Uptake of putative neurotransmitters in the organ of Corti.

In vitro uptake of putative neurotransmitters into the organ of Corti of the guinea pig was studied by autoradiography. After incubation in 3H-glycine the label was heaviest over the inner hair cell, but was not confined to the synaptic region of the cell. After incubation in 3H-GABA, 3H-glutamate and 3H-aspartate, heavy labeling was seen over the fibers and terminals of the efferent olivocochlear bundle. Leucine, an amino acid not thought to be a neurotransmitter, was uniformly taken up by all cochlear structures. The fact that GABA, glutamate and aspartate are taken up into efferents, which are almost certainly cholinergic, suggests that high affinity uptake of these substances is not restricted to terminals in which these substances are released as neurotransmitters.     

Loss of stereocilia in the human organ of Corti.

The process of disappearance of the stereocilia at the top of the outer hair cell was described by observing aged human cochlea under scanning electron microscope. After loss of hair, remnants of hair could be recognized at the top of outer hair cells. As the process continued, the remnants of the stereocilia increase in percentage, while the remaining abnormal hair decrease. As remnants gradually disappear, the W-configuration faded away. At the same time, the diameter of the hair cell top decreased by shrinking. Before the supporting cells made a complete cover, there may be a depression in the reticular lamina due to shrinking of the top of the hair cell. However, if the depression persists, there is the potential danger of reticular lamina rupture. Existence of a breaking point in the stereocilia of the outer hair cell was proposed.     

Ionic coupling among cells in the organ of Corti.

Gap junctions have been demonstrated morphologically among the supporting cells of the mammalian organ of Corti but, in contradistinction to reptiles, evidence for their existence between the supporting cells and hair cells is equivocal. The literature is ambiguous with respect to electrical coupling and dye coupling among the supporting cells, and no coupling of either kind has been demonstrated for the hair cells. We found strong coupling of both kinds among the supporting cells in the cochleas of live Mongolian gerbils and a less stable coupling between the supporting cells and the outer hair cells. The electrical coupling was established by recording alternating receptor potentials in the hair cells and following their decrement in the population of Hensen's cells; the dye coupling, by injecting Lucifer yellow electrophoretically into the hair cells or the supporting cells and investigating its spread to the neighboring cells. The electrical recordings were made by means of microelectrodes filled with either 1.5 or 3 M KCl or 1 M LiCl with 6% Lucifer yellow, the latter used for dye injection. The electrode resistances ranged from about 20 to 60 M omega in the first instance, and from about 50 to 110 M omega, in the second. The electrodes were inserted into the organ of Corti through scala media according to the method of Dallos, Santos-Sacchi and Flock (1982) modified by us. The alternating potential in Hensen's cells was usually larger than in the outer tunnel of Corti and remained practically constant up to the outer margin of the Hensen's-cell population. Its phase was the same as in the outer hair cells. When the dye was injected into a Hensen's cell, it always spread to neighboring Hensen's cells and often to Deiter's cells. Dye injected into outer hair cells (identified according to anatomical and physiological criteria) also spread to Deiter's and Hensen's cells and, usually, to other outer hair cells. Stained cells were identified in surface preparations and, on two occasions, in serial sections from plastic embedded cochleas.     

Hydrops of the organ of Corti

Objectives: The authors would like to confirm a fluid pathway from the scala tympani to the organ of Corti, and to observe its morphological changes.

Methods: A staining solution for succinic dehydrogenase was perfused with phenazine methosulfate in the scala tympani of living guinea pigs (n?=?5) under deep anesthesia. After fixation, the cochleas were eventually embedded in epon. Sections were observed under a light microscope.

Results: Blue-stained tissue is indicative of the pathway taken by the solution. The staining solution entered the organ of Corti through Hensen-Deiters’ slit. The slit widened and Hensen’s cells were pushed laterally. A new space was formed medial to Hensen’s cells. Cortilymphatic hydrops developed.

Conclusion: The Hensen-Deiters’ slit is a pathway of a certain staining solution from the scala tympani to inside the organ of Corti of the guinea pig. The influx of the fluid pushes Hensen’s cells laterally and upward, resulting in a formation of hydrops of the organ of Corti or cortilymphatic hydrops. The hydrops is observed in animals with experimental perilymphatic fistula and with viral labyrinthitis. At the end stage of the hydrops, only the surface of the organ of Corti remains as a thin layer without any cellular elements.     

Ultrastructural changes of the nerve elements following disruption of the organ of Corti. II. Nerve elements outside the organ of Corti.

Various stages of changes in the nerve fibers, spiral ganglion cells, and satellite cells from the guinea pig cochlea 3 to 137 days after perilymphatic perfusion with streptomycin solution (2 and 20%) were observed electron microscopically. Initially, the axoplasms of the cochlear nerve fibers became swollen or pyknotic. Then, the axons disappeared and myelin lamellae disrupted. The Schwann cells shrank and degenerated, though their basement membranes survived for a time. Regeneration of the cochlear nerve fibers began with extension of axonal sprouts into the tube of the basement membrane and surviving Schwann cells, which still contained myelin debris. Only one of the axonal sprouts matured for myelination. These regenerating cochlear nerve fibers were found in the osseous spiral lamina, modiolus and internal auditory meatus, but these fibers atrophied and disappeared afterward. Retrograde degeneration occurred in the olivo-cochlear bundle. Some of the efferent myelinated fibers also showed temporary regeneration.     

Height changes in the organ of Corti after noise exposure.

To determine whether or not exposure to noise causes an alteration in the height of the organ of Corti (OC), 16 cochleas which had been exposed for one or two hours to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 108 dB were examined microscopically as whole mounts. These specimens were divided into four groups: early ears (N = 3) recovered less than 0.6 hours following the exposure; intermediate ears (N = 5) recovered 0.6-4.0 hours; 1-day ears (N = 3) recovered 24 hours; and late ears (N = 5) recovered 2-21 days. Height was measured at three positions across the OC and at multiple percentage locations from apex to base. The OC-height data from the noise-exposed cochleas were compared statistically to those from ten control cochleas. A significant reduction (P < or = 0.01) in OC height at the third outer hair cell (OHC) was first evident in the early ears in the region 65-95% distance from the apex. The height was reduced even further in the intermediate ears and included a region from 15-25% distance from the apex as well as the 65-95% region. In the late ears, heights had returned to control values, except within focal OC lesions. Height at the first row of OHCs was less affected than at the third row, and height at the inner hair cell (IHC) was least affected. These height changes were accompanied by distortion of the shape and position of OHCs, the shape of Deiters' cells and buckling of inner and outer pillar bodies. Sometimes IHCs had distorted shapes and were displaced from their usual positions. Although no functional measures were obtained from these ears, data from the literature indicate that the exposure described above would have produced a sizable threshold shift. Transient reduction in OC height likely accounts for some portion of noise-induced threshold shifts.     

Membrane specializations in the human organ of Corti

The human organ of Corti was investigated with the freeze fracturing technique with the purpose of analysing membrane specializations. Tight junctions were found on hair cells as well as on supporting cells. Inner and outer hair cells were coupled to the supporting cells by rather extensive tight junctions. The tight junctions between the Deiter's cells were comparable to those of the hair cells, while the tight junctions between the Hensen's cells were considerably less extensive. Gap junctions were present coupling all supporting elements in the organ of Corti, small ones preferably in the apical regions of the cells and large ones in the basal region.     

Serotonergic innervation of the organ of Corti

The olivocochlear efferent system of the mammalian cochlea, which is divided into two lateral and medial bundles, contains numerous neuroactive substances (acetylcholine, GABA, dopamine, enkephalins, dynorphins and CGRP). These have been located at the brainstem in neurons belonging to the lateral superior olive (lateral efferent system) or in neurons of the periolivary region around the medial superior olive and the trapezoid body (medial efferent system). All of these substances were found in well-characterized projections corresponding to lateral and medial nerve fibres and terminals which connect to the type I afferent dendrites and the outer hair cells, respectively. All could be involved in the modulation of the auditory process, as is suggested by the cochlear turnover increases observed in some of them (i.e. enkephalins or dopamine) induced by sound stimulation. Recently, the presence and distribution of serotonin-containing fibres has been included in the long list of cochlear neuroactive substances. However, its highly particular peripheral pattern of distribution together with the lack of response to sound stimulation could suggest that serotonergic fibres constitute a previously unknown cochlear innervation.     

Giant cilia in the human organ of Corti

Fusion of cilia, the growth of clumps of fused cilia and giant cilium formation have been studied in the normal human organ of Corti using the scanning electron microscope. These unusual forms are found mainly in the apical portions of the cochlea and appear to precede the loss of normal apical cilia which increases and extends in a basal direction with age. These changes may be due to low frequency noise damage or be a phenomenom of ageing. The mechanism of their formation is discussed in the light of recent experimental work on cell fusion.     

Ultrastructure of the organ of Corti in experimental hydrops

The early ultrastructural changes of the organ of Corti in the guinea pig after obliteration of the endolymphatic sac and duct were examined in a time-sequence study. Initial loss of outer hair cells was followed by inner hair cell degeneration, both starting at the apical part of the cochlea. Morphological changes of the sensory cells were found to start at the endolymphatic surface and were characterized by a variety of sterociliary pathologies, distortion of the cuticular plate and a less prominent contrast-staining of glycocalyx. Further degeneration was marked by pathological changes of intracellular organelles and the afferent and efferent nerve endings. Details of the ultrastructural changes in the organ of Corti are discussed with regard to the pathophysiology of experimental hydrops.     

Loss of stereocilia in the human organ of Corti

Summary The process of disappearance of the stereocilia at the top of the outer hair cell was described by observing aged human cochlea under scanning electron microscope. After loss of hair, remnants of hair could be recognized at the top of outer hair cells. As the process continued, the remnants of the stereocilia increase in percentage, while the remaining abnormal hair decrease.As remnants gradually disappear, the W-configuration faded away. At the same time, the diameter of the hair cell top decreased by shrinking. Before the supporting cells made a complete cover, there may be a depression in the reticular lamina due to shrinking of the top of the hair cell. However, if the depression persists, there is the potential danger of reticular lamina rupture.Existence of a breaking point in the stereocilia of the outer hair cell was proposed.This study was supported by the Research Grant for Specific Diseases of the Ministry of Health and Welfare, Idiopathic Bilateral Sensorineural Hearing Loss Research Committee of Japan     

Vascular supply to the organ of Corti in man

Summary The capillary distribution in the basilar membrane and the osseous spiral lamina was studied in 40 human temporal bones. The capillaries were made visible using a histochemical technique for demonstrating alkaline phosphatase activity. High tone sensorineural hearing loss can be explained by capillary distribution, length of a segment in the outer spiral vessel and the distance between the inner and the outer spiral vessels in different cochlear turns.A portion of this paper was read at the First International Workshop on Sensorineural Deafness on December 2, 1974 in Nashville, Tenn., USA. This research was supported by the Research Grant for Specific Disease of the Ministry of Health and Welfare, Sudden Deafness Research Committee of Japan     

Interaction of light with the organ of Corti

Summary A cochlear postmortem preparation has been developed which allows for hydromechanical studies of the transilluminated organ of Corti. It was found that cochlear hair cells act as optical waveguides. This property of hair cells is important for optical cochlear investigations. It may be used in the study of the motion of single hair cells.