Fig. 4. A hypothetical molecular model depicting the segregation of focal contacts and fibrillar adhesions. (A) Initial adhesions contain both ₅ß₁ integrin (bound primarily to fibronectin) and _vß₃ integrin (bound primarily to vitronectin). Both integrins are associated through different proteins with actin filaments and are subjected to actomyosin-driven contraction forces. (B) Since substrate-attached vitronectin forms a rigid matrix, _vß₃ integrin remains immobile despite the applied contraction force. In contrast, ₅ß₁ integrin is bound to a relatively soft fibronectin matrix and thus translocates centripetally owing to the actomyosin-driven pulling. The translocation of the fibronectin receptor can also stretch the fibronectin matrix and promote fibrillogenesis. Abbreviations: a, actin; , -actinin, F, FAK; fn, fibronectin; m, myosin II; P, parvin/actopaxin; pa, paxillin; ta, talin; te, tensin; vi, vinculin; vn, vitronectin; 51, ₅ß₁ integrin; v3, _vß₃ integrin.