Normal platelets in the blood stream have a discoid shape
and have little or no interaction with other platelets or
Endothelial damage, specifically exposure of underlying
collagen as shown at left, triggers platelet adhesion.
First, von Willebrand's factor(vWF) binds to subendothelial
collagen. This results in conformational changes in vWF
allowing vWF to bind to the GP Ib receptor on platelets.
Exposure of the basement membrane and release of tissue
factor from damaged endothelialcells serves to activate
Following adhesion, proteins and soluble products
interact with platelet membrane receptors to cause
activation. At time of activation, platelets become
spherical and develop cytoplasmic hair-like filipodia. The
GP IIb/IIIa receptor complex Ds conformation allowing
binding of fibrinogen. The contents of the alpha and dense
granules are secreted. All of the above lead to platelet
At left you see vWF bound to collagen; vWF bound to
platelet GP Ib, and fibrinogen bound to activated platelet
GP IIb/IIIa. The fibrinogen acts as a glue binding platelets
The contents of platelet granules are discharged,
exponentially magnifying the accumulation of platelets and
fibrin at the site of injury. This process is known as
platelet aggregation, the end result of which is formation
of a platelet-fibrin plug or thrombus.
Clot retraction is a function in which the central
portion of the platelet attaches to fibrin strands at nodes
(cross-over points)and platelet filipods attach to single
fibrin strands. Actin in the filipods and myosin in the
platelet body contract, shortening the fibrin strands and
shinking the platelet-fibrin plug.This process requires
large amounts of energy (platelet ATP) and calcium.
Add Ca++ by pushing the button below and observe the
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