Models of Fibrin Generation Models of Fibrin Generation. (A) Contact activation-initiated coagulation (The Activated Partial Thromboplastin Time Assay). Arrows indicate reactions involving proteolytic conversion of the zymogens of plasma coagulation factors (Black Roman numerals) to the active forms (Roman numerals followed by a lower case "a"). Non-enzymatic cofactors are indicated by white Roman numerals in black ovals. The process culminates in generation of the protease α-thrombin, which cleaves fibrinogen to form a fibrin clot. The contact activation reactions depicted in Figure 1 are at the top of the sequence in the gray oval. (B) Tissue factor-initiated coagulation. Coagulation is initiated by formation of a complex between factor VIIa in plasma and tissue factor (TF) at a wound site, triggering activation of factor X, and leading to initial thrombin and fibrin formation (black arrows). Factor VIIa/TF also activates factor IX, to sustain factor X activation (white arrows). In some situations, additional activation of factor IX by factor XIa (gray arrow) is required. Source: Factor XI and the Plasma Contact System, The Online Metabolic and Molecular Bases of Inherited Disease Citation: Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson K, Mitchell G. The Online Metabolic and Molecular Bases of Inherited Disease; 2014 Available at: https://ommbid.mhmedical.com/DownloadImage.aspx?image=/data/books/971/ch175fg2.png&sec=62650567&BookID=971&ChapterSecID=62650561&imagename=&gboscontainerID=0 Accessed: December 26, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved