Platelets are small cytoplasmic fragments of megakaryocytes that are involved in blood clotting.
Platelet Structure
Size: 1-4µm in diameter
Shape: Several shapes (spherical, rod-shaped, dumbbell, comma shape, cigar shape, etc.). They become disk-shaped when inactivated.
Components:
- Cell membrane: Contains lipids (phospholipids, cholesterol & glycolipids)
- Microtubules: Polymerized tubulins that provide structural support for inactivated platelets to maintain disk-like shape
-
Cytoplasm: Contains cellular organelles and some chemical substances:
- Proteins:
- Contractile proteins (actin, myosin & thrombostein)
- Von Willebrand factor
- Fibrin stabilizing factor
- Platelet-derived growth factor (PDGF)
- Platelet-activating factor (PAF)
- Vitronectin (serum spreading factor)
- Thrombospondin
- Enzymes:
- Adenosine triphosphatase (ATPase)
- Enzymes necessary for prostaglandin synthesis
- Hormones:
- Adrenaline
- Serotonin
- Histamine
- Cytoplasmic granules: Two types – dense and alpha
- Alpha granules contain:
- Clotting factors – fibrinogen, V & XIII
- Platelet-derived growth factor
- Vascular endothelial growth factor (VEGF)
- Basic fibroblast growth factor (FGF)
- Endostatin
- Thrombospondin
- Dense granules contain:
- Nucleotides
- Serotonin
- Calcium
- Lysozymes
- Alpha granules contain:
- Proteins:
Normal Platelet Count
150,000 – 300,000 /mm3
Variations in Platelet Count
Physiological Variations:
- Age: Less in infants (reaches normal level at 3rd month after birth)
- Sex: No difference between males and females. Reduced during menstruation.
- High altitude: Platelet count increases
- After meals: Count increases
Pathological Variations:
- Thrombocytopenia: Decrease in platelet count
- Thrombocytosis: Increase in platelet count
Platelet Functions
- Adhesiveness: Stickiness to rough surfaces
- Aggregation: Grouping of platelets
- Agglutination: Clumping together of platelets
- Blood clotting: Responsible for the formation of intrinsic prothrombin activator
- Clot retraction: Initiates clot retraction using contractile proteins in its cytoplasm
- Hemostasis: Accelerates hemostasis by secreting serotonin, sealing off damage, and forming a plug to seal damage in blood vessels.
- Repair of ruptured vessel: Secrete Platelet-derived growth factor (PDGF) useful for the repair of ruptured blood vessels
- Defense: Encircles and destroys foreign bodies by agglutination
Platelet Production and Lifespan
Platelets are formed in the bone marrow:
- Pluripotent stem cells give rise to Colony Forming Unit – Megakaryocyte (CFU-M)
- This develops into Megakaryocyte
- Megakaryocytes’ cytoplasm fragments to form little platelet bodies
Production of platelets is influenced by colony-stimulating factors and thrombopoietin. About 1/3 are trapped in the spleen. The average lifespan of platelets is 7-10 days. Platelets are destroyed by the tissue macrophage system in the spleen.
Platelet Activators
- Collagen, which is exposed during damage of blood vessels
- Von Willebrand factor
- Thromboxane A2
- Platelet-activating factor
- Thrombin
- ADP
- Calcium ions
- P-selectin – cell adhesion molecule secreted from endothelial cells
- Convulxin – purified protein from snake venom
Platelet Inhibitors
- Nitric oxide
- Clotting factors: II, IX, X, XI & XII
- Prostacyclin
- Nucleotides which breakdown the ADP
Platelet Disorders
- Thrombocytopenia: Decrease in platelet count
- Thrombocytosis: Increase in platelet count
- Thrombocythemia: Persistent abnormal increase in platelet count
- Glanzmann’s thrombasthenia: Inherited hemorrhagic disorder caused by structural abnormality of platelets
- Transfusions of stored blood which lack platelets reduce platelet count.
- Splenomegaly decreases platelet counts while splenectomy increases platelet counts because platelets are destroyed in the spleen.
