In vitro coagulation is initiated by factor:

The conversion of fibrinogen to fibrin is catalyzed by thrombin. Thrombin is a serine protease that is formed from its circulating precursor, prothrombin, by the action of activated factor X. Factor X can be activated by reactions in either of two systems, an intrinsic and extrinsic system. The initial reaction in the intrinsic system is conversion of inactive factor XII to active factor XII (XIIa). This activation, which is catalyzed by high molecular weight kininogen and kallikrein can be brought about in vitro by exposing the blood to electro negatively charged wettable surfaces such as glass and collagen fibers. Activation in vivo occurs when blood is exposed to the collagen fibers underlying the endothelium in the blood vessels. Active factor XII than activates factor Xi, and active factor XI activates factor IX. Activated factor IX forms a complex with active factor VIII, which is activated when it is separated from Von Willibrand’s factor. The complex of IXa and VIIIa active factor X. phospholipids from aggregated platelets (PL) and Ca are necessary for full activation of factor X.