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Platelets (Thrombocytes)

  1. Structure
    These are colourless, non-nucleated, granulated, spherical, oval or rod-shaped bodies.
    Diameter : 2 to 4 micrometer
    Number : 1.5 to 4 lakhs per cu mm
    Half-life : 4 days; average life span : about 10 days
  2. Formation
    Site : Bone marrow
  3. Development :
    Platelets are formed from giant cells called megakaryocytes. The megakaryocytes themselves are formed from stem cells called megakaryoblasts. The megakaryoblasts → become promegakaryocyte, which become à megakaryocyte → which form platelets.
  4. Release into circulation
    The platelets are formed in the cytoplasm of the megakaryocytes; bits of cytoplasm get detached and are released into the circulation.
    Platelets reside in the spleen for a short period prior to circulation in the blood stream.
    Percentage of the released platelets in the circulating blood : 60% and 75%
    Out of the rest 25 to 40%, most of them are in the spleen; thus, removal of the spleen increases the platelet count.
  5. Ultrastructure
    Platelet membrane
    The platelet membrane is highly invaginated and has a canalicular system; the canaliculi are in contact with the ECF
  6. Receptors on the membrane
    The platelet membrane contain receptors for collagen, ADP, vessel wall von Willebrand factor and fibrinogen.
    A ring of microtubules is present around the periphery of platelets.
  7. Cytoplasm
    The platelet cytoplasm contains actin, myosin, glycogen, lysosomes, and granules.
    The platelet granules are of 2 types :
    1. Dense granules
      These contain the non-protein sub­stances that are secreted in response to platelet activa­tion; the substances include serotonin, ADP, and other adenine nu­cleotides,
    2. Alpha -granules,
      These contain secreted proteins other than the hydrolases in lvsosomes. These proteins include clotting factors and platelet-derived growth factor (PDGF).
  8. PDGF
    It is a dimer made up of A and B subunit polypeptides. Both homodimers (viz. AA and BB) and heterodimer (viz. AB) are produced.
  9. Function
    1. stimulates wound healing
    2. it is a potent mito­gen for vascular smooth muscle.
  10. Von Willebrand factor
    This is a protein present in the vessel wall and in plasma; it is produced by the endothelial cells of blood vessels platelets
  11. Function
    1. helps the platelets to adhere to damaged vessel wall
    2. regulates circulating levels of factor VIII.

Role of platelets in hemostasis


Steps :

  1. Platelet binding
    When a blood vessel is injured, platelets bind to the exposed collagen and to the von Willebrand factor in the wall
    The receptors on the platelet membrane help in this binding.
  2. Platelet activation
    Binding pro­duces platelet activation this causes release of the contents of their granules; ADP is also one of the released substances
  3. Platelet aggregation
    This is mediated by :
    1. ADP
      The released ADP acts on the ADP re­ceptors in the platelet membranes à this produces further accumulation of more platelets (platelet aggregation).
      Platelet ADP receptors :
      These are P2Y1 , P2Y2 , and P2X1.
    2. Platelet activating factor (PAF)


What is PAF?


It is a cy­tokine secreted by neutrophils, monocytes and platelets. It is an ether phospholipid, 1-alkyl-2-acetylglyceryl-3-phosphorylcholine; it is produced from mem­brane lipids. It acts via a G protein-coupled receptor to increase the production of arachidonic acid derivatives, including thromboxane A.


Actions of PAF :

- It helps in platelet aggregation

-It has inflammatory activ­ity.


Factors regulating platelet production :

  1. Colony-stim­ulating factors (CSF): these control the production of megakaryocytes
  2. Thrombopoietin :

This is a circulating protein factor.

Production : It is produced constitutivelv by the liver and kidneys,


  • It helps in maturation of megakaryocytes
  • It helps in feedback regulation of platelet production :

There are thrombopoietin receptors on platelets.

When the number of platelets is low à the free circulating thrombopoietin is increased (because less thrombopoietin is bound to the platelets) à this increases production of platelets.


When the number of platelets is high à the free circulating thrombopoietin is decreased (because more thrombopoietin is bound to the platelets) à this decreases production of platelets.

-  The amino terminal portion of the thrombopoietin molecule has the platelet-stimulating activity

- whereas the carboxyl terminal portion contains many carbohydrate residues and is concerned with the bio-availability of the molecule.


Applied aspects

Thrombocytopenic purpura

This occurs due to low platelet count.


Features :

-  clot retraction is de­ficient

-  there is poor constriction of ruptured ves­sels.

-  easy bruisability and multiple subcutaneous hemorrhages.


Thrombasthenic purpura

In this condition, the platelet count is normal but the platelets are abnor­mal

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