(thrombocytes) are non-nucleated
fragments of megakaryocytes which become "sticky"
under certain circumstances and
adhere to damaged blood vessels. Platelet plug formation
requires platelets and specific plasma proteins. The key
protein is von Willebrand factor (vWf) secreted by megakaryocytes, platelets and endothelial cells. vWf
accumulates at the site of vessel damage.
| The platelet plug begins when
vessel damage exposes subendothelial tissue. vWf binds to collagen fibers and
this changes vWf to make it bind and anchor platelets. The platelets undergo a change in
metabolism and surface properties that causes them to become more
"sticky" and to secrete certain products.
| Two of the secreted
products, serotonin and epinephrine, cause vasoconstriction.
ADP, another secreted product, causes platelet
aggregation by a positive feedback loop. ADP also stimulates thromboxane
A2 production which further increases platelet
| Thromboxane A2 forms
from arachidonic acid derived from the plasma membrane
of the platelet and stimulates ADP secretion,
platelet aggregation and vasoconstriction. Healthy endothelial
cells prevent platelet aggregation by converting arachidonic
acid to prostacyclin and releasing it and nitric
oxide (NO). Both prostacyclin and NO inhibit platelet plug
| Platelets also have a high
concentration of actin and myosin which enables them to
contract and increase the tightness of the platelet