During the amplification phase, a small amount of thrombin (FIIa) produced during the tissue factor (TF)-mediated initiation phase facilitates further coagulation. On the platelet surface, thrombin activates coagulation factors XI (FXI), VIII (FVIII), and V (FV). Activated FXI (FXIa) converts factor IX (FIX) into its active form (FIXa), which then associates with the cofactor FVIIIa. The resulting FIXa:FVIIIa complex, known as the tenase complex, activates factor X (FX) to FXa. FXa subsequently binds to FVa, forming the FXa:FVa complex, also called prothrombinase. The prothrombinase complex converts prothrombin to thrombin, which in turn cleaves and activates additional FXI, FVIII, and FV, creating positive feedback loops (O'Donnell JS et al., 2019; Preston RJS et al., 2019).

Thrombin also interacts with platelet surface receptors, such as protease-activated receptors (PARs), contributing to platelet activation, degranulation, and the recruitment of additional platelets to the injury site. Activated platelets aggregate, forming a platelet plug (Swieringa F et al., 2018; Sang Y et al., 2021). Procoagulant platelets further release clotting factors and expose phosphatidylserine (PS) on their cell membranes, providing surfaces for coagulation factors and promoting the assembly of the tenase (FIXa:FVIIIa) and prothrombinase (FXa:FVa) complexes (Lentz BR 2003; Swieringa F et al., 2018; Sang Y et al., 2021; Majumder R 2022). This generates large amounts of thrombin through FXa:FVa-catalyzed two-site cleavage of prothrombin (FII). While thrombin generation primarily occurs on the surfaces of activated platelets, other PS-bearing cells, such as leukocytes and endothelial cells, may also contribute (Zhang Y et al., 2016; Tong D et al., 2018).

Thrombin produced during the amplification and propagation phases converts soluble fibrinogen into fibrin monomers, which polymerize to form insoluble fibrin fibers. Thrombin also activates FXIIIa, which cross-links fibrin fibers, stabilizing the thrombus (reviewed by Hoffman M & Monroe DM, 2001; Hoffman M, 2003; Roberts HR et al., 2006; Smith SA, 2009; O'Donnell JS et al., 2019; Preston RJS et al., 2019).