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PLATELETS

PLATELETS. OBJECTIVES Platelets Production Platelet Structure Peripheral Zone Sol-Gel Zone Organelle Zone Platelet Function Platelet Function Tests. Platelets Production. Are produced in the B.M. by fragmentation of the Cytoplasm of Megakaryocytes.

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PLATELETS

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  1. PLATELETS OBJECTIVES • Platelets Production • Platelet Structure Peripheral Zone Sol-Gel Zone Organelle Zone • Platelet Function • Platelet Function Tests drmsaiem

  2. Platelets Production • Are produced in the B.M. by fragmentation of the Cytoplasm of Megakaryocytes. • Each Megakaryocytes is responsible for the production of 4000 platelets. • The time interval from differentiation of the stem cell to the production of platelets averages about 10 days. drmsaiem

  3. Platelets production • Platelets production is under the control of humeral agents known as Thrombopoietin. • The normal platelet count is about 250 x 109/l (150-400 x 109/l). • The mean platelet diameter is 1-2 µm and the mean cell volume 5.8 fl. drmsaiem

  4. Platelet Structure drmsaiem

  5. Platelet Structure • Though platelets are anuclear cytoplasmic fragments, they contain a number of organelles, such as mitochondria, microtubules, Dense Body, etc. These organelles can be divided into three defined zones that possess a unique function. • Peripheral Zone Glycocalyx surround the platelet and is considered an important component of the platelet membrane. • Platelet membrane, represents a fluid lipid bilayer composed of glycoprotein, glycolipids, and lipoproteins. drmsaiem

  6. Platelet Structure (Peripheral Zone) • A number of GPs present in this area are responsible for blood group specificity (ABO), tissue compatibility (HLA), and platelet antigenicity. • Platelet membrane glyco-proteins serve as a receptor and facilitate transmission of stimuli across the platelet membrane. • Platelet membrane GPIb appears to be a primary receptor for vWF, which serves to mediate the initial adhesion of platelets to subendothelium. drmsaiem

  7. Platelet Structure • Platelet membrane GPIIb/IIIa functions as a receptor for substance such as fibrinogen, fibronectin, and vWF, thereby mediating plateletaggregation. • The platelet membrane also includes receptors for substance such as ADP, thrombin, epinephrin, and serotonin, which play a role in platelet aggregation. drmsaiem

  8. Platelet Structure Sol-Gel Zone • The term Cytoskeleton is often used to describe this zone. • Microtubules and microfilaments are found within this zone. • Microtubules encase the entire platelet, maintaining its discoid shape. drmsaiem

  9. Platelet Structure • Microfilaments are present in the cytoplasm of the platelet and are composed of actin and myosin-link contractile protein. • Actin is the major contractile protein in the platelet cytosol, accounting for 20-30% of the total platelet protein, where as myosin composes 2-5% of the total platelet protein. drmsaiem

  10. Platelet Structure Organelle Zone • It is responsible for the metabolic activities of the platelet. • Platelets contain three morphologically distinct types of storage granules; Alpha granules, dense granules, lysosomes containing acid hydrolases. • Alpha granules are more numerous (20-200 per platelet) and contain a number of different proteins. • Dense bodies are fewer in number (2-10 per platelet) and represent densely opaque granules in transmission electron microscope (TEM) preparations. drmsaiem

  11. Platelet Structure The contents of the dense body granules are; • Adenosine diphosphate (ADP) • Adenosine triphosphate (ATP) • Calcium • Catecholamines (epinephrine, norepinephrine) • Serotonin • Pyrophosphate • Magnesium. • The contents of both the alpha and dense granules are released during the energy- dependent release reaction. drmsaiem

  12. Platelet Structure Dense Tubular System (DTS) • Important structure present in the cytoplasm of the organelle zone of the platelets. • DTS is derived from the smooth endoplasmic reticulum of immature Megakaryocytes. • The DTS is the site of prostaglandin and thromboxane synthesis and sequestration of calcium. drmsaiem

  13. Platelet Structure • It is primarily the release of calcium from the DTS that triggers platelet contraction and subsequent internal activation of platelets. • Platelet activation is an energy- dependent process that relies on the metabolic function of mitochondria. There are about 10 to 60 mitochondria per platelet require glycogen as their source of energy for metabolism. • Resting platelet ATP (energy) production is generated by glycolysis and the oxidative Kerbs cycle. drmsaiem

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