Platelets: Structure, Composition and Functions

  • Platelets also called thrombocytes are the formed elements of blood.
  • They are small, non-nucleated colorless and moderately refractive bodies.
  • They are considered to be the fragments of the cytoplasm being formed elements of blood.
  • They are about 2.5µ (2 to 4µ) in diameter and 7.5 cu µ (7 to 8 cu µ) in volume.
  • They are of various shapes. They may be spherical or rod shaped and become oval or disk shaped when inactivated.
  • Sometimes they have dumbbell shape, comma shape, cigar shape or any other unusual shape.
  • Filo-podia (processes) are absence in inactivated platelets whereas present in activated platelets.

Structure and Composition

  • Platelets are constituted by cell membrane, microtubules and cytoplasm.

A) Cell Membrane

  • Platelet has 6 nm thick cell-membrane.
  • An open canalicular system is formed by extensive invagination of cell membrane.
  • This canalicular system is a delicate tunnel system from where the contents of platelets granules are extruded.
  • Phospholipids, cholesterol and glycolipids are the major lipids found in the cell membrane of platelets whereas glycocalyx are carbohydrates and glycoproteins are major proteins.
  • Glycoproteins and phospholipids are functionally more important as compared to others.


  1. They prevent the adherence of platelets to normal endothelium whereas accelerates the adherence to collagen and damaged endothelium in ruptured blood vessels.
  2. They also form the receptors of ADP (adenosine di-phosphate) and thrombin.


  1. They accelerate the clotting reactions and forms the precursors of thromboxane A2 and other prostaglandin- related substances.

B) Microtubules

  • A ring around cytoplasm below the cell membrane is formed by microtubules which are made up of polymerized proteins called tubulin.
  • The inactivated platelets get structural support to maintain the disk- like shape.

C) Cytoplasm

  • Cellular organelles, golgiapparatus, endoplasmic reticulum, mitochondria, microtubule micro-vessels, filaments and granules are present in the cytoplasm of platelets.
  • Some chemical substances such as proteins enzymes, hormonal substances etc. are also present in it.


  1. Contractile proteins like actin and myosin are responsible for contraction of platelets and third protein called thrombosthenin is responsible for clot retraction.
  2. Adherence of platelets and regulation of plasma level of factor VIII is done by von Willebrand factor.
  3. Fibrin–stabilizing factor acts as a clotting factor.
  4. Platelet derived growth factor (PDGF) repairs damaged blood vessels and promotes wound healing. It also works as potent mytogen (chemical promoting mitosis) for smooth muscles fibres of blood vessels.
  5. Platelet activating factor (PAF) causes aggregation of platelets when the blood vessels get injured so that blood loss in excess is prevented.
  6. Vitronectin (serum spreading factor) promotes adhesion of platelets and spreading of tissue cells in culture.
  7. Thrombospondin helps by inhibiting angiogenesis (formation of new blood vessels from preexisting vessels).


i) Adenosine triphosphate (ATPase)

ii) Enzymes necessary for synthesis of prostaglandins.

Hormonal substances

i) Adrenaline

ii) 5-hydroxytryptamine (5-HT; serotonin)

iii) Histamine

Other chemical substances

i) Glycogen

ii) Substances like blood group antigens

iii) Inorganic substances such as calcium, copper, magnesium and iron.

Platelets: The chameleons of cancer biology    Figure 6 from Investigation of platelet adhesion properties of Von-Willebrand-Factor drug candidates | Semantic Scholar

Image source: Theconversation                       Image source: Semanticscholar


Platelets granules

  • There are two types of platelets granules. They are
  1. Alpha granules contain clotting factors (fibrinogen, V and XIII), platelet-derived growth factor, vascular endothelial growth factor (VEGF), basic fibroblast growth factor (FGF), endostatin, thombospondin.
  2. Dense granules contain nucleotides, serotonin, phospholipid, calcium lysosomes.

Normal count and Variations

1.Normal platelets count is 250,000/cu mm of blood. It ranges between 200,000 and 4,00,000/cu mm of blood.

2.Physiological Variations

  • Platelets are less in number in infants (1,50,000 – 2,00,000/cu mm ) and reaches normal level at 3rd month after birth.
  • No difference on the basis of sex or gender in the count though reduced in female during menstruation.
  • Increasing platelets count in high altitude.
  • Platelets count increases after taking food.

3. Pathological variations

  • Platelets count differs in different infection cases.

Properties of platelets

  • There are three important properties of platelets. They are:

a) Adhesiveness

i) It is the property of platelets by which it can stick to a rough surface.

ii) Endothelium gets damaged during injury of blood vessel due to which sub-endothelial collagen is exposed.

iii) Platelets get activated and adhere to collagen after coming in contact with it.

iv) There is interaction between von Willebrand factor released from damaged endothelium and a receptor protein called glycoprotein lb that lies on the surface of platelets membrane after adhering with collagen.

v) Collagen, thrombin, ADP, Thromboxane A2, calcium ions, P-selectin and vitronectin are factors accelerating adhesiveness.

b) Aggregation (Grouping of platelets)

i) After adhesion, aggregation occurs which is followed by activation of more number of platelets.

ii) This activation occurs by substances released from dense granules of platelets.

iii) Platelets change their shape as it gets activated and elongation of long filamentous pseudopodia takes place called as processes or filopodia.

iv) This structure called filopodia helps in aggregation of platelets together.

v) ADP, thromboxane A2 and platelet-activating factor (PTA) helps in accelerating the activation and aggregation process.

c) Agglutination

i) There is clumping of platelets together by the actions of some platelet agglutinins and platelets activating factor.

Functions of Platelets

  • Platelets are normally inactive and execute their actions only after activated.
  • Many substances are released by the platelets immediately after activation which is known as platelet release reaction.
  • Functions of platelets are actually the functions of the released substances.

I) Role in blood clotting

  • Intrinsic pro-thrombin activator is formed by platelets that help in the onset of blood clotting.

II) Role in clot retraction

  • Blood cells along with platelets are entrapped in the fibrin threads as the blood clots.
  • Contractile proteins namely actin, myosin and thrombosthenin are useful in clot retraction that are present in the cytoplasm of platelets.

III) Role in prevention of blood loss (hemostasis)

  • Hemostasis occurs generally by three ways. They are
  1. Secretion of 5-HT by platelets cause the constriction of blood vessels.
  2. Platelets seal the damage in blood vessels like capillaries by its adhesive property.
  3. Temporary plug is also formed by platelets due to which there is seal of any damage in blood vessels.

IV) Role in repair of ruptured blood vessel

  • PDGF (platelet derived growth factor) helps in the repairing of the endothelium and other structures of the ruptured blood vessels which is formed in the cytoplasm of platelet.

V) Role in defense mechanism

  • Platelets can encircle the foreign bodies and also destroys them by their agglutination property.

Platelets - Function, Thrombocytopenia, Aggregation, Count/Microscopy

Image source: Microscopemaster

Development of Platelets

  • Bone marrow is the place where platelets and all other blood cells are formed from.
  • At first, pluripotent stem cell gives rise to the colony forming unit-megakaryocyte (CFU-M) that gets developed to megakaryocyte.
  • Pseudopodium is formed then by the cytoplasm of megakaryocyte which then detaches in a portion to form platelet after which enters the circulation.
  • Colony-stimulating factors and thrombopoietin influences the production of platelets.
  • Monocytes and T-lymphocytes secrete colony-stimulating factor and whereas liver and kidney secretes thrombopoietin which is a glycoprotein like erythropoietin.

Lifespan and Fate of Platelets

  • The life span of platelets is about 10 days in average as it may vary between 8-11 days.
  • They are destroyed by tissue macrophage system present in spleen.
  • There is decrease in platelets count in condition like splenomegaly (enlargement of spleen) and platelets count increases in case of splenectomy i.e., removal of spleen.




Platelets: Structure, Composition and Functions