Endolymph

Abstract: Vertebrate hair cells, the primary receptors of auditory, vestibular and lateral-line organs, occur in epithelia which separate fluids of differing ionic composition. The apical surfaces of hair cells, on which the mechanosensitive hair bundles are situated, face a high-K+ fluid (termed endolymph in the inner ear); the basolateral surfaces instead contact fluid (perilymph or a related substance) of a composition similar to that of other extracellular fluids1–3. The universal occurrence of high-K+ fluid on the apical surfaces of hair cells in vertebrates has been taken as evidence that it is important for the transduction process, in particular that it relates to the ionic specificity4 of the conductance change5 underlying the receptor potential. There is, however, conflicting experimental evidence regarding this specificity. K+ has generally been thought to carry the receptor current, as replacement of endolymph with perilymph in the guinea pig cochlea abolishes the extracellularly recorded microphonic potential6. Yet microphonic potentials, as well as intracellular receptor potentials, have been recorded in other preparations when the apical surfaces of the hair cells faced instead a high-Na+ saline, and thus when the electrochemical gradient for K+ was near zero5,7. Ca2+ has also been proposed to carry the receptor current8, but its concentration is quite low in endolymph3, particularly that of the mammalian cochlea9. We present evidence here that the receptor current in a vertebrate hair cell is carried in vivo by K+, but that the transduction channel is in fact nonspecific, being permeable to Li+, Na+, K+, Rb+, Cs+, Ca2+, and at least one small organic cation.

Abstract: PURPOSE To assess the value of a three-dimensional Fourier transformation MR technique "CISS" (constructive interference in steady state) in imaging the inner ear. SUBJECTS We studied 50 normal inner ears (40 axial, 10 coronal) and 10 pathologic inner ears in 60 patients. RESULTS The cochlea, semicircular canals, and vestibulum were visualized in detail. Cranial nerve VII and the cochlear, superior vestibular, and inferior vestibular branch of cranial nerve VIII were identified in 90%, 94%, 80%, and 88% of the cases, respectively. A vascular loop was recognized inside the internal auditory canal in 6%, and in the porus in 30%, of the cases. The high signal of the cerebrospinal fluid and labyrinthine fluids (perilymph and endolymph) on the CISS images made excellent delineation of tumors in the cerebellopontine angle and internal canal possible and allowed detection of tumoral labyrinth involvement. The thin sections, high resolution of the images, and capability of producing multiplanar and three-dimensional reconstructions often offered additional information. CONCLUSIONS The CISS sequence allows detailed study of the normal and pathologic inner ear and promises to be highly valuable in the demonstration of the vascular loop.

Abstract: Most clinicians accept cupulolithiasis as the pathophysiological mechanism underlying benign paroxysmal positional vertigo (BPPV.) According to this theory, a cupular deposit induces a gravitational effect on the posterior canal crista. Posterior semicircular canal occlusion is a new operative procedure for treating incapacitating BPPV. It is postulated that canal occlusion abolishes endolymph movement within the canal, effectively fixing the cupula and rendering it unresponsive to both angular and linear acceleration (gravity). During two recent canal occlusions, abundant "free-floating particles" were identified within the posterior canal endolymph. When changing the position of the canal in the earth vertical plane, these free-floating particles would move under the influence of gravity. The hydrodynamic drag of the particles would induce endolymph movement with cupular displacement leading to the typical response. This finding supports an alternate explanation to cupulolithiasis as the pathophysiological mechanism underlying BPPV.

Abstract: The stria vascularis of the mammalian cochlea is composed primarily of three types of cells. Marginal cells line the lumen of the cochlear duct and are of epithelial origin. Basal cells also form a continuous layer and they may be mesodermal or derived from the neural crest. Intermediate cells are melanocyte-like cells, presumably derived from the neural crest, and are scattered between the marginal and basal cell layers. The marginal cells form extensive interdigitations with the basal and intermediate cells in the normal adult stria. The stria also contains a rich supply of blood vessels. We investigated the role of melanocytes in the stria vascularis by studying its development in a mouse mutant, viable dominant spotting, which is known to have a primary neural crest defect leading to an absence of recognisable melanocytes in the skin. Melanocytes were not found in the stria of most of the mutants examined, and from about 6 days of age onwards a reduced amount of interdigitation amongst the cells of the stria was observed. These ultrastructural anomalies were associated with strial dysfunction. In the normal adult mammal, the stria produces an endocochlear potential (EP), a resting dc potential in the endolymph in the cochlear duct, which in mice is normally about +100 mV. In our control mice, EP rose to adult levels between 6 and 16 days after birth. In most of the mutants we studied, EP was close to zero at all ages from 6 to 20 days. Melanocyte-like cells appear to be vital for normal stria vascularis development and function. They may be necessary to facilitate the normal process of interdigitation between marginal and basal cell processes at a particular stage during development, and the lack of adequate interdigitation in the mutants may be the cause of their strial dysfunction. Alternatively, melanocytes may have some direct, essential role in the production of an EP by the stria. Melanocytes may be important both for normal strial development and for the production of the EP. We believe this is the clearest demonstration yet of a role for migratory melanocytes other than their role in pigmentation.