Ultrastructure and immunohistochemical identification of the extracellular matrix of the chinchilla cochlea.

The molecular composition and three-dimensional organization of the extracellular matrix (ECM) was studied by immunofluorescent microscopy, transmission and scanning electron microscopy in three connective tissue structures of the cochlea: the spiral limbus, basilar membrane and spiral ligament. Type II collagen, fibronectin, tenascin, chondroitin sulfate proteoglycans, alphav and beta1 integrins were immunolocalized in the ECM of these connective tissue structures. Electron micrographs showed a continuum of cross-striated collagen fibrils having a similar diameter and axial periodicity that spread from the spiral limbus via the basilar membrane and into the spiral ligament. Some of collagen fibrils were aggregated laterally into bundles. Bundle images, and their digital Fourier transformations, showed a major 67-nm axial D-repeat characteristic for collagen fibrils. Transmission electron microscopy showed numerous proteoglycans associated with the collagen fibrils. The spiral limbus, basilar membrane and spiral ligament demonstrated regional differences in molecular composition and structural organization of their ECM. The glycoproteins fibronectin, tenascin and alphav integrin were immunolocalized mainly in the basilar membrane. Collagen fibrils of the spiral limbus and spiral ligament did not appear to be strongly oriented. However, most of the collagen fibrils in the basilar membrane were arranged into radially directed bundles. Collagen fibrils in the basilar membrane were also surrounded by a homogeneous matrix, which was immunoreactive to fibronectin and tenascin antibodies. A more complete understanding of the composition and structural organization of the ECM in these connective tissue structures in the cochlea provides a foundation upon which micromechanical models of cochlear function can be constructed.