Platelets are 2.5 µm in average normal diameter and have a discod shape. The resting platelet is divided into three zones:

• Peripheral zone: responsible for adhesion and aggregation. Consists of fluffy glycocalyx coat, cytoskeleton and platelet membrane. Contains absorbed coagulation factors I, V, VIII, XI, XII, receptors for ADP, thrombin, vWF, collagen, fibrinogen, fibrin, fibronectin, epinephrine, PAF, thrombospondin, thromboxane A2, prostacyclin, epinephrine, serotonin and glycosyl transferase.
• Sol-Gel zone: responsible for contraction and support microtubule system. Contains the connecting system called the open canalicular system and the dense tubular system.
• Organelle zone: contains the dense body system, non-metabolic ADP, serotonin, catecholamines, calcium, alpha granules; platelet factor 4, platelet mitogenic factor, fibrinogen, beta thromboglobulin, lysosomal granules, mitochondria and glycogen granules.

The following animation shows more information on platelet ultrastructure:

• Platelet Morphology

(Requires Adobe Flash Player™. Click here for free download)

Electron micrographs from a resting platelet (left, x10,000), or from an activated platelet showing pseudopodia emission (right, x5,000). Click on pictures to enlarge.
(Pictures have been kindly provided by Dr. J. White).

See bibliografy

Last update: April 26, 2008