PLT Function

Overview of Platelet Function

Definition: Platelets (thrombocytes) are small, anucleate cell fragments that play a critical role in hemostasis (blood clotting), thrombosis (blood clot formation), and inflammation
Primary Function: To initiate and participate in the formation of a platelet plug at sites of vascular injury, thereby preventing excessive bleeding
Secondary Functions: Contributing to the coagulation cascade and fibrin clot formation
Normal Platelet Count: 150,000 - 450,000 /μL (150-450 x 10^9/L)
Thrombocytopenia: Platelet count < 150,000 /μL
Thrombocytosis: Platelet count > 450,000 /μL

Platelet Structure

Plasma Membrane:
- Phospholipid bilayer with embedded glycoproteins and receptors
- Glycocalyx: Outer coating containing glycoproteins that mediate adhesion to subendothelial matrix and other platelets
Cytoplasm:
- Granules:
  - Alpha Granules: Contain clotting factors (e.g., fibrinogen, von Willebrand factor [vWF]), growth factors (e.g., platelet-derived growth factor [PDGF]), and adhesion molecules (e.g., P-selectin)
  - Dense Granules (δ-granules): Contain ADP, ATP, serotonin, and calcium
  - Lysosomes: Contain hydrolytic enzymes
- Open Canalicular System (OCS): System of channels that connect the platelet interior to the external environment, facilitating the release of granular contents and increasing surface area
- Dense Tubular System (DTS): Storage site for calcium ions and enzymes involved in prostaglandin and thromboxane synthesis
- Microtubules and Actin Filaments: Provide structural support and facilitate shape change, movement, and clot retraction

Key Steps in Platelet Function

Adhesion:
- Mechanism:
  - Platelets adhere to the exposed subendothelial matrix at sites of vascular injury
  - Von Willebrand factor (vWF) acts as a bridge between the platelet glycoprotein Ib/IX/V receptor (GPIb/IX/V) and collagen in the subendothelium
  - Other adhesion molecules: Collagen receptors (e.g., GPVI), integrins (e.g., α2β1)
- Disorders of Adhesion:
  - Bernard-Soulier Syndrome: Deficiency of GPIb/IX/V complex, leading to impaired vWF binding
  - von Willebrand Disease: Deficiency or dysfunction of vWF, leading to impaired platelet adhesion
Activation:
- Mechanism:
  - Adhesion triggers platelet activation, leading to a cascade of intracellular signaling events
  - Release of ADP and thromboxane A2 (TXA2) amplifies the activation process
  - Activation leads to:
    - Shape change: Platelets transform from discoid shape to spherical shape with pseudopodia (filopodia)
    - Granule release (degranulation): Secretion of granular contents into the surrounding environment
    - Increased expression of GPIIb/IIIa receptors on the platelet surface
- Key Receptors and Signaling Molecules:
  - ADP receptors (P2Y1 and P2Y12): Bind ADP, promoting platelet activation and aggregation
  - Thromboxane A2 (TXA2): A potent platelet activator and vasoconstrictor, synthesized from arachidonic acid by cyclooxygenase (COX) and thromboxane synthase
  - Thrombin: A potent activator of platelets via protease-activated receptors (PARs)
  - Collagen: Activates platelets via the GPVI receptor
- Disorders of Activation:
  - Storage Pool Deficiencies: Deficiencies in platelet granules (e.g., dense granule deficiency, alpha granule deficiency)
  - Gray Platelet Syndrome: Deficiency of alpha granules, leading to impaired platelet function
Aggregation:
- Mechanism:
  - Activated platelets bind to each other, forming a platelet plug (primary hemostatic plug)
  - The GPIIb/IIIa receptor (integrin αIIbβ3) on activated platelets binds to fibrinogen, which acts as a bridge between adjacent platelets
  - This cross-linking of fibrinogen molecules promotes platelet aggregation
- Disorders of Aggregation:
  - Glanzmann Thrombasthenia: Deficiency of the GPIIb/IIIa receptor, leading to impaired platelet aggregation
Secretion (Release Reaction):
- Mechanism:
  - Activated platelets release the contents of their granules (alpha granules and dense granules)
  - ADP and TXA2 amplify platelet activation and promote further aggregation
  - Serotonin contributes to vasoconstriction
  - Clotting factors (e.g., fibrinogen, vWF) contribute to secondary hemostasis
- Disorders of Secretion:
  - Storage pool deficiencies (e.g., Wiskott-Aldrich syndrome)
Stabilization of the Platelet Plug:
- Mechanism:
  - The coagulation cascade is activated, leading to the formation of fibrin
  - Fibrinogen binds to the activated GPIIb/IIIa receptors, forming a fibrin meshwork that stabilizes the platelet plug
  - Clot retraction: Platelets contract, pulling the edges of the damaged vessel together and strengthening the clot
- Role of Thrombin: Thrombin is a key enzyme in the coagulation cascade and also a potent platelet activator, creating a positive feedback loop

Regulation of Platelet Function

Prostacyclin (PGI2):
- Produced by endothelial cells
- Inhibits platelet activation and aggregation
- Causes vasodilation
Nitric Oxide (NO):
- Produced by endothelial cells
- Inhibits platelet adhesion, activation, and aggregation
- Causes vasodilation
ADPase (CD39):
- An enzyme on endothelial cells that degrades ADP, reducing platelet activation
Thrombomodulin-Thrombin Complex:
- Thrombomodulin binds to thrombin, changing its substrate specificity
- The thrombomodulin-thrombin complex activates protein C, which inactivates factors Va and VIIIa, inhibiting coagulation
Inhibitory Receptors on Platelets:
- Platelets express receptors that can inhibit platelet activation (e.g., FcγRIIB, GPVI)

Platelet Function Tests

Platelet Count:
- Measures the number of platelets in the blood
- Used to detect thrombocytopenia and thrombocytosis
Bleeding Time:
- Historical test (now rarely used)
- Measures the time it takes for bleeding to stop after a standardized skin incision
- Prolonged bleeding time suggests a platelet disorder or von Willebrand disease
Platelet Function Analyzer (PFA-100):
- Measures platelet function under high shear stress
- Blood is passed through a small aperture coated with collagen and either epinephrine or ADP
- Measures the time it takes for a platelet plug to form and occlude the aperture (closure time)
- Prolonged closure time suggests a platelet disorder or von Willebrand disease
Platelet Aggregation Studies:
- Measures the ability of platelets to aggregate in response to various agonists (e.g., ADP, collagen, epinephrine, thrombin, arachidonic acid, ristocetin)
- Agonists are added to platelet-rich plasma, and the change in light transmission is measured as platelets aggregate
- Abnormal aggregation patterns can help identify specific platelet disorders
Flow Cytometry:
- Measures platelet surface markers (e.g., GPIIb/IIIa, GPIb/IX/V)
- Can assess platelet activation and binding of agonists
Thromboelastography (TEG) and Thromboelastometry (ROTEM):
- Viscoelastic tests that assess the overall hemostatic function of blood
- Measure the strength and elasticity of the developing clot
- Provide information about platelet function, coagulation factors, and fibrinolysis

Disorders of Platelet Function

Quantitative Disorders:
- Thrombocytopenia: Decreased platelet count
- Thrombocytosis: Increased platelet count
Qualitative Disorders:
- Inherited Platelet Function Disorders:
  - Bernard-Soulier Syndrome: Deficiency of GPIb/IX/V complex, impairing vWF binding
  - Glanzmann Thrombasthenia: Deficiency of GPIIb/IIIa receptor, impairing platelet aggregation
  - Storage Pool Deficiencies: Deficiencies in platelet granules (e.g., dense granule deficiency, alpha granule deficiency)
- Acquired Platelet Function Disorders:
  - Uremia: Accumulation of uremic toxins in kidney failure impairs platelet function
  - Medications: Aspirin, clopidogrel, and other antiplatelet drugs inhibit platelet function
  - Myeloproliferative Neoplasms (MPNs): Abnormal platelet function is common in ET and PMF
  - Von Willebrand Disease (vWD): Deficiency or dysfunction of vWF, impairing platelet adhesion

Key Terms

Platelet (Thrombocyte): Small, anucleate cell fragment involved in hemostasis
Adhesion: Platelet attachment to the subendothelial matrix
Activation: Platelet stimulation leading to shape change, degranulation, and increased GPIIb/IIIa expression
Aggregation: Platelet-to-platelet binding
Secretion: Release of granular contents from platelets
vWF (von Willebrand Factor): A protein that mediates platelet adhesion
GPIIb/IIIa: A platelet surface receptor that binds fibrinogen and mediates platelet aggregation
Thromboxane A2 (TXA2): A potent platelet activator and vasoconstrictor
Prostacyclin (PGI2): An inhibitor of platelet activation and aggregation
PFA-100: Platelet function analyzer
Thromboelastography (TEG): A viscoelastic test of hemostasis

PLT Production

PLT Destruction