothelial growth factor/vascular permeability factor; VWF-von Willebrand factor 1. Diagram platelet structure, including glycocalyx, plasma membrane, filaments, microtubules, and granules. 2. Illustrate platelet adhesion, including the role of von Willebrand factor 3. Illustrate platelet aggregation, including the role of fibrinogen 4. List the secretions of platelet dense bodies and α-granules 5. Demonstrate the relationship of platelets and the plasma coagulation mechanism. [1]: #corresp-1