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PLATELETS
OBJECTIVES
Platelets Production
Platelet Structure
Peripheral Zone
Sol-Gel Zone
Organelle Zone
Platelet Function
Platelet Function Tests
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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 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 Structure
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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)
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 Structure
Platelet membrane GPIIb/IIIa functions as a receptor for
substance such as fibrinogen, fibronectin, and vWF,
thereby mediating platelet aggregation.
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 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 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 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 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 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 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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