Platelets (thrombocytes) correc
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Platelets (thrombocytes) correc Presentation Transcript

  • 1. PLATELETS (THROMBOCYTES) Lecture by GK Mbassa
  • 2. Purpose of knowing structure, biochemistry and functions of platelets
    • Understand qualitative platelet abnormalities
    • Gain knowledge on hemostasis for treatment of diseases
  • 3.
    • Know platelets role in tumor metastases, atherosclerosis and inflammation resulting from cytoplasmic fragments of megakaryocytes, e.g. arachidonic acid.
  • 4. Morphology of platelets
    • Heterogenous in blood smears; discoid, spheroid, elongated, flat
    • Granular organelles distributed in cytoplasm.
    • Some organelles in centre (granulomere)
  • 5.
    • Platelet cytoplasm is hyalomere, which is clear
    • Platelet is bounded by thin membrane, smooth or having fine projections
  • 6.
    • EDTA anticoagulant minimizes platelets clumping
    • Platelets may clump to other cells (erythrocytes and neutrophils), called satellitism.
  • 7.
    • Platelet volume in dog, pig, man is 7.6 – 8.3 fl, in cattle, equine, sheep, rat, guinea pig, mouse it is 3.2. – 5.4 fl, while in the cat it is 15.1 fl,
    • Platelet counts vary (1- 10 x 1011/l)
  • 8.
    • Larger platelets are metabolically and functionally more active than small platelets.
    • Scanning electron microscope show platelets to have discoid or lentiform shape, with smooth surfaces, slightly biconcave surface, has shallow indentations at external openings of the open canalicular system
  • 9.
    • Surface projections represent protractions of platelets granules
  • 10.
    • Surface features of platelets are similar in most species.
    • Platelets diameter length is 1.3 – 4.7 µm in dog, cat, equines, cow, sheep and goat.
    • Platelet thickness is 0.5 µm
  • 11.
    • Transformed platelets acquire pseudopods or projections, found also in normal blood Surface projections occur very fast when blood is taken out of vessel, vary in number and sizes between species
  • 12. Ultrastructural features of the platelet
    • Unit membrane covered with amorphous material (external or exterior coat)
    • Bundles of microtubules in matrix beneath membrane
  • 13.
    • Internal structure comprises of heterogenous granules (alpha-granules)
    • Clycogen particles
    • Dense granules
    • Mitochondria
    • Lysosomes
    • Peroxisomes
  • 14.
    • Poorly developed Golgi complex
    • Endoplasmic reticulum (rarely)
    • Spongy like channels, called open canalicular system)
    • Open canalicular system communicate with substance of platelet, open to surface at invaginations.
  • 15.
    • Open canalicular system is lined by unit membrane, covered by external coat
    • Another system of platelet channels is the dense tubular system.
    • Dense tubular system occurs under marginal band of microtubules and appears to open to surface, but does not open on the platelet surface.
  • 16.
    • Platelets of many animals have similar morphology.
    • Platelets have two types of granules, (1) alpha-granules, and (2) dense granules.
    • Dense and alpha granules are homogeneously distributed, but vary in electron density, number and size.
  • 17. Functional organization of the platelet
    • The platelet is divided into four structural regions
    • (1) Peripheral zone
    • (2) Sol-gel zone
    • (3) Organelle zone
    • (4) Membrane system
  • 18. Peripheral zone
    • Composed of external (exterior) coat, unit membrane, sub-membraneous area
    • Functions, maintain platelet integrity, receive and transmit stimuli triggering platelet responses (adhesions, aggregations)
  • 19.
    • Exterior coat has glycoproteins (glycocalyx) contains mucopolysaccharides and Mg2+ dependent AT Pase, plasma proteins (fibrinogen, IgG, IgM), coagulation factors (vitamin K-dependent factors, factors V and VIII)
  • 20.
    • Glycoproteins have receptors for platelet activation and aggregation.
    • Seven glycoproteins recognised, including glycoprotein 1b (reaction site for von Willebrand factor, a component of coagulation factor VIII) necessary platelet adhesion to endothelium on injured blood vessel
  • 21.
    • Platelet membrane; maintains platelet integrity, rich in phospholipids.
    • Platelet phospholipids function in blood coagulation (eg
  • 22. Sol-gel zone
    • Represented by matrix of platelet cytoplasm, contains microfilaments and microtubules, which function as cytoskeletal elements.
    • Microfilaments and microtubules maintain discoid platelet shape, form contractile system for shape change, pseudopod formation, internal contractions and granule secretion.
    • Microfilaments also function in clot retraction .
  • 23.
    • Microfilaments are also associated with thrombosthenin, a contractile protein (has actin-myosin)
    • Microtubule tubulin dissolves at 4oC, when exposed to colchicine or vinca alkaloids, leading to platelet shape irregularities.
  • 24. Organelle zone
    • Composed of all internal platelet components, except microtubules, microfilaments (sol-gel zone) components and membrane system.
  • 25.
    • Main component of organelle zone are platelet granules, that are morphologically and biochemically heterogeneous, azurophilic granules (alpha-granules under electron microscope)
  • 26.
    • Alpha-granules are membrane bound, oval, round, electron dense, contain platelet factor 4 (antiheparin), congulation factor V, fibrinogen, beta-thromboglobulin (a thrombin-sensitive protein), fibronection, factor VIII- related antigen, and a mitogenic or growth factor.
    • Platelets in von Willebrand disease lack factor VIII related antigen
  • 27.
    • Electron dense granules, called delta granules, or dense bodies contain non metabolic pool ATP and ADP, Ca2+, mono-amines (serotonin, histamine).
    • Dense granules vary with species.
  • 28. External coat Microfilaments Alpha granule Open canalicular system Dense tubular system Microtubules Golgi complex Lysosome
  • 29.
    • Lysosomal granules contain acid hydrolases; acid phosphatase, β-glucuromidase, Contraction of microtubules forces all internal organelles towards the centre squeezing or without squeezing their contents to the exterior via open canalicular system.
    • Platelet activation triggers secretion of various platelet constituents.
  • 30. Membrane system
    • Memberane system comprises the
    • Open canalicular system
    • Dense tubular system
    • Open canalicular system provides a passage for externalization of platelet secretory products and internalization of substances from plasma into the platelet.
  • 31.
    • Dense tubular system provides a site for sequestration of Ca2+ and localization of enzymes needed for prostaglandin synthesis
    • Release of Ca2+ from the dense tribular system triggers platelet aggregation
  • 32. Platelet constituents Platelet location Constituent Function Exterior coat Fibrinogen Platelet aggregation Membrane Arachidonic acid Prostaglandin synthesis Plaletet factor 3 (phospholipid) Enhances Coagulation cAMP Inhibits release reaction
  • 33. Platelet constituents Microtubules Tubulin Cytoskeleton Contractility Microfilaments Thrombosthenin Shape change, clot reaction, release reaction Alpha-granules Beta-thromboglobulin Impedes prostacylin production from endothelial cells Catalase Enzymic process
  • 34. Factor VIII-relatedantigen Platelet adhesion to subendothelium Fibrinogen Shape change, clot reaction, release reaction Beta-thromboglobulin Impedes prostacylin production from endothelial cells Fibronectin Adherence to extracellular matrix Promote wound healing