Learning Objective

By the end of this article, you should be able to describe the structure, function, and clinical relevance of platelets, explain the stages of platelet plug formation, understand the mechanisms of platelet activation and aggregation, and recognize disorders of platelet number or function and their clinical consequences.

Overview

Platelets, also called thrombocytes, are anucleate cellular fragments derived from megakaryocytes. Although they lack a nucleus and nuclear DNA, they contain mitochondria and other organelles, enabling them to participate actively in hemostasis.

Lifespan: 7–10 days
Normal platelet count: 150–400 × 10⁹/L
Sequestration: Up to 30% of platelets reside transiently in the spleen, ready for rapid mobilization.
Development: Stimulated by thrombopoietin from megakaryocytes.

This article explores platelet structure, function, regulation of clot formation, and clinical implications.

Structure of Platelets

Platelets originate from megakaryocytes, the largest progenitor cells of the bone marrow derived from common myeloid progenitors.

Granules:

Alpha-granules: Contain high-molecular-weight proteins including von Willebrand Factor (vWF), Factor V, and fibrinogen.
Dense granules: Contain small molecules like ATP, ADP, serotonin, and calcium ions.

Surface Receptors:

Agonist receptors: Recognize stimulatory molecules such as collagen, thrombin, and ADP.
Adhesion receptors: Facilitate binding to the vessel wall, other platelets, or leukocytes. Example: Glycoprotein IIb/IIIa, targeted by antiplatelet drugs like tirofiban.

Function of Platelets

Platelets are critical for hemostasis through adhesion, activation, and aggregation, forming the initial platelet plug.

1. Adhesion

Vessel injury exposes the endothelium and collagen fibers.
vWF binds exposed collagen and platelet vWF receptors, promoting adhesion.
Collagen exposure also activates the coagulation cascade, producing thrombin (Factor IIa).
Thrombin converts fibrinogen (Factor I) into insoluble fibrin, stabilizing the platelet plug.
vWF also stabilizes Factor VIII, preventing its rapid degradation and supporting the intrinsic coagulation pathway.

2. Activation

Platelet binding to collagen triggers GPIIb/IIIa activation via GPCR signaling.
Activated platelets secrete ADP and thromboxane A2, amplifying platelet activation.
Morphological changes increase platelet surface area for aggregation.

3. Aggregation

Activated platelets express GPIIb/IIIa receptors, binding fibrinogen or vWF.
Fibrinogen crosslinks platelets to form a stable platelet plug.

Fibrinolysis

Platelet plug formation is positive feedback, requiring regulation to prevent excessive clotting.
Plasminogen, produced in the liver, is activated to plasmin by Factors XIa and XIIa.
Plasmin degrades fibrin into fibrin-degradation products (D-dimers).
Clinical relevance: Negative D-dimer tests help exclude DVT/PE, though elevations can occur in surgery, trauma, or pregnancy.

Clinical Relevance

Antiplatelet Therapy

Clopidogrel: ADP receptor antagonist; prevents platelet activation and aggregation. Used in secondary prevention of stroke or MI.
Aspirin: Irreversibly inhibits cyclo-oxygenase → blocks thromboxane production → prevents platelet aggregation. Used in ACS and high-risk pregnancy (pre-eclampsia).

Von Willebrand Disease (vWD)

Autosomal dominant disorder with low vWF levels, causing impaired platelet adhesion and reduced Factor VIII stability.
Clinical presentation: epistaxis, easy bruising, bleeding gums, and menorrhagia.
Management: Desmopressin (releases endogenous vWF and Factor VIII), tranexamic acid (antifibrinolytic).

Thrombocythemia

Definition: Platelet count >450 × 10⁹/L; severe cases >1000 × 10⁹/L.
Primary: Idiopathic.
Secondary: Bleeding, infection, inflammation, trauma, and hyposplenism.
Risks:
- Arterial thrombosis: Stroke, TIA, peripheral emboli
- Venous thrombosis: DVT, PE, Budd-Chiari syndrome
Despite high numbers, platelets may be dysfunctional, increasing bleeding risk.
Management: Aspirin for counts <1000 × 10⁹/L; more potent therapy if >1000 × 10⁹/L.