BRIEF REVIEW OF THE FIBRONEXUS AND ITS SIGNIFICANCE FOR MYOFIBROBLASTIC DIFFERENTIATION AND TUMOR-DIAGNOSIS

This brief review details the structure, nature, and distribution of t he fibronexus, and discusses its significance for myofibroblastic diff erentiation and tumor diagnosis. The fibronexus is a cell surface spec ialization consisting of intracellular actin filaments and extracellul ar fibronectin filaments associated with subplasmalemmal plaque materi al. The fibronexus represents an intercellular junction between myofib roblasts, but in particular is a device for providing contact between myofibroblasts and matrix that mediates continuity between intracellul ar contractile filaments and extracellular matrix proteins. Immunoelec tron microscopy in particular has shown that the intracellular filamen ts contain actin. The extracellular filaments contain fibronectin and collectively form the fibronectin fibril. The plaque probably contains such proteins as vinculin, talin, alpha-actinin, and integrin. Under appropriate biologic development and fixation conditions, the fibronec tin fibril of the fibronexus is characterized by and distinguished fro m lamina by enhanced density, a rigid appearance, failure to adhere cl osely to the contours of the cell surface (except focally near the pla que material), and a longitudinally filamentous substructure. Confirma tion of the presence of a fibronectin fibril may be obtained by the fi nding of intense cell surface staining with an antifibronectin antibod y. Problems in identifying the fibronexus may be encountered, however, due to poor development and fixation, in which case the filamentous s ubstructure may be inapparent. The fibronexus is such a typical featur e of and is often so conspicuous in myofibroblasts that it can be rega rded as, perhaps essential for the interpretation of myofibroblastic d ifferentiation. Structures with a similar appearance have been documen ted in fundamentally nonmyofibroblastic cells; these include aortic an d scleral spur smooth muscle cells and endothelium. Uncertainties rema in in the protein composition of the fibronexus, the nature of its con tact with the matrix, and its relationship to similar structures seen in nonmyofibroblastic cells. Immunoelectron microscopy provides a pote ntial means of clarifying some of these questions.