Reevaluation of fibronectin-collagen interactions in tissues: an immunocytochemical and immunochemical study

Ultrastructural immunolocalization of fibronectin in epiphyseal and metaphyseal bone of young rats

Fibronectin is a well known glycoprotein of extracellular connective tissue matrices due to a specific amino acid-sequence (RGD) suggested to act as an attachment factor in cell-cell or cell-matrix interactions. Although also present in bone, little is known about the role of fibronectin in this tissue. To obtain data for discussions on function we used ultrastructural immunolocalization techniques to quantitatively examine the distribution of fibronectin in various bone matrix compartments. The study was focused on three different stages of endochondral ossification in growing long bones of young rats. The results show large amounts of fibronectin in mature bone tissue. At a higher magnification, an obvious fibronectin association to individual fibrils of collagen type I was demonstrated. Intracellular labeling was observed in Golgi-related vesicles in some active osteoblasts of metaphyseal bone, indicating local synthesis of fibronectin. In contrast to previous suggestions based on light microscopic observations, the labeling of bone or cartilage matrices facing the surface of all cell types were low. The pattern is clearly different from that of osteopontin and bone sialoprotein, two other bone matrix proteins with the same cell-binding sequence. Our results indicate that fibronectin at these stages of development participates in matrix organization rather than being an important link between cartilage or bone matrix and adjacent cells.

Immunocytochemical localization of collagen types I, III, IV, and fibronectin in the human dermis. Modifications with ageing

The distribution of collagen types I, III, IV, and of fibronectin has been studied in the human dermis by light and electron-microscopic immunocytochemistry, using affinity purified primary antibodies and tetramethylrhodamine isothiocyanate-conjugated secondary antibodies. Type I collagen was present in all collagen fibers of both papillary and reticular dermis, but collagen fibrils, which could be resolved as discrete entities, were labeled with different intensity. Type III collagen codistributed with type I in the collagen fibers, besides being concentrated around blood vessels and skin appendages. Coexistence of type I and type III collagens in the collagen fibrils of the whole dermis was confirmed by ultrastructural double-labelling experiments using colloidal immunogold as a probe. Type IV collagen was detected in all basement membranes. Fibronectin was distributed in patches among collagen fibers and was associated with all basement membranes, while a weaker positive reaction was observed in collagen fibers. Ageing caused the thinning of collagen fibers, chiefly in the reticular dermis. The labeling pattern of both type I and III collagens did not change in skin samples from patients of up to 79 years of age, but immunoreactivity for type III collagen increased in comparison to younger skins. A loss of fibronectin, likely related to the decreased morphogenetic activity of tissues, was observed with age.

Basement membrane and fibronectin matrix are distinct entities in the developing mouse blastocyst

Immunocytochemical techniques were used to study the distribution of fibronectin, type IV collagen (collagen-IV), and laminin in four different stages of mouse blastocyst development. Immunoreactivity for collagen-IV and laminin is present in a granular pattern inside the inner cell mass (ICM) cells in stage 1 blastocysts, while these blastocysts are negative for fibronectin. Fibronectin immunoreactivity appears extracellularly under the trophectoderm (TE) in stage 2 blastocysts, in the form of homogeneously distributed dots, and/or fibrils located preferentially close to cell boundaries. It is followed by the appearance of both collagen-IV and laminin immunoreactivity in patches on the basal side of the TE in stage 3 blastocysts. These patches are initially localized under the central region of TE cells, thus in a location clearly different from that of fibronectin-positive fibrils. As development proceeds the collagen-IV- and laminin-positive patches become larger, covering, by stage 4, an extensive portion of the inner lining of the blastocoel. Fibronectin-positive material is still present in a fibrillar form in stage 3 blastocysts, but is generally reduced to thin strands by stage 4. These results indicate that fibronectin is independent of the mouse blastocyst basement membrane, but may play a transient role in cell adhesion during its deposition. In addition, the results suggest that the ICM plays a major role in the production of collagen-IV and laminin, while the basal surface of TE cells is the primary site of basement membrane assembly.

Expression of fibronectin receptor (integrin) in the uterus of rats in relation to the estrous cycle

The expression and localization of fibronectin receptor (integrin), fibronectin, laminin and collagen type IV in the endometrium of the rat uterus during each period of the estrous cycle were investigated by immunofluorescent microscopy. Fibronectin receptor was observed at the epithelial cells of the endometrium and at vascular endothelial cells. At proestrus, when epithelial cells actively migrate, fibronectin receptor was observed at the basal and lateral epithelial cell surfaces. During estrus, fibronectin receptor had begun to disappear, little fibronectin receptor was observed at metestrus or diestrus. No prominent changes in the localization of fibronectin (seen at the vascular endothelial cells and in the stroma) or of laminin and collagen type IV (seen at the muscles and at the basement membranes of the epithelial and vascular endothelial cells) were observed in relation to the estrous cycle. Thus, uterine epithelial cells, like epithelial cells of the healing cornea, increase their expression of fibronectin receptor during active migration, probably facilitating their attachment to stromal fibronectin. This fibronectin-fibronectin receptor mechanism may underlie epithelial repair, whether the defect results from physiological processes or from an insult.

Association of fibronectin with the microfibrils of connective tissue

The association of fibronectin with the microfibrils of connective tissue was examined in the zonular fibers of the mouse eye by immunohistochemical methods at the light and electron microscopic level. Mouse eyes fixed in formaldehyde were embedded either in paraffin for immunostaining by the peroxidase-antiperoxidase (PAP) method or in Lowicryl for immunolabeling by antirabbit globulin antibodies bound to 5 or 15 nm gold particles. Ultrastructural studies were also carried out after glutaraldehyde perfusion. Both the PAP and immunogold procedures demonstrated the association of fibronectin with microfibrils. After immunolabeling with 5 nm gold particles, examination at high magnification localized fibronectin to fine filaments that appeared to be attached to the surface of microfibrils. The filaments extended outward singly or formed loose aggregates. Their diameter ranged from 1.2 to 3 nm, with a mean of 1.5 nm. Because of their similarity to the fibronectin molecules previously described after rotary shadowing, the filaments were likely to be fibronectin molecules themselves. Since fibronectin is known to have high affinity for the amyloid P component, a model is presented in which fibronectin filaments are bound to the amyloid P component making up the tubular core of microfibrils in mice. Evidence is presented that fibronectin filaments may link microfibrils to one another and thus insure the continuity and strength of zonular fibers. More generally, it is likely that connective-tissue microfibrils, whether or not inserted into elastic fibers, are bonded through fibronectin to surrounding cells, collagen fibrils, or proteoglycans, and thus insure cohesion among connective tissue elements.