Scanning electron microscopy of the connective tissue along the lateral wall of the mouse cochlear duct with special reference to the external sulcus cells.

The external sulcus cells in the lateral wall of the cochlear duct insert cell cords into the connective tissue. To examine the shape, arrangement, and distribution of the sulcus cell cords in the cochlear duct in adult mice, the external sulcus cells were removed by chemical maceration methods, and the connective tissue exposed. The holes for the insertion of the sulcus cell cords along the entire course of the cochlear duct could thus be observed by scanning electron microscopy. A zone perforated with the holes was seen in the lateral region of the area between the stria vascularis and the lateral edge of the basilar membrane, this zone extending from the basal end of the hook to the helicotrema. In the hook and basal half turn, the holes possessed large fusiform openings and branched in the connective tissue toward the outer bony wall. In the apical turn, the holes displayed small round openings and were shallow, having a small number of branchings. The width of the perforated zone, the population density of the openings of the holes and the area proportion of the openings per unit surface area in the lateral wall decreased from the base to the apex of the cochlear duct. The regional differences in the organization of the sulcus cell cords suggest that the external sulcus cells function in close relation to regional auditory functions in the cochlear duct.