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PLATELETSPLATELETS

OBJECTIVESOBJECTIVESPlatelets ProductionPlatelets Production

Platelet StructurePlatelet Structure

Peripheral ZonePeripheral Zone

Sol-Gel ZoneSol-Gel Zone

Organelle ZoneOrganelle Zone

Platelet FunctionPlatelet Function

Platelet Function TestsPlatelet Function Tests

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Platelets ProductionPlatelets Production

Are produced in the B.M. by fragmentation of the Are produced in the B.M. by fragmentation of the Cytoplasm of Megakaryocytes.Cytoplasm of Megakaryocytes.

Each Megakaryocytes is responsible for the Each Megakaryocytes is responsible for the production of 4000 platelets.production of 4000 platelets.

The time interval from differentiation of the stem The time interval from differentiation of the stem cell to the production of platelets averages about cell to the production of platelets averages about 10 days.10 days.

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Platelets productionPlatelets production

Platelets production is under the control of Platelets production is under the control of humeral agents known as Thrombopoietin.humeral agents known as Thrombopoietin.

The normal platelet count is about 250 x 10The normal platelet count is about 250 x 1099/l /l (150-400 x 10(150-400 x 1099/l)./l).

The mean platelet diameter is 1-2 µm and the The mean platelet diameter is 1-2 µm and the mean cell volume 5.8 fl.mean cell volume 5.8 fl.

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Platelet StructurePlatelet Structure

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Platelet StructurePlatelet Structure

Though platelets are anuclear cytoplasmic Though platelets are anuclear cytoplasmic fragments, they contain a number of organelles, fragments, they contain a number of organelles, such as mitochondria, microtubules, Dense Body, such as mitochondria, microtubules, Dense Body, etc. These organelles can be divided into etc. These organelles can be divided into three three defined zonesdefined zones that possess a unique function. that possess a unique function.

Peripheral ZonePeripheral Zone

Glycocalyx surround the platelet and is considered Glycocalyx surround the platelet and is considered an important component of the platelet membrane.an important component of the platelet membrane.

Platelet membrane, represents Platelet membrane, represents a fluid lipid bilayera fluid lipid bilayer composed of composed of glycoproteinglycoprotein, , glycolipidsglycolipids, and , and lipoproteinslipoproteins..

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Platelet Structure (Platelet Structure (Peripheral Zone)Peripheral Zone)

A number of A number of GPsGPs present in this area are present in this area are responsible for blood group specificity (ABO), responsible for blood group specificity (ABO), tissue compatibility (HLA), and platelet tissue compatibility (HLA), and platelet antigenicity.antigenicity.

Platelet membrane Platelet membrane glyco-proteinsglyco-proteins serve as a serve as a receptorreceptor and facilitate and facilitate transmission of stimuli transmission of stimuli across the platelet membrane.across the platelet membrane.

Platelet membrane Platelet membrane GPIbGPIb appears to be a primary appears to be a primary receptor for receptor for vWFvWF, which serves to , which serves to mediate the mediate the initial adhesion of platelets to subendothelium.initial adhesion of platelets to subendothelium.

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Platelet StructurePlatelet Structure

Platelet membrane Platelet membrane GPIIb/IIIaGPIIb/IIIa functions as a functions as a receptor for substance such as fibrinogen, receptor for substance such as fibrinogen, fibronectin, and vWFfibronectin, and vWF, thereby mediating platelet, thereby mediating platelet aggregationaggregation..

The platelet membrane also includes The platelet membrane also includes receptors receptors for substance such as ADP, thrombin, for substance such as ADP, thrombin, epinephrin, and serotonin, which play a role in epinephrin, and serotonin, which play a role in platelet platelet aggregationaggregation..

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Platelet StructurePlatelet Structure

Sol-Gel ZoneSol-Gel Zone

The term The term CytoskeletonCytoskeleton is often used to describe is often used to describe this zone.this zone.

Microtubules and microfilamentsMicrotubules and microfilaments are found within are found within this zone.this zone.

Microtubules encase the entire platelet, Microtubules encase the entire platelet, maintaining its maintaining its discoid shapediscoid shape..

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Platelet StructurePlatelet Structure

Microfilaments are present in the cytoplasm of Microfilaments are present in the cytoplasm of the platelet and are composed of the platelet and are composed of actinactin and and myosinmyosin-link -link contractile proteincontractile protein..

ActinActin is the major contractile protein in the is the major contractile protein in the platelet cytosol, accounting for 20-30% of the platelet cytosol, accounting for 20-30% of the total platelet protein, where as total platelet protein, where as myosinmyosin composes composes 2-5% of the total platelet protein.2-5% of the total platelet protein.

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Platelet StructurePlatelet Structure

Organelle ZoneOrganelle Zone

It is responsible for the It is responsible for the metabolic activitiesmetabolic activities of the of the platelet.platelet.

Platelets contain three morphologically distinct Platelets contain three morphologically distinct types of storage granules; types of storage granules; Alpha granulesAlpha granules, , dense dense granulesgranules, , lysosomeslysosomes containing acid hydrolases. containing acid hydrolases.

Alpha granules are more numerous (20-200 per Alpha granules are more numerous (20-200 per platelet) and contain a number of different platelet) and contain a number of different proteins.proteins.

Dense bodies are fewer in number (2-10 per Dense bodies are fewer in number (2-10 per platelet) and represent densely opaque granules in platelet) and represent densely opaque granules in transmission electron microscope (TEM) transmission electron microscope (TEM) preparations.preparations.

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Platelet StructurePlatelet Structure

The contents of the The contents of the dense bodydense body granules are; granules are;

Adenosine diphosphate (Adenosine diphosphate (ADPADP))Adenosine triphosphate (Adenosine triphosphate (ATPATP))CalciumCalciumCatecholamines (epinephrine, norepinephrine)Catecholamines (epinephrine, norepinephrine)SerotoninSerotoninPyrophosphatePyrophosphateMagnesium.Magnesium.

The contents of both the alpha and dense The contents of both the alpha and dense granules are released during the energy- granules are released during the energy- dependent release reaction.dependent release reaction.

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Platelet StructurePlatelet Structure

Dense Tubular System (DTS)Dense Tubular System (DTS) Important structure present in the cytoplasm of Important structure present in the cytoplasm of

the organelle zone of the platelets.the organelle zone of the platelets. DTS is derived from the smooth endoplasmic DTS is derived from the smooth endoplasmic

reticulum of immature Megakaryocytes.reticulum of immature Megakaryocytes.

The DTS is the site of The DTS is the site of prostaglandinprostaglandin and and thromboxanethromboxane synthesis and sequestration of synthesis and sequestration of calcium.calcium.

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Platelet StructurePlatelet Structure

It is primarily the release of It is primarily the release of calcium calcium from the DTS from the DTS that triggers platelet that triggers platelet contractioncontraction and subsequent and subsequent internal internal activationactivation of platelets. of platelets.

Platelet activation is an energy- dependent process Platelet activation is an energy- dependent process that relies on the metabolic function of mitochondria. that relies on the metabolic function of mitochondria. There are about 10 to 60 mitochondria per platelet There are about 10 to 60 mitochondria per platelet require glycogen as their source of energy for require glycogen as their source of energy for metabolism.metabolism.

Resting platelet ATP (energy) production is generated Resting platelet ATP (energy) production is generated by glycolysis and the oxidative Kerbs cycle.by glycolysis and the oxidative Kerbs cycle.