Platelet microanatomy

The following zones are recognised: s

Peripheral zone

Exterior coat or glycocalyx Unit plasma membrane System of filaments Open canalicular system

Sol-gel zone

Submembrane fibres Dense tubular system Microtubules Microfilaments

Organelle zone

Mitochondria Lysosomes Peroxisomes Alpha-granules Dense bodies

Membrane zone

Surface-connected open canalicular system Dense tubular system The contractile apparatus of the platelet is responsible for filopodium and lamelli-podium formation, secretion and clot retraction.

The circulating blood level ranges from 150 000-400 000/cu.mm. About 2/3rds of the platelet mass circulates in the plasma and the other 1/3rd is sequestered within the spleen. The circulating platelet is inactive. Under normal circumstances it does not interact with other cells during its normal survival of 9-10 days. The turnover rate is around 10% per day.

Normal platelet function comprises adherence and aggregation, release of procoagulant factors and provision of phospholipid surface for the assembly of enzyme complexes.

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