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PubMed1986

Ionic environment of cochlear hair cells.

The scala media of the adult cochlea in mammals comprises a morphologically closed compartment sealed with tight junctions of the intermediate to tight types. The unique ionic composition of endolymph is maintained by the stria vascularis through active reabsorption of sodium and active secretion of potassium against ionic gradients. The subtectorial space is only a partially closed compartment which communicates with the endolymph via holes in the tectorial membrane at its outer insertion to the organ of Corti. Hardesty's membrane divides the subtectorial space into two compartments: one facing the surfaces of inner hair cells and one facing the surfaces of outer hair cells. In the study of comparative anatomy, hair cells, e.g. in the lizard, basilar papilla are of two types: those covered with a tectorial membrane and those being free-standing lacking the tectorial membrane. The ionic environment of the hair cell surface seems to be the same, independent of whether covered with a tectorial membrane or not. The tectorial membrane itself is semipermeable to ions in the endolymphatic space. Only the surface structures of the hair cell with the sensory hairs facing the subtectorial space are exposed to the high concentration of potassium, whereas the remaining parts of the hair cell are surrounded by a fluid having a more normal extracellular type of ionic composition (cortilymph/perilymph). During embryonic development the ionic composition of endolymph develops in parallel with the morphologic maturation of the stria vascularis. A completely mature composition of endolymph is reached before any electrophysiological potentials in the cochlea can be elicited. The sensory hair surface of hair cells has reached a mature morphology prior to the maturation of endolymph. In several species the tectorial membrane is morphologically only partially mature when the increase of the potassium concentration of endolymph starts. Drugs primarily affecting the stria vascularis causing a transient change of the ionic composition of endolymph result in a transient dysfunction of inner ear potentials. If the ionic changes persist for longer time, morphological changes can occur in both the stria vascularis and the hair cells of the organ of Corti. Whether such changes are primarily caused by the ototoxic drug itself or by changes in the ionic composition of endolymph has to be explored further.

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PubMed2001

Regulation of endolymphatic fluid volume.

Direct measurements of the dispersal of markers in endolymph have failed to support previously established hypotheses of endolymph homeostasis, specifically longitudinal flow, radial flow, and dynamic flow theories. Rather, they suggest that in the normal state endolymph is maintained without a significant involvement of volume flow at all. Ions appear to be transported into and out of the endolymphatic space in a similar manner to that for a single cell, with each ion transport process contributing to the electrolyte pool. In abnormal volume states, however, longitudinal volume flow of endolymph may contribute to homeostasis. Procedures that enlarge the endolymphatic space result in endolymph flow toward the base of the cochlea, contributing to the removal of electrolytes and volume. Similarly, procedures that decrease cochlear endolymph volume induce apically directed flow in the cochlea, contributing to the addition of electrolytes and volume to the endolymphatic space. The endolymphatic sac responds to endolymph volume disturbance, showing op posite responses to volume increases and decreases. Although evidence is still limited, the endolymphatic sac appears to act as a "bidirectional overflow" system. While volume disturbances originating from out-of-balance transport processes anywhere in the labyrinth may be corrected by the sac, dysfunction of the sac itself is likely to have a substantial effect on endolymph status.

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