Figure 2415

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 epithe-

media epithelial cell lial cell, which faces the scala media and is bathed by endolymph. x8,400.

Several other named cell types of unknown function are also present in the spiral organ. Interested readers should consult specialized texts for more-detailed descriptions.

The hair cells are arranged in an inner and an outer row of cells

The inner hair cells form a single row of cells throughout all 23/4 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. 24.18). 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. 24.19a). 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. 24.19b). These processes flatten near the apical ends of the hair cells and collectively form a complete plate surrounding each hair cell (Fig. 24.20).

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. 24.17 and 24.19b). The extracellular fluid in this intercellular space is corti-lymph. 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

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