Relation of Tectorial Membrane to Corti's Organ, and Nature of Cortilymph and Infratectorial Fluids: a Scanning Electron-Microscopic and X-Ray Microanalysis Study.

Abstract

This study was undertaken in order to define more precisely theposition of the tectorial membrane in relation to the reticular lamina and the hair cells, both by direct observation in the frozen-hydratedbulk cochlea of guinea pig and gerbil and by x-ray microanalysis of therelative distribution of elements in the surrounding extracellular spaces.The cochlea was surgically exposed and frozen in situ with freon-22,which had been cooled to -160(DEGREES)C by liquid nitrogen. After freezing, the specimen was kept in liquid nitrogen until it was viewed in the scanning electron microscope. It was removed from the skull by a series of saw cuts and precisely shaped to fit a specially designed specimen holder.Morphological and analytical studies were conducted on etched specimensat -160(DEGREES)C, using a 4 (mu)m('2) scanning raster with an accelerating voltage of(' )9 KeV at 0.1 nanoamperes. In the gerbil and the guinea pig the tectorial membrane in all four turns of the cochlea is in contact with the reticular lamina, from the supporting cells of the inner hair cells to the Deiters-Hensen junction. This pattern of contact of the hydrated membrane is in contrast to that of the dehydrated membrane, which is usually seen elevated above the organ of Corti. The change in membrane position and conformation was documented by a series of fifty micrographs taken of the membrane and organ of Corti during sublimation in the scanning electron microscope and made into a 16-mm time-lapse motion picture. During sublimation contact between the membrane and the reticular lamina was broken. The membrane was seen to shrink appreciably and to be lifted off the organ of Corti. The x-ray microanalysis study in the gerbil and guinea pig shows a statistically significant lower level of K and Cl in the inner sulcus and in the tectorial membrane than in endolymph. In addition, the gerbil shows a statistically significant lower level of Na and Cl in the tunnel of Corti than in perilymph. Because K and Cl levels in the infratectorial fluid and the tectorial membrane were both lower than in endolymph, K/Cl ratios failed to show a difference between the compartments, just as they had failed to do so in Flock's study. Electrophysiological studies were undertaken in an attempt to settle the controversy over the magnitude and interpretation of the dc resting potentials in the various extracellular spaces of the organ of Corti. A small negative potential was recorded in the inner sulcus, similar to that in the tunnel of Corti, indicating that, like the reticular lamina, the membrane also encloses a space that is separated from the endolymph with its high positive potential. Tanaka et al. had reported that the inner sulcus is not sealed off from the endolymph, because after perfusing scala media with a solution of cobalt chloride they found cobalt ions in the infratectorial fluid. In this study it has been shown that their finding must have been an artifact. After perfusion of scala media with cobalt chloride, the tectorial membrane was found to be detached from the reticular lamina, and the concentrations of K, Cl, and Co were the same in the infratectorial space as in the endolymph. The results of these and other experiments described constitute evidence that the tectorial membrane is in direct contact with the reticular lamina so as to seal off the inner sulcus from scala media, and that there are four distinct cochlear fluids, viz., perilymph, endolymph, Cortilymph, and the fluid of the inner sulcus (infratectorial fluid).