Model of fibrin polymerization

• Thrombin (factor IIa) binds to fibrinogen and cleaves fibrinopeptide A and B (FPA and FPB) to generate fibrin monomers consisting of one E domain (open circle) and two D domains (closed circles).
• Adjacent monomers self-assemble to form protofibrils through interaction of complementary sites between the D and E domains (D-E contact).
• FPB release exposes additional determinants complimentary with binding sites on the D domain, resulting in lateral growth and thickening of the protofibrils.
• Factor XIIIa induces formation of covalent bonds (crosslinking) between adjacent D-domains (D-D contact, shown as double lines) to stabilize the fibrin polymer.
• Additional crosslinking interactions involve the alpha-C region domain contribute to lateral aggregation (not shown).

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