HEMA 2 L2

• Megakaryopoiesis involves the maturation and structure of megakaryocytes
• Megakaryoblast originates from the Hematopoietic Stem Cell and is stimulated by Thrombopoietin for differentiation
• Promegakaryocyte is somewhat immature but capable of protein synthesis and has a developed network of membrane within the cytoplasm
• Megakaryocyte is a stage following promegakaryocyte, with multiple nuclei and abundant cytoplasm containing small granules
• Metamegakaryocyte is the 4th stage of maturation, very large with decreased nuclear-cytoplasmic ratio, and can shed 1000 to 4000 platelets
• Platelet production is stimulated by Thrombopoietin, Interleukin-3, and Granulocyte-macrophage colony stimulating Factor (GM-CSF)
• Thrombopoietin is produced by the liver and stimulates megakaryocytes for platelet production
• Identification of maturation stage can be challenging due to the large size and ploidy of megakaryocytes and metamegakaryocytes
• Platelet functions include adhesion to injured vessels, aggregation at the site of injury, promoting coagulation, releasing biochemicals important in hemostasis, and contracting the platelet-fibrin clot
• Platelets play a central role in the response to vessel injury and are the smallest microscopically visible element in the peripheral blood film
• Platelets have a discoid shape, appear light violet with granular specs, and are small fragments of megakaryocyte cytoplasm
• Retraction process involves stabilizing platelets and platelet-fibrin attachment to participate in vascular response to injury and debulking the clot to reestablish blood flow
• Platelets under the microscope appear as dense blue to purple particles with granules that stain with graded intensity during Romanowsky stain preparation
• Peripheral zone consists of the outer membrane. Submembrane area links the membrane and inner cell body
• Sol-gel zone (cytoskeleton) underlies the submembrane filaments and constitutes the matrix or muscle & skeletal portion of the platelet
• Intraplatelet matrix supports the platelet discoid shape
• Serves as a stable gel component to regulate the arrangement of internal organelles and microtubular system in platelets
• Contains actin and myosin, which interact to form Actomyosin (thrombosthenin) important for clot retraction
• Organelle Zone (OZ) constitutes the major portion of the platelet cytoplasm
• Includes electron dense granules, alpha granules, peroxisomes, lysosomes, and mitochondria
• 3 types of granules in OZ:

  • Dense/Delta granules contain ADP, ATP, serotonin, calcium, and magnesium
  • Alpha/L granules contain clotting factors, Von Willebrand factor, Platelet factor 4, B-thromboglobulin, Thrombospondin, and Platelet-derived growth factor (PDGF)
• Platelet membrane system components:

  1. Canalicular System serves as delivery routes for substances ingested by platelets and route of extrusion of substances released from stimulated platelets

  2. Dense Tubular System important for influencing the microtubule supporting the discoid platelet shape and site for prostaglandin synthesis
• Platelet life span and turnover rate:

  • Life span: 5 to 10 days
  • In normal recipients: 2 to 9 days
  • Spends 5 days in circulation, 5 days in the bone marrow
• 2 groups of platelets:

  • Young platelets are hemostatically more effective
  • Old platelets are hemostatically less effective
  • Platelets collide randomly with one another in circulation
  • 30% of the total circulating platelet population is normally sequestered in the spleen
• Platelet events:

  1. Adhesion

  2. Release Reaction and Activation

  3. Aggregation
• Platelet adhesion:

  • Platelets do not adhere to intact endothelial cells under homeostatic conditions
  • Adhere to detached endothelial cells and subendothelial network of collagen fibrils, fibronectin, and basement membrane during injury
  • Requires von Willebrand factor (vWF) as a link between platelet glycoprotein receptor Ib (1b) and subendothelial connective tissue
• Platelet release reaction and platelet activation:

  • Severe vessel injury exposes deeper vessel wall structures
  • Platelets undergo release reaction facilitating platelet aggregation and activating blood coagulation
  • Various substances released affect clotting, inflammation, and repair at the site of injury
  • Platelet-derived growth factor (PDGF) influences smooth muscle cell proliferation and vessel wall hyperplasia
• Platelet aggregation:

  • Stimulated by ADP, thrombin, TX A2, and collagen
  • Mediated by the microtubular system and contraction of the microfilament system within platelets
  • ADP, thrombin, and TX A2 are independent mediators of platelet aggregation
• Stimuli for platelet aggregation:

  • ADP released from dense granules in response to collagen stimulation
  • Thrombin induces platelet aggregation by stimulating release of ADP and activating phospholipids
  • Thromboxane A2 promotes platelet aggregation directly and synergistically with other stimuli
• Formation of Thromboxane A2:

  • Activated platelets liberate arachidonic acid
  • Cyclooxygenase stimulates formation of Prostaglandins G2 and H2
  • Thromboxane synthase stimulates formation of Thromboxane A2
  • Thromboxane A2 promotes platelet aggregation and acts synergistically with other stimuli
• Modulation of the hemostatic response at the site of vessel wall injury:

  • Endothelial cells and platelets play key roles in the response
  • Relative amounts of Thromboxane A2 and PGI2 released from platelets and endothelial cells modulate the hemostatic response