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  • Platelets are components of blood, they are small, anucleate cells that play a critical role in haemostasis and thrombosis
  • The more rapidly platelets achieve this, the lower the amount of blood lost



  • Normal count – 100-300/150-400 × 109/L


  • Platelets ordinarily circulate in the bloodstream in a quiescent state
  • But undergo ‘explosive’ activation following damage to the vessel wall, thereby exposing their receptors
  • Leading to rapid formation of a platelet adhesion and aggregation or primary plug and occlusion of the site of damage



  • The blood platelets are fragments of the cytoplasm of the megakaryocyte, one of the largest cells in the body reaching up to 50µm in diameter
  • They are characterized by large nuclear contents and are formed chiefly in the bone marrow
  • They are derived from pluripotential stem cells, the earliest recognized platelet precursor being burst forming unit BFU-meg
  • Under the influence of thrombopoietin, which is produced in the liver (95%) and kidney, and cytokines such as IL-3 & IL-11, the BFU-meg develop into megakaryocyte colony forming unit
  • The committed progenitor cells cease to undergo classical mitosis but develop by endomitotic reduplication, where the nucleus but not the cell divides and the cytoplasm increases
  • The rapid increase in cytoplasm is accommodated by progressive folding or invagination of its membrane
  • It has been proposed that this process accounts for the appearance of demarcation membranes which eventually form the boundary of individual platelets
  • The fully matured megakaryocyte begins to extend pseudopodia, which penetrates through the wall of adjacent marrow sinusoids
  • They are then broken off by the force of the blood flow giving rise to individual platelets of large cytoplasmic fragments
  • The proplatelets are transported through the bloodstream via the heart to the lung where final fragmentation occur
  • The time interval from differentiation of the human stem cell to the production of platelets averages 10 days
  • Each megakaryocyte gives rise to as many as 1000-5000 platelets

Diagrammatic Representation of Stem Cells and Cell Lines That Arise from It


Platelet Formation





  • About 20-30% of the platelets are sequestered in the spleen
  • Normal half-life ranges between 8-14 days
  • Effete[1] platelets are recognized and taken up by cells of reticuloendothelial system in the liver, spleen and bone marrow


  • Platelets are discoid in shape with dimensions of approximately 3.0 by 0.5µm
  • And a mean volume of 7-11fl
  • The platelet mitochondria are the site of adenosine triphosphate production via oxidative pathway the energy being needed to support the cell functions
  • The granule contents contribute to platelet function, such as promoting vessel constriction, blood coagulation
  • And a variety of non-haemostatic defense mechanisms such as leukocyte recruitment (chemotaxis), mitogenesis[2][3] and vessel repair
  • The dense bodies which contain 5-HT60, ADP and Calcium ions do play a pivotal role in the formation and maintenance of the platelet plug
  • Platelets also have several surface protein antigens HPA[4]  Platelets also expresses ABO and HLA[5] class-1

Platelet Ultrastructure



  • The platelet membrane is vital to the cell’s function
  • It contain a number of specific glycoprotein receptors through which platelets interact with aggregating agents, inhibitors, coagulation factors such as fibrinogen, vWF and thrombin and consequently with vessel wall and with each other
  • The glycoproteins belong to several different gene families the best defined being the integrins and the Leucin-rich GP family
  • Platelet membrane also contain phospholipids which are concerned with prostaglandin secretion and Calcium mobilization within the cell and localization of procoagulant activity on the outer surface of platelets

Important Platelet Membrane Glycoproteins

                                                      NAME LIGAND                                        FUNCTION






GPV 7-Transmembrane GPs


Plt-collagen adhesion


Plt-subendothelial microfibril


Plt spreading, fibroblast attachment to vessel wall


Plt-plt aggregation


Plt aggregation

Hydrolyzed by thrombin Thrombin, adrenaline


Plt aggregation and secretion



  • Platelets contain 3 storage granules;
    • Dense granules (7/platelet)
    • Alpha granules which is the most numerous (80/platelet) and
    • Lysosomes
  • These granules rapidly release their contents upon platelet activation via canalicular system

Dense bodies

  • ADP – Aggregation, vasoconstriction
  • ATP – Degrade to ADP
  • 5-HT – Vasoconstriction, Aggregation
  • Calcium  Pyrophosphate ?

Alpha Granules

  • PF4                                                 – Heparinoid neutralization
  • B-thromboglobulin                         – ?Chemotaxis
  • Thrombospondin                         – ?Aggregation
  • PDGF[6]                                     – [7]Mitogenesis, vessel repair
  • vWF                                                 – Adhesion
  • Fibrinogen                                     – Aggregation, coagulation
  • Factor V                                     – Prothrombinase activity
  • Fibronectin                                     – Fibroblast and platelet adhesion
  • PAI-I[8]                                    – Inhibition of fibrinolysis
  • α2-antiplasmin                                     – Inhibition of fibrinolysis


  • The main function of the platelet is the formation of mechanical plugs during normal haemostatic response to vascular injury
  • In the absence of platelets, spontaneous leakage of blood through small vessels may occur
  • Central to their functions are the adhesion, aggregation and release reaction as well as procoagulant activity
  • The immobilization of platelets at the site of vascular injury requires specific plateletvessel wall (adhesion) and platelet-platelet (aggregation) interaction both partly mediated through VWF

Platelet Activation Following Vascular Injury





Platelet Procoagulant Activity

  • After platelet aggregation and release, the exposed membrane phospholipid (Platelet factor 3) is available for 2 reactions in the coagulation cascade
    • The Tenase, and
    • Prothrombinase complexes
  • Both phospholipid-mediated reactions are Calcium ion dependent
  • The phospholipid surface forms an ideal template for the crucial concentration and orientation of these proteins



  • Acquired thrombocytopenias
    • Drug induced/chemicals
    • Isoimmune thrombocytopenias
    • BM infiltration
    • Infections
  • Inherited thrombocytopenias
  • Thrombocytopathies
    • Disorders of platelet adhesion
    • Disorders of platelet aggregation

Acquired Thrombocytopenia

  • Drugs
    • Ethanol
    • Thiazides
    • Chlorpropramide
    • Tolbutamide
    • Estrogens
    • Steroids
    • Quinine
    • Quinidine
    • Hydralazine
  • Bone marrow infiltration
    • Leukemia
    • Gauchers dx[9]
    • Myelofibrosis
  • Infection
    • Due to invasion of megakaryocytes
    • Seen in babies of women with TORCH[10] infections
  • Others

Inherited Thrombocytopenias

  • Wiskott Aldrich syndrome
    • An X-linked recessive disorder characterized by eczema, defective cellular and humoral immunity and thrombocytopenia with small platelets
    • Congenital amegakaryocytic thrombocytopenia
  • May-hegglin anomaly
    • Autosomal dominant, rare condition with basophilic inclusions (döhle bodies) in granulocytes and neutropenia
    • There is ineffective thrombopoiesis with large platelets
  • Grey platelet syndrome
    • Autosomal dominant condition characterized by large platelets and selective deficiency in number and content of platelet alpha granules
    • Pronounced thrombocytopenia is seen with severe bleeding
  • Montreal platelet syndrome
    • Mutation in vWF gene leading to spontaneous platelet aggregation, thrombocytopenia and giant platelets


Disorders of Platelet Adhesion

  • Bernard-Soulier Syndrome
    • Autosomal recessive bleeding disorder characterized by giant platelets, mild to moderate thrombocytopenia and prolongation of bleeding time disproportionate to level of thrombocytopenia
    • It is common with consanguineous kindreds
    • Skin, muscle and visceral bleeding is common, with severe menorrhagia and epistaxis
    • There is deficiency of gp1b[13], with poor binding to sub-endothelial tissues and vWF
    • Platelets do not aggregate with Ristocetin

Disorders of Platelet Aggregation

  • Glanzmann Thrombasthenia
    • An autosomal recessive bleeding diathesis[14] characterized by prolonged bleeding time, normal platelet count, absence of aggregation in response to agonists e.g. ADP, Collagen, Thrombin, Arachidonic acid
    • Aggregation induced by Ristocetin and vWF is seen
    • Common in consanguineous kindreds
    • Mucosal bleeding, easy and spontaneous bruising, subcutaneous haematomas and petechiae[15] are seen
    • Abnormality of GPIIb-IIIa
  • Storage Pool Disease
    • Hemansky pudlak and
    • Chediak higashi syndromes
    • Deficiency of platelet dense granules associated with easy and spontaneous bruising, mucocutaneous haemorrhages, epistaxis73 and haematuria


  • Platelet Count
  • Bleeding time ü IVY’s method
    • Standardized Template

(Normal range – 2-7 minutes)

  • Adhesion test
    • Retention in glass bead column
  • Aggregation test
    • Turbidiometric technique using ADP, Collagen, Ristocetin, Adrenaline, Thrombin and Arachidonic acid
  • Investigation of Granular Contents and Release
    • Dense bodies-Electron microscopy
    • Prostaglandin pathway
    • Thromboxane B2 Radioimmunoassay
    • Prothrombin consumption index


[1] Exhausted

[2] Induction of cellular mitosis

[3] -Hydroxytryptamine (Serotonin)

[4] HPA – Human Protein Antigen

[5] HLA – Human Leukocyte Antigen

[6] PDGF – Platelet Derived Growth Factor

[7] Induction of cellular mitosis

[8] Platelet Activator Inhibitor-I

[9] dx – disease

[10] Torch infections are a set of perinatal infections which can cause anomalies or foetal loss

[11] SLE – Systemic Lupus Erythematosus

[12] ITP – Immune/Idiopathic thrombocytopenic purpura

[13] Glycoprotein 1b

[14] Predisposition

[15] Red or purple, flat, pinhead spots that occur in the skin or mucous membranes 73 Nosebleed