FUNCTION OF PLATELETS OR THROMBOCYTES

Platelets or Thrombocytes

(Platelets =small plate; thrombo = lump or clot; cytes = cells)

1. Platelets are the smallest blood cells, colourless, spherical, oval or rounded granulated bodies; 

2. 2-5 µm in diameter with an average volume 5.8 μm³; 

3. Leishman staining shows a faint blue cytoplasm with distinct reddish purple granules; nucleus is not

present.

THROMBOPOIESIS or MEGAKARYO-CYTOPOIESIS i.e. DEVELOPMENT OF PLATELETS

Site of origin: bone marrow

Steps: Pluripotent stem cell

                     ↓

Committed stem cell (polyploid precursor cell)

                    ↓

Megakaryoblast (Stage I)

                    ↓

Pro-megakaryocyte (Stage II)

                    ↓

Granular megakaryocyte (Stage III)

                    ↓

              Platelets

Megakaryocytes are giant cells with 35-160 um diameter; multinucleated (contain irregular ring of lobed nuclei) with dense granular cytoplasm. Platelet formation begins with the formation of micro-vesicles which join to form a demarcation membrane for platelets. Platelets are formed within the cytoplasm of granular megakaryocyte by pinching off bits of cytoplasm and are released into the circulation when the cell dies. One megakaryocyte forms 2,000-4,000 platelets by this process.

Platelets production is regulated by 'feedback'

mechanism by Colony stimulating factor and thrombopoietic stimu lating factor (TSF) or thrombopoietin, whose origin is not known. It is probably released in circulation following excessive destruction of platelets or when the thrombocytopenia develops. It promotes the production of megakaryocytes from committed stem cells.

FUNCTIONS

1. HAEMOSTASIS i.e. spontaneous arrest of bleeding by physiological process Haemostasis mechanism includes: 

(i) Platelet adhesion i.e. when a blood vessel is injured, platelets adhere to the exposed collagen, laminin and

von-Willebrand factor in the vessel wall.

(ii) Platelet activation - platelets binding to exposed collagen initiate platelet activation, also produced by ADP and thrombin.

The activated platelets:

(a) change the shape i.e. put out pseudopodia

(b) discharge their granule contents, and

(c) stick to each other, called platelet aggregation.

(iii) Platelets aggregation is also increased by 'platelet activating factor' (PAF), a cytokine secreted by neutrophils, monocytes and platelets cell membrane 

lipids.

( Normally platelets are present in an inactive state in the circulation, unless they become active, there can be no haemostasis).

(iii) Platelets aggregation activates phos pholipase 'C', which in turn activates phospholipase A2 this causes release of 'arachidonic acid' from membrane phospholipids, and which in turn gets converted to thromboxane A2 and prostacyclin.

(a) Thromboxane A2 causes: 

¶ further increase in platelets aggregation, which along with platelets adhesion helps in formation of temporary haemo static plug; this causes stoppage of bleeding from the injured blood vessel and maintains the integrity of the vascular tree.

¶ release of platelets contents i.e. nor-epinephrine and 5-HT, both are vasoconstrictor agents, therefore, play important role for haemostasis in contracting large blood vessels.

(b) Prostacyclin inhibits thrombo xane A, formation and thus prevents further platelets aggrega tion, keeping platelets plug localised (i.e. prevents intravas cular spread of clot).

(Aspirin by inhibiting thromboxane A, formation prevent platelets aggregation., Therefore, aspirin in low doses is of value in preventing myocardial infarction and stroke)

2. BLOOD COAGULATION

The loose aggregation of platelets in the temporary haemostatic plug is bound together and converted into 'definitive' haemostatic plug by fibrin. The clotting mechanism responsible for formation of fibrin involves a complex series of reactions in which platelets play a

major role.

3. CLOT RETRACTION

Within 5-30 minutes of fibrin clot formation, clot retracts i.e. it contracts down to 40% of its original volume. This is produced by contraction of attached platelet's pseudopodia which contain the contractile actomyosin like protein, thrombosthenin. The compact clot is a more effective haemostatic plug.

4. PHAGOCYTIC FUNCTION

platelets help in 'phagocytosis' of carbon particles, viruses and immune complexes.

5. STORAGE AND TRANSPORT FUNCTION

platelets store 5-HT and histamine, which are released when the platelets disintegrate and act on the blood vessel. Platelets can take up 5-HT against a concentration gradient.