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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
PLATELETS

OBJECTIVES

  • Platelets Production
  • Platelet Structure

Peripheral Zone

Sol-Gel Zone

Organelle Zone

  • Platelet Function
  • Platelet Function Tests

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platelets production
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.

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platelets production3
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.

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platelet structure5
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.

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platelet structure peripheral zone
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.

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platelet structure7
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.

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platelet structure8
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.

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platelet structure9
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.

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platelet structure10
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.

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platelet structure11
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.

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platelet structure12
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.

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platelet structure13
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.

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