Platelet Characteristics
- Anuclear cellular fragments.
- Produced by megakaryocytes within bone marrow, lungs, other tissues.
- Average size: 1-4 μm.
- Mean platelet volume (MPV): 8.96 ± 1.5 fL.
- Distribution: One-third sequestered in spleen; two-thirds in systemic circulation.
- Lifespan: 7-10 days.
- Primary function: Bolster primary hemostasis; prevent hemorrhage after vascular injury.
Primary Hemostasis Phase
Adhesion
- Vascular injury exposes subendothelial matrix components (collagen, von Willebrand factor).
- Plasma von Willebrand factor (vWF) binds exposed collagen.
- vWF interacts with platelet glycoprotein Ib (GPIb) complex.
- Interaction tethers platelets to injury site.
- Platelet collagen receptors GPVI and α2β1 bind collagen firmly.
Activation
- Adhesion triggers complex cellular signaling cascades.
- Generates thromboxane A2 from arachidonic acid via cyclooxygenase enzyme.
- Conformational change occurs in major integrin αIIbβ3 (GPIIb-IIIa) receptor via “inside-out” signaling.
- Enhances avidity for vWF, vessel wall ligands, and fibrinogen.
Secretion
- Platelet activation prompts rapid release of storage granule contents.
- Dense granules liberate adenosine diphosphate (ADP), adenosine triphosphate (ATP), calcium ions, serotonin.
- Alpha granules secrete fibrinogen, vWF, factor V, high-molecular-weight kininogen, fibronectin, α-1-antitrypsin, α-thromboglobulin, platelet factor 4 (PF4), platelet-derived growth factor.
Aggregation
- Active αIIbβ3 receptor binds circulating fibrinogen.
- Fibrinogen serves as bridging ligand connecting adjacent platelets.
- Recruits additional platelets; forms reversible, temporary hemostatic plug.
Role In Secondary Hemostasis
Coagulation Surface Provision
- Platelet activation externalizes internal membrane phospholipids (primarily phosphatidylserine).
- Provides catalytic phospholipid reaction surface for coagulation factor assembly.
- Facilitates assembly of extrinsic and intrinsic Xase complexes.
- Scaffolds prothrombinase complex (Factor Xa/Va/Calcium) assembly.
Thrombin Generation
- Surface factor assembly creates localized “thrombin burst”.
- Thrombin converts soluble fibrinogen into insoluble fibrin network.
- Stabilizes initial platelet plug into definitive, highly cross-linked hemostatic barrier.
Additional Hemostatic Functions
Vasoconstriction
- Released serotonin and histamine increase local blood vessel constriction.
Clot Retraction
- Platelet contractile proteins and cytoskeleton mediate clot retraction.
- Constricts plug; limits overall clot size.
Heparin Neutralization
- Neutralizes local heparin activity.
Platelet Receptors And Granules
| Component | Physiologic Function | Associated Clinical Pathology |
|---|
| Glycoprotein Ib (GPIb) | Binds von Willebrand factor for adhesion | Bernard-Soulier syndrome |
| Glycoprotein IIb/IIIa (αIIbβ3) | Binds fibrinogen for aggregation | Glanzmann thrombasthenia |
| Glycoprotein VI (GPVI) | Binds subendothelial collagen | GPVI receptor defect |
| Alpha Granules | Store vWF, fibrinogen, Factor V, PF4 | Gray platelet syndrome |
| Dense Granules | Store ADP, ATP, Calcium, Serotonin | Hermansky-Pudlak syndrome, Chediak-Higashi syndrome |
Differential Diagnosis Of Platelet Disorders
Quantitative Disorders (Thrombocytopenia)
- Increased Destruction (Immune): Immune thrombocytopenic purpura (ITP), drug-induced, systemic lupus erythematosus, neonatal alloimmune thrombocytopenia.
- Increased Consumption (Microangiopathy): Hemolytic uremic syndrome, thrombotic thrombocytopenic purpura, disseminated intravascular coagulation, Kasabach-Merritt syndrome.
- Decreased Production: Congenital amegakaryocytic thrombocytopenia, aplastic anemia, marrow infiltration (leukemia, neuroblastoma), Wiskott-Aldrich syndrome.
- Sequestration: Hypersplenism (portal hypertension, Gaucher disease).
Qualitative Disorders (Platelet Dysfunction)
- Adhesion Defects: Bernard-Soulier syndrome (abnormal GPIb), von Willebrand disease (vWF deficiency).
- Aggregation Defects: Glanzmann thrombasthenia (abnormal GPIIb/IIIa), congenital afibrinogenemia.
- Granule Secretion Defects: Gray platelet syndrome (alpha granule absence), Hermansky-Pudlak syndrome (dense granule absence).
- Acquired Dysfunction: Uremia, severe liver disease, cardiopulmonary bypass, medication effect (aspirin, NSAIDs).