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Following vascular perfusion performed in vivo on anesthetized guinea pigs, method of preparation of semithin and ultrathin sections of Corti organ was presented. For ultrastructural studies cochlear ducts were embedded in toto in epon.
The behaviour of the functional unit haircell - peripheral neurit before and after the dissection of the cochlear nerve in two cases of cerebello-pontine angle tumors is described. An immediate alteration does not seem to be found when the blood supply via internal acoustic meatus is not severed. The survival time after the dissection of the acoustic nerve is somewhat more than seven weeks. This is longer than related morphological findings indicate in the cat where the morphological survival time of fibre population I (95% of all fibres) is not more than four weeks. On the other hand, morphological survival time of fibre population II (5% of all fibres) in cat is up to 14 months. The functional survival time in man under pure tumor pressure is longer than 10 years. Discrepancies between morphological and functional findings may indicate that morphological and functional behaviour do not necessarily coincide.
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We report on findings in guinea pigs with objectively tested normal hearing ability. At the apical end of the Corti organ only in the inner row of hair cells the stereociliae are detectable by scanning electron microscopy. Here the row is interrupted several times. Near the helicotrema we find first the inner row of the outer hair cells, then the middle row and finally the outer row. At the beginning of the D-turn their arrangement is disordered. Normally, at the basal end of the D-turn all rows of inner and outer hair cells show their typical formation. This finding should be taken into consideration when making a cochleogram for proving of experimental noise damages.
For the first time ever, the cochlear tissue of the human embryo has been successfully grown in vitro in two cases, and neuro-specific enolase (NSE) immunoreactivity was studied in one of these tissues. The outer hair cells were arranged in three rows, and the inner hair cells in one row and in better order than the outer hair cells. After NSE immunostaining, the outer spiral bundle, tunnel fiber, outer hair cells, inner hair cells, and the spiral ganglion cells showed positive staining. This data suggests that the human embryonic cochlea has nearly reached complete maturation by 16 weeks, and that the tissue of the Corti organ can be differentiated and matured in vitro.
The organ of Corti is a complex structure containing a single row of inner hair cells (IHCs) and three rows of outer hair cells (OHCs), supported respectively by one row of inner phalangeal cells and three rows of Deiters' cells. When fetal rat organ of Corti explants are cultured, supernumerary OHCs and supernumerary Deiters' cells are produced, without any additional cell proliferation. Analysis of semi- and ultrathin sections revealed that supernumerary OHCs are produced at the distal edge of the organ of Corti. Quantitative analysis of cell types present in the organ of Corti demonstrates that when the number of OHCs increases: (i) the total number of cells remains constant; (ii) the number of Deiters' cells increases; (iii) the number of tectal cells decreases and of Hensen's cells decreases. Using specific HC markers, i.e. jagged2 (Jag2) and Math1, we showed that in addition to existing OHCs, supernumerary OHCs, tectal cells and Hensen's cells expressed these markers in embryonic day 19 organ of Corti explants after 5 days in vitro. The results of this study suggest that Hensen's cells retain the capacity to differentiate into either tectal cells, which differentiate into OHCs, or into undertectal cells which differentiate into Deiters' cells.
In shrews (family Soricidase) the organ of Corti was examined by means of the surface specimen technique. The analysis of the qualitative deviations from the normal cellular pattern of Corti organ was based on more than 160 ears of shrews (Sorex araneus, S. minutus, Neomys fodiens, and Crocidura suaveolens). The dislocation, rotation, malformation of hair cells, and one finding of the giant outer hair cell are described. The quantitative analyses of the population of hair cells are based on 61 ears of Sorex araneus divided into 2 age groups. The mean values of aplasias and of missing and supernumerary hair cells in dependence on individual rows and half-turns of cochlea are given. The loss of hair cells depends on the age of the animal. The findings of aplasias and supernumerary outer hair cells (especially those of atypical 4th row) are relatively frequent in shrews. The organ of Corti in shows has a very regular arrangement; deviations of all types are scant and thus we may hold the ideal cochleogram for the norm. The found deviations seem to have no functional meaning.
3-137 days after disruption of the guinea pig organ of Corti by perilymphatic perfusion with 20% streptomycin (SM), ultrastructural changes of the nerve fibers in the organ were observed. Most of nerve fibers began to degenerate after a latent period of 4 days. On the other hand, a number of fibers survived reactively enlarged and later developed into myelinated and unmyelinated fibers by becoming enclosed in Schwann cells which entered the organ of Corti through the habenula perforata. Regeneration and sprouting of the surviving nerve fibers also occurred. The fibers became mature, but atrophied after 60 days and then gradually disappeared. The regenerating fibers were mainly of the myelinated and unmyelinated efferent type. Retrograde degeneration occurred in both afferent and efferent fibers. In the less damaged organ of Corti perfused with 2% SM or Ringer's solution, Schwann cell invasion was not found.
The organ of hearing is tuned as expressed both in the vibratory response of the cochlear partition and in the resulting receptor potentials of the sensory cells. We now demonstrate a sharply tuned response, consisting of a position shift of the surface of the organ of Corti, occurring during the presentation of a tone. The magnitude of the position shift exceeds that of the vibratory response to the stimulus. The shift is most pronounced in the region of the outer hair cells, and its affected by an inhibitor of outer hair cell motility. We conclude that the response is induced by the action of the outer hair cells.
The organ of Corti is highly ordered, with a single row of inner hair cells and three rows of outer hair cells. The number of hair cells produced was thought to be limited by the time of their terminal mitosis (i.e. E14 in the mouse). However, exogenous application of retinoic acid has been shown to stimulate the formation of supernumerary hair cells in organ of Corti explants from E13 to E16 mouse embryos. Using late embryonic and neonatal rat organ of Corti explants, we investigated the potential for production of supernumerary hair cells in more mature auditory sensory epithelia. When newborn rat organ of Corti explants were cultured under control conditions, an area of supernumerary hair cells was observed in a segment of organ of Corti that was at the junction between the basal and middle turns. In these areas of supernumerary hair cells the number of hair cells increased per unit of length, but remained constant per surface unit, further demonstrating the supernumerary character of this phenomenon. Organ of Corti explants treated with epidermal growth factor (EGF) showed a 50% increase in the length of the organ of Corti segment containing supernumerary hair cells. Upregulation of supernumerary hair cell formation by EGF was found to start and be maximal at birth (P0) and to disappear by 2 days after birth (P2). Treatment of EGF stimulated P0 explants with an antimitotic drug, cytosine arabinoside (ARAc), demonstrated that the production of supernumerary hair cells occurred independently of cell division.
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.
Immunohistological examination of guinea pig cochleas was performed using a panel of 25 monoclonal antibodies directed to various lacto-, ganglio- and globo-series carbohydrate epitopes as well as mucin-type epitopes. Lacto-series structures were found to be localized at specific sites of the tectorial membrane (TM) and Corti's organ, i.e. alpha 1-->3 fucosyl type 2 chain (Le(x)) at Kimura's membrane, marginal band and covering net of TM; alpha 1-->2, alpha 1-->3 difucosyl type 2 chain (Le(y)) at covering net; and sialosyl-Le(x) and sialosyl-i at Kimura's membrane and sensory epithelia, particularly sensory tips of hair cells of Corti's organ. In striking contrast, ganglio-series structures (GM3, GD3, GD2, 9-O-Ac-GD3) were detected at spiral ganglion cells, neuronal fibres and stria vascularis, but were completely absent from Corti's organ and most of the TM. Other epitope structures defined by various antibodies were not detectable at any location. The functional roles of lacto-series carbohydrate epitopes expressed at TM and Corti's organ remain unknown. However, the expression of Le(y) (but not other structures) in association with developmental deficiency of TM induced by 6-N-propyl-2-thiouracil in rats suggests that Le(y) plays some role in normal TM development. The presence of Le(x) at Kimura's membrane and sialosyl-Le(x) at hair cell sensory tips of Corti's organ suggests the intriguing possibility that these fucosylated/sialosylated carbohydrate structures play some role in interactions (either attractive or repulsive) of these inner ear components, which have been implicated in the physiology of hearing, i.e. the conversion of sound waves to nerve impulses.
The entire organ of Corti on guinea pigs were observed by SEM, which has a high resolving power and can be used for a three dimensional study on the fine structure of the cochlea. After the animals were sacrificed, primary perfusion with 3% glutaraldehyde through the oval and round windows was completed in 5 minutes and with 1% OsO4 for the next fixation. After microdissection, the entire organ of Corti was exposed and remained on the modiolus. The OTO technique and critical point drying were used to prepare SEM specimens. The organ of Corti of guinea pig was found to be very similar to that of human being. The basilar membrane widened from the base to the apex of the cochlea. The sensory hair cells, inner and outer, had different appearances marked by shallow U-shaped IHC and W-shaped OHC cilia. The longest layer of OHC cilia left imprints on the undersurface of the tectorial membrane. The relationship between the hair cells and supporting were cells shown. We conclude that the whole specimen of the organ of Corti is more advantageous than piece specimens in acoustic physiological and pathological research by integrated anatomical pattern.
Scanning electron microscopy (SEM) study of the innervation of the organ of Corti has been carried out for many years. Various methods have been used in attempts to observe more details of the endings of nerve fibers on the sensory hair cells. In this study, a new method of microdissection of Corti's organ was done using a small piece of double-sided adhesive tape. This method is simple and facilitates observation of Corti's organ from the basal to the apical turns. Using it we found variations of the nerve endings and the Deiters' cells in each half turn beginning from the lower basal turn to the upper fourth turn. Nerve endings at the lower poles of outer hair cells are arranged in clusters, and are reduced in size and number as the upper turn is approached. On the contrary, the Deiters' cells are increased in size toward the upper turn, therefore nerve endings in the upper turn are often covered by Deiters' cells. Small nerve endings high on the side of the outer hair cell were also observed. Although there was clear evidence of ultrastructural change in the hair cells' cilia after administration of ototoxic drugs or after hyperstimulation by sound, there was no evidence of concomitant ultrastructural change in the nerve endings. Nerve endings appeared to be more resistant than the hair cells.
Experimental organ of Corti destruction results in (1) secondary loss of all type I spiral ganglion neurons, (2) development of type III spiral ganglion neurons, (3) degeneration of most cochlear nerve myelinated fibers, and (4) terminal degeneration in the ventral and dorsal cochlear nuclei. The first signs of degenerative changes occur by eight days after organ of Corti destruction and degeneration debris remains until 28 weeks after destruction.