Adhesion & Aggregation

Initial attachment of platelets onto vascular subendothelium is a critical step for hemostasis. Several factors are known to participate in platelet-subendothelium interactions: subendothelial and plasma adhesive proteins, their receptors on platelet membrane, and rheological factors. Alteration of any of these factors may imply disorders of physiologic hemostasis, leading to thrombosis or bleeding episodes.

Laminin, von Willebrand factor, fibronectin, and different types of collagen are the main components of subendothelial structures. It is known that the binding of von Willebrand factor to subendothelium and to platelet glycoprotein Ib is of critical importance for platelet attachment to subendothelial components. Subsequent platelet spreading and aggregate formation is mediated by platelet glycoprotein IIb-IIIa.

The contribution of platelets to hemostasis does not depend exclusively on adhesive and cohesive functions mediated by membrane receptors. Activated platelets offer a phospholipid surface of critical importance for the activation of coagulation mechanisms.

The following animations shows the process of platelet attachment on a thrombogenic surface, shape change, pseudopodia emission, spreading, adhesion and aggregation (electron micrographs of platelets have been provided by Dr. J. White):

The following animation shows more information on platelet ultrastructure:

Platelet Spreading

Adhesion and Aggregation

Normal endothelium. Platelets do not interact with healthy endothelial cells

Platelets attached on a thrombogenic surface

Cross-section of a thrombus

Electron micrographs showing some attached and aggregating platelets on a thrombogenic surface (left, x1,000) and a cross-section of a thrombus (right, x5,000). Click on pictures to enlarge.

(Pictures have been kindly provided by Drs. J. White and G. Escolar. For image uses, please see Use of Content at Legal Information).

Bibliography

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Heemskerk JW, Sakariassen KS, Zwaginga JJ, Brass LF, Jackson SP, Farndale RW, Biorheology Subcommittee of the SSCotI. Collagen surfaces to measure thrombus formation under flow: possibilities for standardization. J Thromb Haemost. 2011; 9: 856-8. 10.1111/j.1538-7836.2011.04230.x.

Michelson AD. The Clinical Approach to Disorders of Platelet Number and Function.  Platelets: Academic Press, 2013, 813-8.

Ruggeri ZM, Jackson SP. Platelet Thrombus Formation in Flowing Blood.  Platelets: Academic Press, 2013, 399-423.

Nurden AT, Nurden P. Inherited disorders of platelet function: selected updates. J Thromb Haemost. 2015; 13 Suppl 1: S2-9. 10.1111/jth.12898.