The spiral organ of Corti is a complex epithelial layer on the floor of the scala media (Fig. 25.18 and Plate 109, page 948). It is formed by the following:  Inner hair cells (close to the spiral lamina) and outer hair cells (farther from the spiral lamina) Inner phalangeal (supporting) cells and outer pha
Several other named cell types of unknown function are also present in the spiral organ.
The hair cells are arranged in inner and outer rows of cells.
The inner hair cells form a single row of cells throughout all 2.75 turns of the cochlear duct. The number of cells forming the width of the continuous row of outer hair cells is variable. Three ranks of hair cells are found in the basal part of the coil (Fig. 25.19). The width of the row gradually increases to five ranks of cells at the apex of the cochlea.
The phalangeal and pillar cells provide support for the hair cells.
Phalangeal cells are supporting cells for both rows of hair cells. The phalangeal cells associated with the inner hair cells surround the cells completely (Fig. 25.20a). The phalangeal cells associated with the outer hair cells surround only the basal portion of the hair cell completely and send apical processes toward the endolymphatic space (Fig. 25.20b). These processes flatten near the apical ends of the hair cells and collectively form a complete plate surrounding each hair cell (Fig. 25.21).
FIGURE 25.18 • Photomicrograph of the vestibular duct and spiral organ of Corti. This higher-magnification photomicrograph of the cochlear duct shows the structure of the spiral organ of Corti. Relate this structure to the inset, which labels the structural features of the spiral organ. 180. Inset. Diagram of the sensory and supporting cells of the spiral organ of Corti. The sensory cells are divided into an inner row of sensory hair cells and three rows of outer sensory hair cells. The supporting cells are the inner and outer pillar cells, inner and outer (Deiters’) phalangeal cells, outer border cells (Hensen’s cells), inner border cells, Claudius’ cells, and Btcher’s cells. (Modified from Goodhill V. Ear, Diseases, Deafness, and Dizziness. Hagerstown, MD: Harper & Row, 1979.)
The apical ends of the phalangeal cells are tightly bound to one another and to the hair cells by elaborate tight junctions. These junctions form the reticular lamina that seals the endolymphatic compartment from the true intercellular spaces of the organ of Corti (Figs. 25.18 and 25.20b). The extracellular fluid in this intercellular space is cortilymph. Its composition is similar to that of other extracellular fluids and to perilymph.
Pillar cells have broad apical and basal surfaces that form plates and a narrowed cytoplasm. The inner pillar cells rest on the tympanic lip of the spiral lamina; the outer pillar cells rest on the basilar membrane. Between them, they form a triangular tunnel, the inner spiral tunnel (see Fig. 25.18).
The tectorial membrane extends from the spiral limbus over the cells of the spiral organ of Corti.
The tectorial membrane is attached medially to the modiolus. Its lateral free edge projects over and attaches to the organ of Corti by the stereocilia of the hair cells. It is formed from the radially oriented bundles of collagen types II, V, and IX embedded in a dense amorphous ground substance. Glycoproteins unique to the internal ear, called otogelin and tectorin, are associated with the collagen bundles. These proteins are also present in the otolithic membranes overlying the maculae of the utricle and saccule, as well as in the cupulae of the cristae in the semicircular canals.