FINE MORPHOLOGY OF THE SENSORY CELLS IN THE ORGAN OF CORTI OF MAN

Evolutionary and Developmental Biology Provide Insights Into the Regeneration of Organ of Corti Hair Cells

We review the evolution and development of organ of Corti hair cells with a focus on their molecular differences from vestibular hair cells. Such information is needed to therapeutically guide organ of Corti hair cell development in flat epithelia and generate the correct arrangement of different hair cell types, orientation of stereocilia, and the delayed loss of the kinocilium that are all essential for hearing, while avoiding driving hair cells toward a vestibular fate. Highlighting the differences from vestibular organs and defining what is known about the regulation of these differences will help focus future research directions toward successful restoration of an organ of Corti following long-term hair cell loss.

Scanning Electron Microscopy of the Human Organ of Corti

The human cochlea has been preserved from post-mortem autolysis by perfusion with a fixative shortly after death. Subsequent staining with osmium permits dissection of this structure from the temporal bone. (Temporal bones were obtained from eight patients). When prepared for examination in the scanning electron microscope, the auditory sensory cells are found to be located in the band-like organ of Corti which extends the length of the cochlea. The sensory cells have a cluster of stereocilia projecting from their free upper surface and because of this are called hair cells. The hair cells are divided into two separate groups: a single row of inner hair cells, which show little variation in their surface appearance along the length of the cochlea, and three or four rows of outer hair cells whose cilia change in conformation and increase in length along the cochlea.

Ultrastructure of the Odontocete organ of Corti: scanning and transmission electron microscopy

The morphological study of the Odontocete organ of Corti, together with possible alterations associated with damage from sound exposure, represents a key conservation approach to assess the effects of acoustic pollution on marine ecosystems. By collaborating with stranding networks from several European countries, 150 ears from 13 species of Odontocetes were collected and analyzed by scanning (SEM) and transmission (TEM) electron microscopy. Based on our analyses, we first describe and compare Odontocete cochlear structures and then propose a diagnostic method to identify inner ear alterations in stranded individuals. The two species analyzed by TEM (Phocoena phocoena and Stenella coeruleoalba) showed morphological characteristics in the lower basal turn of high-frequency hearing species. Among other striking features, outer hair cell bodies were extremely small and were strongly attached to Deiters cells. Such morphological characteristics, shared with horseshoe bats, suggest that there has been convergent evolution of sound reception mechanisms among echolocating species. Despite possible autolytic artifacts due to technical and experimental constraints, the SEM analysis allowed us to detect the presence of scarring processes resulting from the disappearance of outer hair cells from the epithelium. In addition, in contrast to the rapid decomposition process of the sensory epithelium after death (especially of the inner hair cells), the tectorial membrane appeared to be more resistant to postmortem autolysis effects. Analysis of the stereocilia imprint pattern at the undersurface of the tectorial membrane may provide a way to detect possible ultrastructural alterations of the hair cell stereocilia by mirroring them on the tectorial membrane.

Ultrastructural changes in the hydropic cochlea of the guinea-pig

Transmission electron microscopy of the cochlear organ of Corti in experimental endolymphatic hydrops revealed two principal features. Starting 1 month after induction of hydrops, osmiophilic inclusions thought to represent lipofuscin accumulation were frequently observed in the subcuticular cytoplasm of the outer hair cells along the length of the cochlea. Starting 3 months after induction of hydrops the efferent terminals on the outer hair cells appeared to be vacuolated. These data suggest that oxidative insult is likely to contribute to the pathology associated with endolymphatic hydrops and thus that free radical scavengers might be useful in the treatment of Menière's disease patients. In addition the early modification of the efferent innervation of the hydropic cochlea might underlie the known hypersensitivity to various insults, including noise stimulation, glycerol administration and hypoxia.

Freeze-fracture organization of hair cell synapses in the sensory epithelium of guinea pig organ of Corti

The fine structure of both the afferent and efferent hair cell synapses in the sensory epithelium of guinea pig organ of Corti was examined by freeze-fracture electron microscopy. In the afferent synapse, barlike aggregates of intramembrane particles (IMPs) of about 10 nm in diameter were seen on the P-face of the afferent presynaptic membrane directly beneath the presynaptic dense projection which is located in the active zone of the presynaptic membrane. Small and large depressions have been seen on the presynaptic membrane. The former were observed in the proximity of the barlike aggregates, while the latter were observed some distance from the aggregate. In outer hair cells, IMPs of about 10 nm in diameter were seen on the P-face of the afferent postsynaptic membrane at a density of 3,000/microns 2. In the efferent synapse, many aggregates composed of from several to tens of large IMPs of 13 nm in diameter were observed on the presynaptic membrane. These aggregates were localized to small membrane depressions, which tended to be deeper as particle number per aggregate increased. Dense populations of IMPs of about 9 nm in diameter were observed on the P-face of the efferent postsynaptic membrane at a density of 4,000/microns 2. A fenestrated subsynaptic cistern completely covers the efferent postsynaptic membrane. Moreover, the subsynaptic cistern spans several efferent postsynaptic membranes when efferent synapses are gathered in a group. In the afferent and efferent synapses of hair cells, specializations of the synaptic membranes were represented by marked aggregates characteristic of IMPs. In the efferent synapse, IMP movement inside the synaptic membrane was proposed in relationship to retrival of synaptic vesicle membrane. Structural relationship between the subsynaptic cistern and efferent postsynaptic membrane was revealed.

Comparative anatomy of the cochlea and auditory nerve in mammals

The numbers and structure of hair cells; afferent, efferent, and reciprocal synapses as seen at the base of hair cells; innervation patterns of first order cochlear neurons; and number and morphology of spiral ganglion cells will be discussed and compared in the guinea pig, rat, cat, monkey and man. Despite many similarities both in the organ of Corti and the spiral ganglion in these species, there are a number of differences which may have important physiologic implications. In the organ of Corti, the major differences among species are the length and width of the basilar membrane, the number of inner and outer hair cells, and the length of hairs on both inner and outer hair cells. Significant differences in the innervation pattern of the inner hair cell among these species include the number of afferent nerve terminals per inner hair cell, the degree of branching of afferent fibers, and the number of synapses per afferent nerve terminal. Among outer hair cells, the number of afferent nerve terminals per outer hair cell, presence or absence of a pre-synaptic body, presence or absence of reciprocal synapses, the number of efferent terminals per outer hair cell, and the presence of dendodendritic synapses in outer spiral bundles may be differences important physiologically. In the spiral ganglion, there are significant differences in the number of spiral ganglion cells, the number of cochlear nerve fibers, the percentage of spiral ganglion cells which are myelinated, and the presence of synapses on spiral ganglion cells.

Cochlear morphology from Würzburg (1951) to Turin (1987): old and new aspects

A review is given of the methods employed in human temporal bone pathology, from the camera lucida drawings of the last century to the sophisticated techniques of today.

Selected pathological findings in the human cochlea

Out of a material of 45 patients with known audiograms where the inner ears had been fixed with an aldehyde within 7 h after death, 4 cases were chosen for detailed morphological examination. The general findings in the ageing human cochlea are presented as well as the findings in the 4 specific cases.

A scanning electron microscopy study of post mortem autolytic changes in the human and rat cochleas

The use of SEM as an adjunct to TEM and electrophysiological examination of the cochlea is now well established. It has provided a relatively simple method of assessing the effect of noise, ototoxic drugs, electrical stimulation, etc. on the surface features of the organ of corti. A controlled experiment was undertaken to document the surface autolytic changes in rat cochleas by fixation at intervals up to eight hours post mortem. These were compared with human material fixed between 40 minutes and eight hours post mortem. The results complement previous light and transmission electron microscopy studies, gives insight into the optimal and acceptable fixation times for the two species and act as a guide for the interpretation of human material.

A study of postmortem autolysis in the human organ of Corti

Forty-six human temporal bones from 24 individuals were removed at autopsy and prepared for electron microscopy. The adequacy of histologic preservation was evaluated by light and electron microscopy. Characteristic autolytic changes included vacuolization of afferent neurons and neural poles of inner and outer hair cells, lysis of limiting membranes of hair and supporting cells, swelling of endoplasmic reticulum, and dissolution of mitochondrial cristae. The rate of autolysis varied significantly within cellular components of the inner ear. The neural poles of hair cells demonstrated more rapid autolysis than apical poles and nerve terminals showed more autolysis than myelinated nerve fibers. Postmortem time and the cause of death affected the adequacy of histologic preservation. Fixation in patients dying of pneumonia, hypoxia, head injury, or malignancy tended to be poor, whereas the fixation achieved in patients dying of cardiac disease with postmortem time of under 140 minutes was generally good.

Preservation of the Human Cochlea

Human cochleas processed with a varying interval between death and initial fixation were examined by transmission and scanning electron microscopy. The specimens were fixed by paraformaldehyde-glutaraldehyde fixatives in buffer. Cochleas fixed less than two hours postmorten showed excellent morphology in general. With increasing time lag between death and fixation the results became more variable. In some specimens fixed up to six hours postmorten, however, the morphology still was well preserved, but as the interval increased, a greater variability was introduced. The conclusion is that specimens fixed up to six hours postmortem can show excellent electron microscopic morphology. Limited information can be obtained up to 12 hours postmortem, after which the results tend to be unreliable.

A freeze-fracture study of afferent and efferent synapses of hair cells in the sensory epithelium of the organ of Corti in the guinea pig

Afferent and efferent synapses of hair cells in the organ of Corti of the guinea pig have been examined in freeze-fracture replicas. Afferent synapse. In the inner hair cells, intramembranous particles 10 nm in diameter are aggregated on the ridge on the P-face of the presynaptic membrane directly beneath the synaptic rod. In the outer hair cells, in which the synaptic rod is located in the presynaptic cytoplasm underneath the presynaptic membrane, small aggregations of intramembranous particles 10 nm in diameter can be found on the P-face of the presynaptic membrane corresponding to the site of the presynaptic dense projection. Intramembranous particles 10 nm in diameter are also densely aggregated on the P-face of the postsynaptic membrane of the outer hair cells. Efferent synapse of the outer hair cells. Large intramembranous particles 13 nm in diameter are distributed in clusters composed of four to ten particles on the P-face of the presynaptic membrane. In the P-face of the postsynaptic membrane, disc-like aggregations of intramembranous particles 9 nm in diameter are found. The subsynaptic cistern covers the cytoplasmic surface of the postsynaptic membrane of the efferent synapse; it may cover more than one postsynaptic membrane when several efferent synapses are in close proximity to one another.

Serial section reconstruction of the neural poles of hair cells in the human organ of Corti. I. Inner hair cells

Study of the anatomy of the cochlea, and in particular the morphology of synaptic relationships between hair cells and cochlear neurons, is essential for elucidation of the mechanisms of transduction of mechanical acoustic signals into electrical neural events. Because considerable gaps remain in our understanding of the microscopic anatomy of these synapses, particularly in the human, a reconstruction of neural pole of inner hair cells of the human organ of Corti was performed. The data are based on 526 serial sections from the basal turn (10 mm region) and 356 serial sections from the middle turn (26 mm region). This provided complete data on 3 and partial data on 5 inner hair cells. Afferent terminals on inner hair cells were variable in size, ranging 1 to 20 micrometers in diameter. Branching of large fibers to produce multiple terminals innervating from 1 to 3 inner hair cells was common. Each inner hair cell received approximately 6 to 8 different nerve terminals. In addition, each terminal possessed a variable number of synaptic contacts. Junctional membrane specialization consisted of synapses, desmosomes, coated vesicles and arrays of microtubules and membrane cisternae. Specialization at synapses consisted of asymmetrical membrane thickening. At inner hair cells the postsynaptic membrane was thicker than the presynaptic membrane. Eighty-three percent of synapses had presynaptic bodies. Vesiculated efferent terminals synapsed on afferent fibers at the base of inner hair cells, but never directly on the inner hair cell. These anatomical data demonstrate distinct differences between the human and animal inner ear, which are important in the interpretation of neurophysiological data in animals and the formulation of hypotheses that involve assumptions crossing species.

Fine structure of the sensory epithelium of guinea-pig organ of Corti: subsurface cisternae and lamellar bodies in the outer hair cells

The fine structure of subsurface cisternae and lamellar bodies in the outer hair cells of the guinea-pig organ of Corti was studied with thin sections and freeze-fracture replicas. Subsurface cisternae in the outer hair cells consist of multilayers along the lateral plasma membrane of the cell. The outermost layer is a flattened cistern in the upper part of the supranuclear region, but comprises a series of tubules in the lower part. Deeper layers are fenestrated cisternae in which disc-like areas are found in the upper part of the supranuclear region. Lamellar bodies consist of concentric layers of fenestrated cisternae and are located in the apical cytoplasm beneath the cuticular plate. They are continuous with the subsurface cisternae. In the supranuclear cytoplasm, bulges of the subsurface cisternae and the lamellar bodies are found. Dilated cisternae are also present. Some dilated cisternae contain many small vesicles, which display acid phosphatase activity. The dilated cisternae are considered as forms of the bulges undergoing transformation into multivesicular bodies. The possible role of the lamellar bodies, and the origin and fate of the subsurface cisternae are discussed.

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.

Scarpa's ganglion in the cat

A quantitative analysis of characteristics of neurons in Scarpa's ganglion was performed with light and electron microscopy. Plotting the major axes of 340 neurons as a function of their nuclear diameters indicated that there are two types of cells: Large neurons with large nuclei and small ones with small nuclei. Small neurons have an excentric nucleus with indentations of the nuclear membrane. Large neurons have a spherical and central nucleus. Almost all cells are myelinated. Occasional unmyelinated cells or those surrounded by many myelin layers represent normal variations of large or small neurons. The inferior portion of the ganglion containing about one-third of the total number of neurons and innervating most of the saccular macula and the posterior crista has more than 10% small neurons. The superior portion of the ganglion with two-thirds of the total number of neurons and innervating the remaining sensory epithelia has about 5% small neurons. The significance of this observation concerning innervation patterns of vestibular sensory epithelia is discussed.

Scanning electron microscopy of the normal human cochlea

Preservation of the fine structures of the human cochlea has been achieved by perfusing the cochlea with fixative shortly after death. Following the dissection of the temporal bone the surface of the organ of Corti and stria vascularis has been examined in the scanning electron microscope. The surfaces of the inner and outer hair cells can be seen and the stereocilia projecting from their surfaces closely examined. The number and length of the stereocilia of the outer hair cells changes linearly with distance along the cochlear duct. The surface of the stria vascularis is similar to that seen in other animals.

Scanning electron microscopy of the human cochlea--the organ of Corti

The surface of the organ of Corti from normally hearing adult humans has been examined with the scanning electron microscope. It is possible to construct cytocochleograms and to derive a regression line with confidence limits to represent the distribution of the sensory hair cells. Examining individual hair cells more closely, the number of cilia on each hair cell, decreased linearly with distance, from the base of the cochlea. However, the length of the longest cilia on each outer hair cell increased linearly with distance.

Stapes anomaly and cochlear sensory cell changes. A scanning electron microscope study

This paper reports the pathological findings in the cochleae of a 66-year-old man examined under a scanning electron microscope (SEM). Columella-shaped stapes, which were the only anomalous changes in the middle ear cleft, were found in both ears. Beside this, giant hair formation and fusion of stereocilia in the inner sensory cells were also observed in the apical part of the lower basal turn. Since the patient had no history of ototoxic drug use or other ear disease episodes, it is suggested that these inner hair cell changes might have been caused by the conductive disorder accompanying the stapes anomaly. Giant hair formation was discussed from the inner ear findings of 14 other cases examined by a SEM.

Ultrastructural study of the human spiral ganglion

Electron microscopic study of the human spiral ganglion was conducted on 17 ears from 12 individuals aged 9 months to 92 years. Two types of neurons, large and small, with distinct cytological characteristics were found. Both types of neurons were myelinated and unmyelinated; however, a majority of the population was unmyelinated (94%). The distribution of myelinated neurons varied greatly among individuals, though a slight increase in their population was noted in aged individuals, the highest count being 28% in a specimen from an individual 75 years old. The small neurons constituted 6% of the population, and their cytoplasms were highly filamentous in both myelinated and unmyelinated types. The findings of this study provide no evidence as to the functional significance of the myelination of spiral ganglion cells.

Organ of Corti in the human fetus: scanning and transmission electronmicroscope studies

The organ of Corti in the five-month human fetus was studied by transmission and scanning electronmicroscopy. Differentiation of the surface organization of the organ of Corti into a single row of inner and three to four rows of outer hair cells was complete at this stage except at the apical end. The morphological aspects of the hair bundles changed with maturation of the sensory cells; the inner hair cells preceded the outer hair cells in cytodifferentiation at a given location.

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.

Electron microscopical and histochemical studies of outer hair cells in normal rabbits

The outer hair cells of the rabbit were electron microscopically and histochemically investigated. The results clearly showed that the outer hair cells of the rabbit have no agglomeration of mitochondria in its infranuclear region. This anatomical fact means that the functional mechanism in the outer hair cells of the rabbit may be substantially different from that of other animals which have agglomeration of mitochondria. From the results of the histochemical study, it is reasonably supposed that the anaerobic energy metabolism may ordinarily play the leading part in sustaining the cellular functions in the infranuclear region of the outer hair cells of the rabbit.

Extra internal hair cells. A scanning electron microscopic study

The extra internal hair cell (EIHC) of the human cochlea was observed by means of a scanning electron microscope. The EIHC was found not infrequently in all turns of the human cochlea. It was located medial to the IHC row. The inner pillar cells showed an abnormal structure. The anatomical relationships between the displaced IHC and EIHC, and the inner pillar cell were classified into five types. The origin of these anomalies is discussed form an embryological viewpoint.

Fine structure of the afferent synapse of the hair cells in the saccular macula of the goldfish, with special reference to the anastomosing tubules

The fine structure of the afferent synapse has been studied in the hair cells of the goldfish saccular macula. A spherical dense body which is surrounded by synaptic vesicles is observed in association with the presynaptic membrane. An alternating, parallel arrangement of dense bars and of rows of synaptic vesicles is observed on the presynaptic membrane beneath the dense body. Each row consists of five to six immediately available synaptic vesicles, and five to six such rows of vesicles are observed per synapse. Sometimes anastomosing tubules are found around the dense body. The tubules are formed by direct infolding of the plasma membrane. Many coated vesicles are found at the periphery of the anastomosing tubules. A possible role of the anastomosing tubules in the turnover of the synaptic vesicle membrane is discussed.

The fine structure of lipofuscin in the human inner ear

Lipofuscin inclusions in the human membranous labyrinth were studied by electron microscopy. Lipofuscin is morphologically an irregularly shaped, membrane-bound inclusion consisting of an electron-dense structure. The most common component was a fine, granular, osmiophilic substance which was always associated with a homogenous, spherical structure resembling a lipid droplet. The combination of these two components was frequently observed in the human inner ear. Distended inclusions containing lipofuscin components were also observed within the supporting cells, saccular, utricular and ampullar wall, the epithelial cells of the transitional zone and in the dark cells. Lipofuscin is closely associated with lysosome and is known to accumulate in the tissue as a result of aging. The high lysosomal activity possibly may result in lipofuscin formation in the human inner ears. Also some other unknown metabolic conditions may provide the deposits of lipofuscin.

Electron microscopic cochlear observations in bilateral Ménière's disease

Electron microscopic study of the cochleas of an individual with bilateral Ménière's disease revealed the presence of many abnormal sensory cells in the apical regions of the cochleas. The pathological alterations were greater in the left ear with the greater hearing loss. There were some giant cilia, fusion of cilia, and loss of cilia. The outer hair cells contained diffuse cuticular bodies near or basal to the nuclei. Many outer hair cells were retracted away from the reticular membrane. The population of the nerve endings appeared normal. The inner hair cells of the apical turns appeared essentially normal. The spiral ganglia were normal in number and morphology at the apical turns, and a majority of their cell bodies were of the unmyelinated or partly myelinated types. The stria vascularis showed atrophy; however, the magnitude of this change was consistent with that known to occur in the aging ear. In the distended areas Reissner's membrane showed areas devoid of mesothelial cells, as well as atrophic epithelial cells. The blood vessels were no different from other human cochlear vessels. These abnormalities seen in the stereocilia, the outer hair cells, and Reissner's membrane are a matter of fact. We must admit, however, that not enough electron microscopic studies have been performed on ears from aging individuals or ears with other pathologies to state that these changes are unique to Ménière's disease. Furthermore, there is currently insufficient knowledge to predict whether these changes affect auditory function.

Fluorocitrate ototoxicity. A morphologic and cytochemical model for primary neural degeneration in the guinea pig cochlea

Fluorocitrate, an inhibitor of the tricarboxylic acid cycle at the aconitase reaction, produces a time and dose related neural dystrophy in the guinea pig cochlea. There is direct inhibition of succinic dehydrogenase activity but not nicotinamide adenine dinucleotide dehydrogenase and cytochrome oxidase via cytochrome c activities. The dystrophic neural changes morphologically are similar to those noted in primary neural degeneration and neural presbycusis in man. Neural degeneration in aging appears to be the result of a dissociation of biochemical reactions preventing the proper utilization of organic fuel molecules for generation of energy and direct or indirect inhibition of respiration.

Degenerative changes in the human vestibular sensory epithelia

A study of the vestibular end organs from humans of different ages is presented. The inner ears were exposed by microdissection, and the vestibular sensory regions were either sectioned and studied with light or electron microscopy, or prepared and studied with the surface specimen technique. A change, which can be related to aging, is the accumulation of lipofuscin inclusions in sensory and supporting cells, especially pronounced in the type I sensory cell. Changes of the hair bundles, such as disarrangement of cilia, increased fragility of cilia and formation of giant cilia, have also been observed in aged individuals. In three cases there was a history of vestibular disturbance of vertigo. All three cases had shown caloric hypo-reactivity. In two cases, one with a history of herpes zoster oticus and another with a brain stem glioma, no morphological changes which could be attributed to the diseases, were found. The third case showed degeneration of macula utriculi and the lateral and superior cristae, possibly as a result of vascular disturbance.