Fibronectin (FN) has been localized in the rat glomerulus using indirect immunolabeling . It was demonstrated in frozen sections by immunofluorescence, in sections of fixed kidneys by both peroxidase and ferritin-labeled antibodies, and in isolated glomerular basement membranes (GBM) with ferritin-labeled antibodies . Complementary and convergent results were obtained with these approaches . FN was most abundant in the mesangial matrix where it was especially concentrated at the interface between the endothelial and mesangial cells. In the peripheral capillary loop, FN was also detected in the laminae rarae (interna and externa) of the GBM -i.e ., between the endothelial and epithelial cells, respectively, and the GBM. These findings indicate that FN is an important constituent of the glomerulus, and they are compatible with the assumption that, in the glomerulus, as in cultured cells, FN is involved in cell-to-cell (mesangial-mesangial, mesangial-endothelial) and cell-to-substrate (mesangial cellmesangial matrix, epithelium-GBM, endothelium-GBM) attachment .Fibronectin (FN) (22) is a glycoprotein associated with fibrillar elements on the cell surface (11) and in the extracellular matrix (3,14, 33) which is involved in cell-to-cell and cell-to-substrate attachment (36). A similar, immunologically indistinguishable protein known as cold-insoluble globulin (CIG) (37), or plasma fibronectin, is found circulating in the blood .FN appears to be a major connective tissue protein, widely distributed in normal tissues (34). A close alignment or codistribution has been noted between extracellular fibronectin and intracellular actin (15, 33), and the possibility of a transmembrane linkage between the two has been proposed (15). Thus, a complex network' involving actin, FN, and probably other components (e.g., proteoglycans [7, 301) has been envisaged which connects adjacent cells, mediates cell anchorage to the extracellular matrix, and transmits deformation generated by the contractile cells. Because the glomerulus is under a high and variable hydrostatic pressure, it seemed possible that such a network may operate to maintain glomerular integrity and to regulate its blood flow (28). By immunofluorescence, FN has already been localized in the mesangium of newborn (24), adult (5,29, 31, 32,34,38), or diseased glomeruli (25, 31, 32) . By electron microscopy, a prominent fibrillar material is observed in the mesangial matrix (MM) (9), the nature of which is unknown. Some investigators described immunoreactivity for fibronectin (5,29, 31, 32) in the peripheral regions of the capillary loops, whereas others reported that it was absent (25,34,38) . In these regions, an extensive fibrillar network is also ' This network has been referred to as a "fibronexus" (33) .THE JOURNAL OF CELL BIOLOGY " VOLUME 87 DECEMBER 1980 691-696 ©The Rockefeller University Press " 0021-9525/80/12/0691/06 $1 .00 observed in the laminae rarae of the glomerular basement membrane (GBM), which appears to anchor the epithelial and endothelial...