FIGURE 25.15 • Photomicrograph of cochlear canal. This photomicrograph shows a section of the basal turn of the cochlear canal. The osseous spiral lamina (OSL) and its membranous continuation, the basilar membrane (BM), as well as the vestibular membrane (VM) divide the cochlear canal into three parallel compartments: The scala vestibuli, the cochlear duct (CD), and the scala tympani. Both the scala vestibuli and the scala tympani are filled with perilymph, whereas the cochlear canal is filled with endolymph. Note the three walls of the cochlear canal, which are formed by the basilar membrane inferiorly, the stria vascularis (SV ) and underlying spiral ligament (SL) laterally, and the vestibular membrane superiorly. The spiral organ of Corti resides on the inferior wall of the cochlear canal. Dendrites of the cochlear nerve (CN) fibers that originate in the spiral ganglion (SG) enter the spiral organ of Corti. The axons of the cochlear nerve form the cochlear part of the vestibulocochlear nerve. 65.
with the lumen of the saccule and contains the spiral organ of Corti, which rests on its lower wall (see Fig. 25.15).
The scala vestibuli and the scala tympani are perilymphcontaining spaces that communicate with each other at the apex of the cochlea through a small channel called the helicotrema (see Fig. 25.14). The scala vestibuli begins at the oval window, and the scala tympani ends at the round window.
The scala media is a triangular space with its acute angle attached to the modiolus.
In transverse section, the scala media appears as a triangular space with its most acute angle attached to a bony extension of the modiolus, the osseous spiral lamina (see Fig. 25.15). The upper wall of the scala media, which mesothelial cellperilymph of scala vestibuli endolymph of scala media epithelial cell
FIGURE 25.16 • Transmission electron micrograph of the vestibular (Reissner’s) membrane. Two cell types can be observed: a mesothelial cell, which faces the scala vestibuli and is bathed by perilymph, and an epithelial cell, which faces the scala media and is bathed by endolymph. 8,400.
separates it from the scala vestibuli, is the vestibular (Reissner’s) membrane (Fig. 25.16). The lateral or outer wall of the scala media is bordered by a unique epithelium, the stria vascularis. It is responsible for production and maintenance of endolymph. The stria vascularis encloses a complex capillary network and contains three types of cells (Fig. 25.17). The marginal cells, primarily involved in K transport, line the endolymphatic space of the scala media.
FIGURE 25.17 • Transmission electron micrograph of the stria vascularis. The apical surfaces of the marginal cells (M) of the stria are bathed by endolymph (E) of the scala media. Intermediate cells (I) are positioned between the marginal cells and the basal cells (B). The basal cells separate the other cells of the stria vascularis from the spiral ligament (SpL). 4,700.
Intermediate pigment-containing cells are scattered among capillaries. The basal cells separate stria vascularis from underlying spiral ligament. The lower wall or floor of the scala media is formed by a relatively flaccid basilar membrane that increases in width and decreases in stiffness as it coils from the base to apex of the cochlea. The spiral organ of Corti rests on the basilar membrane and is overlain by the tectorial membrane.
The spiral organ of Corti is composed of hair cells, phalangeal cells, and pillar cells.