What is a pivotal molecule associated with the external surfaces of aggregated platelets and involved in intrinsic extrinsic mechanisms blood clotting?

The pivotal molecule associated with the external surfaces of aggregated platelets and involved in intrinsic and extrinsic mechanisms of blood clotting is fibrinogen.

Fibrinogen is a large, complex glycoprotein that is synthesized in the liver. It consists of two identical polypeptide chains, each of which has a molecular weight of approximately 340,000. Fibrinogen is normally present in plasma at a concentration of approximately 2-4 mg/mL.

When blood vessels are damaged, platelets adhere to the damaged endothelium and aggregate to form a platelet plug. Fibrinogen binds to the aggregated platelets and is converted to fibrin by the action of thrombin. Fibrin is an insoluble protein that forms a meshwork of strands that traps red blood cells and plasma to form a blood clot.

Fibrinogen is involved in both the intrinsic and extrinsic mechanisms of blood clotting. In the intrinsic pathway, fibrinogen is converted to fibrin by the action of thrombin, which is generated by the activation of factor XII (Hageman factor). In the extrinsic pathway, fibrinogen is converted to fibrin by the action of thrombin, which is generated by the activation of factor VII (tissue factor).

Fibrinogen is an essential component of the blood clotting process. Without fibrinogen, blood clots would not be able to form and bleeding would occur unchecked.