FIBRONECTIN PROVIDES A CONDUIT FOR FIBROBLAST TRANSMIGRATION FROM COLLAGENOUS STROMA INTO FIBRIN CLOT PROVISIONAL MATRIX

After injury, the wound space is filled with a fibrin/fibronectin clot containing growth factors released by platelets and monocytes, In res ponse to these factors, fibroblasts migrate into the fibrin clot and c ontribute to the formation of granulation tissue, The functional mecha nisms allowing fibroblasts to leave the collagenous matrix of normal c onnective tissue and invade the provisional matrix of the fibrin clot have not been fully defined, To investigate these mechanisms we establ ished a new in vitro model which simulates specific aspects of early w ound healing, that is, the migration of fibroblasts from a three-dimen sional collagen matrix into a fibrin clot, This transmigration could b e induced by physiological concentrations of platelet releasate or pla telet-derived growth factor BB (PDGF-BB) in a concentration-dependent manner, At 24 hours irradiated fibroblasts invaded the fibrin gel almo st as well as non-irradiated cells, indicating that transmigration was independent of proliferation, Plasminogen and its activators appear t o be necessary for invasion of the fibrin clot since protease inhibito rs decreased the amount of migration, These serine proteases, however, were not necessary for exit from the collagen gel as fibroblasts migr ated out of the collagen gel onto a surface coated with fibrin fibrils even in the presence of inhibitors, Removal of fibronectin (FN) from either the collagen gel or the fibrin gel markedly decreased the numbe r of migrating cells, suggesting that FN provides a conduit for transm igration, Cell movement in the in vitro model was inhibited by RGD pep tide, and by monoclonal antibodies against the subunits of the alpha 5 beta 1 and alpha v beta 3 integrin receptor, Thus, the functional req uirements for fibroblast transmigration from collagen-rich to fibrin-r ich matrices, such as occurs in early wound healing, have been partial ly defined using an in vitro paradigm of this important biologic proce ss.