- Small (0.5-3Ξm) irregular anucleate cells
- Contain:
- mitochondria, ATP and glycogen
- Dark (dense) granules
- ADP, ATP, calcium, serotonin
- alpha granules
- VWF, fibrinogen, clotting factor V, XIII
- lysosomes
- Microtubules and surface-connected canaliculi
- External glycocalyx layer (thick, 20-30nm)
- Released as fragments of megakaryocytes
- Regulated by thrombopoietin
- Destroyed by Kupffer cells in the liver + spleen
- about â
in spleen at any one time
- Adhesion to the denuded surface collagen via VWF, as well as directly
- Aggregation (platelet to platelet) mediated by fibrin and VWF â platelet plug
- Activation, which means
- Degranulation (release of vasoactive and platelet-activating mediators)
- Thromboxane A2 â ðĶ 5HT, ADP release â vasoconstriction etc
- +ve feedback loop
- Shape change (flattening and extension of cellular projections)
- Phosphatidylserine exposure on the platelet surface, which is essential for clotting factor binding
Factors that oppose platelet activity
- Prostacyclin (see Epoprostenol)
- Nitric oxide
- Fibrin degradation product