Platelet Structure and Function

Platelet Generation (In Bone Marrow)

megakaryoblast Þ pro-megakaryocyte Þ immature megakaryocyte Þ megakaryocyte

megakaryocyte nucleus undergoes extensive endomitosis (typically reaching 16-32N)

cytoplasmic differentiation (fragmentation of megakaryocyte cytoplasm) produces platelets – in circulation 8-10 days

result is anucleated, discoid cell 2-4 m m in diameter

thrombocytopenia = decreased platelet count, thrombocytosis = increased count

Platelet Structure and Function – Four Structural Regions:

(1) Peripheral Zone (unit membrane Þ coagulation) – exterior coat, phospholipid membrane

submembrane region (pseudopod formation), glycoproteins (GPIb, GPIIb-IIIa etc.)

(2) Sol-gel Zone – cytoskeleton, microtubules – maintain shape

(3) Organelle Zone – alpha/dense granules, lysosomes, mitochondria – granule constituents stimulate adjacent platelets

alpha granules: fibrinogen, von Willabrand factor, factor V, fibronectin, thrombosporidin, IgG, b -thromboglobulin,

platelet factor 4, PDGF, TGF-b , albumin

dense granules: ATP, ADP, Ca⁺⁺, serotonin

(4) Membrane System – calcium stores, enzymes for Prostaglandin synthesis

open canalicular system(transport subst. away) and dense tubular system (Ca⁺⁺/Mg⁺⁺ ATPase/prostaglandin synth)
open canalicular system provides an enormous expansion of the surface area/volume ratio

Forming a Hemostatic Plug – Four Steps

function determines laminar flow Þ platelets contact endothelium Þ shape change (pseudopodia) Þ adhere, secrete, aggregate, contract

(1) Adhesion – attachment of cells on a surface at the site of endothelial injury

subendothelial elements (e.g., collagen) are exposed and platelets spread along the surface
interaction of subendothelium, von Willebrand factor (vWF) and GPIb (platelet surface membrane vWF receptor)

(2) Secretion – release of granular contents and prostaglandin metabolites (e.g., thromboxane A₂) from adherent platelets

these interact with receptors on adjacent platelets and lead to their incorporation into the evolving aggregate

activation by ADP, collagen, epinephrine, etc. causes membrane phospholipase A₂ to release arachidonic acid

converted into PGE₂/H₂ Þ thromboxane A₂, a stimulus for amplification of platelet activation

Note: endothelial cells use PGE₂/H₂ to make PGI₂, which counteracts thromboxane A₂. However, since these cells have a nucleus, they can produce more phospholipid, whereas platelets cannot. As a result, universal inhibition of prostaglandin synthesis (by aspirin) preferentially disrupts platelet function.

independently, phospholipase C cleaves (PIP2) to inositol triphosphate (IP3) and diacylglycerol (DAG)

IP3 leads to efflux of calcium from the dense tubular system and influx of calcium from the ext. membr.
DAG causes shape change and secretion of granules via activation of protein kinase C

release many substances from the intracellular granules

dense granules: ADP and serotonin (platelet-derived vasoconstrictor)

alpha granules: platelet factor 4 (neutralizes heparin), b -thromboglobulin (unknown function), platelet derived growth factor (stimulates smooth muscle and fibroblasts), vWF, factor V, fibrinogen, thrombospondin, IgG and fibronectin

(3) Aggregation – process by which platelets interact with one another to form a hemostatic plug

GPIIb-IIIa complex binds vWF, undergoes Ca⁺⁺-dependent structural change, then acts as receptor for fibrinogen
fibrinogen allows bridging of activated platelets – resting unstimulated platelets bind neither vWF nor fibrinogen
other adhesive proteins (fibronectin, thrombospondin) stabilize platelet aggregates as they are formed

(4) Contraction – initial hemostatic plug stops blood but is unstable and must be followed by a fibrin clot (from fibrinogen)

platelets then undergo an additional contractile response using actin-myosin apparatus (like smooth muscle)
actin binds GPIIb-IIIa, which is bound to fibrin; thus, clot retraction (consolidation) occurs

Promotion/Control of Fluid Phase Coagulation

plasma membranes of resting platelets are asymmetrically localized (negatively charged lipids on inner surface)

activation causes phosphatidylserine to relocate to outer surface, leading to activation of factor X and prothrombin

alpha granule secretion leads to high concentrations of procoagulant proteins (fibrinogen, factors V, VIII, XI and XIII)

enhances fibrin formation and stabilization

excessive aggregation is inhibited by prostacyclin made by endothelial cells

prostacyclin causes adenylate cyclase to be activated – sequesters calcium into the dense tubular system

endothelial cells also make PGI₂ and nitric oxide, vasodilators which inhibit platelet aggregation

Tests of Primary Hemostasis

Platelet Count – automated cell counter, part of a complete blood count

normal values are 150-450 x 10⁹/L; 50-75 is usually sufficient, spontaneous bleeding below 20

Bleeding Time (obsolete) – time to stop bleeding from deliberately incised wound (Ivy method – incision on the forearm)

prolonged with thrombocytopenia, qualitative defects of platelets (congenital or acquired) and thrombocythemia

Platelet Aggregation Studies – aggregation effect of collagen, ADP, epinephrine, ristocetin, arachidonic acid, thrombin

aggregated platelets transmit more light