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:
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. For image uses, please see Use of Content at Legal Information).
White JG. Platelet Structure. Platelets: Academic Press, 2013, 117-44.
Escolar G, Krumwiede M, White JG. Organization of the actin cytoskeleton of resting and activated platelets in suspension. Am J Pathol. 1986; 123: 86-94.
Diaz-Ricart M, Palomo M, Fuste B, Lopez-Vilchez I, Carbo C, Perez-Pujol S, White JG, Escolar G. Inhibition of tyrosine kinase activity prevents the adhesive and cohesive properties of platelets and the expression of procoagulant activity in response to collagen. Thromb Res. 2008; 121: 873-83. 10.1016/j.thromres.2007.08.006.