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