Proliferation of vertebrate inner ear supporting cells.

Using two S phase markers, we determined the cell-cycle behavior of inner ear supporting cells from two species, the chicken and the oscar. The results indicate that chicken utricular supporting cells divide once and do not return to the cell cycle for at least 7 days. In contrast, supporting cell progeny in the oscar saccule return to S phase after 5 days. While both the chicken utricle and oscar saccule show ongoing supporting cell proliferation, these data indicate that there may be a dedicated recycling population of supporting cells in the oscar saccule but not in the chicken utricle that is responsible for hair cell production. An expulsion of proliferative cell progeny in the chicken utricle after 7 days may be a driving force for proliferation, as well as an explanation for why hair cell numbers do not increase in the chicken utricle with age. This was not seen in the oscar saccule, possibly explaining how this end organ increases in size throughout the adult life of the animal. The absence of S phase cell expulsion, however, does not rule out the role of cell death in the oscar saccule.     

Biophysical basis for inner ear decompression sickness.

Isolated inner ear decompression sickness (DCS) is recognized in deep diving involving breathing of helium-oxygen mixtures, particularly when breathing gas is switched to a nitrogen-rich mixture during decompression. The biophysical basis for this selective vulnerability of the inner ear to DCS has not been established. A compartmental model of inert gas kinetics in the human inner ear was constructed from anatomical and physiological parameters described in the literature and used to simulate inert gas tensions in the inner ear during deep dives and breathing-gas substitutions that have been reported to cause inner ear DCS. The model predicts considerable supersaturation, and therefore possible bubble formation, during the initial phase of a conventional decompression. Counterdiffusion of helium and nitrogen from the perilymph may produce supersaturation in the membranous labyrinth and endolymph after switching to a nitrogen-rich breathing mixture even without decompression. Conventional decompression algorithms may result in inadequate decompression for the inner ear for deep dives. Breathing-gas switches should be scheduled deep or shallow to avoid the period of maximum supersaturation resulting from decompression.     

Creatine kinase in epithelium of the inner ear.

Epithelium of the inner ear in the gerbil and mouse was examined immunocytochemically for presence of creatine kinase (CK). Marginal cells of the cochlear stria vascularis and dark cells and transitional cells of the vestibular system were found to contain an abundance of the MM isozyme (MM-CK). CK in these cells concurs with that which is coupled to Na,K-ATPase in other cells and is considered to supply ATP for the Na,K-ATPase that mediates the high KCl of endolymph. Inner hair cells revealed content of the BB isozyme and in this respect resembled the energy-transducing photoreceptor cells in retina. In addition, outer phalangeal (Deiters') cells stained for both MM- and BB-CK whereas inner phalangeal cells evidenced content of only the BB isozyme. Immunolocalization of CK appeared similar in mouse and gerbil inner ear. Specificity of the staining was affirmed by observations in agreement with those reported for CK in various cell types and by staining with antisera from more than one source.     

Mechanical filtering of sound in the inner ear.

We have studied the distortion generated by the cochlea to gain insight into the mechanisms responsible for the sharp tuning or 'frequency selectivity' of the inner ear. We used two stimulating tones of moderate intensity which are progressively separated in frequency, and measured the ear canal cubic distortion components which are generated as a consequence of the stimulus interaction in the cochlea. We inferred that the distortion is generated from the frequency region of the higher of the two stimulus tones and that it is then band-pass filtered by a structure which is tuned to a frequency just over half an octave below that of the high-frequency tone. We suggest that the structure responsible for this band-pass filtering is the tectorial membrane, and we conclude that our results support theories of cochlear mechanics in which resonances due to the tectorial membrane interact with those of the basilar membrane to enhance the frequency selectivity of the inner ear.     

Physiology and pathophysiology of inner ear melanin

The presence of melanin in the inner ear was established more than a century ago, but the exact biological function of the pigment in the labyrinth has yet to be determined. In this brief review, the correlation of pigmentation and inner ear function, as well as the presumed role of melanocytes in hereditary diseases are discussed. Special attention was drawn to the composition of melanin and its presumed function as a biological reservoir for divalent ions and as an ion exchanger, as well as an intracellular buffering system for calcium. It is pointed out that melanin is capable of binding ototoxic drugs. Finally, morphological responses of melanocytes to local disturbance of Ca++ homeostasis in the inner ear are described as 1) intracellular movement and intraepithelial deposition of melanosomes; 2) cell motility; 3) neomelanogenesis; and 4) enhanced exocytotoxic/endocytotic activity. The possible consequences of this malfunction of the melanocytes on the inner ear function are discussed.     

Molecular mechanisms of inner ear development

The inner ear is a structurally complex vertebrate organ built to encode sound, motion, and orientation in space. Given its complexity, it is not surprising that inner ear dysfunction is a relatively common consequence of human genetic mutation. Studies in model organisms suggest that many genes currently known to be associated with human hearing impairment are active during embryogenesis. Hence, the study of inner ear development provides a rich context for understanding the functions of genes implicated in hearing loss. This chapter focuses on molecular mechanisms of inner ear development derived from studies of model organisms.     

Effects of noise on inner ear

The effects of noise on health depend both on individual factors and characteristics of sound exposure. In case of acoustic trauma, reversible or irreversible lesions of inner ear components are possible. Most often there is immediately an acute tinnitus and hearing loss. Audiometric tests demonstrate hearing loss on the high frequency, generally focused on 4 kHz. Immediate treatment is recommended even there is no currently indicator of the ability to restore hearing. New perspectives on treatment are directed to local treatment and/or using new procedure as antioxidative treatment. Occupational and leisure are the two conditions in which chronic exposure to noise is found. Detection and prevention of noise-induced hearing loss is easier in case of industrial workers than in case of noise exposition for musicians and other sounds and stage technicians or concert managers, and of course non-professional with exposure to amplified music.     

Ontogeny and regeneration in the inner ear

Brain Cell Biology -     

Involvement of insulin-like growth factor-I in inner ear organogenesis and regeneration.

The verterbrate inner ear is an excellent model system to study signalling mechanisms in embryonic development. During the last years, insulin-like growth factor-I (IGF-I) has attracted attention in relation to the regulation of inner ear ontogenesis. IGF-I and its high-affinity tyrosine-kinase receptor are expressed during early stages of inner ear development. IGF-I is a powerful mitogen for the otic vesicle, where it stimulates cell-division and mitogenic signalling cascades. Later in development, IGF-I also promotes survival and neurogenesis of the otic neurones in the cochleovestibular ganglion (CVG). The actions of IGF-I are associated with the generation of lipidic messengers and the activation of Raf kinase, which results in the rapid induction of the expression of the proliferative cell nuclear antigen (PCNA) and the nuclear proto-oncogenes c-fos and c-jun. Regulation of organogenesis involves a dynamic balance of the mechanisms regulating cell division, differentiation and death. A model is proposed where this balance is the consequence of the action of IGF-I and NGF, which converge in Raf activation or suppression. The combinatorial expression of jun and Fos family members in particular domains of the otic vesicle would be the final result of such cascade. Some of these mechanisms may be also implicated in otic regeneration.     

Patterning and morphogenesis of the vertebrate inner ear

The positional cues for formation of individual inner ear components are dependent on pre-established axial information conferred by inductive signals from tissues surrounding the developing inner ear. This review summarizes some of the known molecular pathways involved in establishing the three axes of the inner ear, anterior-posterior (AP), dorsal-ventral (DV) and medial-lateral (ML). Signals required to establish the AP axis of the inner ear are not known, but they do not appear to be derived from the hindbrain. In contrast, the hindbrain is essential for establishing the DV axis of the inner ear by providing inductive signals such as Wnts and Sonic hedgehog. Signaling from the hindbrain is also required for the formation of the ML axis, whereas formation of the lateral wall of the otocyst may be a result of first establishing both the AP and DV axes. In addition, this review addresses how genes induced within the otic epithelium as a result of axial specification continue to mediate inner ear morphogenesis.     

Effect of EDTA on cytokeratin detection in the inner ear.

Immunohistochemical studies on the epithelium of the adult inner ear are difficult to perform without decalcification of the bony capsule. In this study, we examined the effect of decalcifying agents on the immunoreactivity of various cytokeratin antigens in the cochlear duct epithelium of 2-day-old rats, allowing the comparison of fresh and decalcified specimens. Decalcification of unfixed tissue in a solution containing EDTA or EGTA and polyvinylpyrrolidone, at pH 7.4 and 4 degrees C for a maximum period of 2 days, not only preserved the antigen epitopes but even enhanced the staining intensities in comparison with fresh specimens. This enhancement effect, caused by chelating agents and found to be blocked by prior fixation with acetone, is suggested to be caused by unmasking of the antigenic epitopes.     

Histochemistry of glycogen in the inner ear

Synopsis The effect of fixation and processing upon the morphological appearance of glycogen within the outer hair cells of the guinea-pig was investigated using two methods. In each method, tissue was fixed for 12 h in cold phosphate-buffered 4% paraformaldehyde and eventually dehydrated in ethanol, embedded in Epon 812, and cut into 4 m sections. In procedure A, after complete processing, the sections were tained using the periodic acid-Schiff reaction (PAS) or the periodic acid-thiocarbo-hydrazide-osmium tetroxide (PATCO) reaction which resulted in the appearance of listinct, coarse granules in the cytoplasm of the outer hair cells. Diastase digestion on one of the two matched sections after Epon removal and prior to staining, confirmed the granules to be glycogen. In procedure B, after primary fixation, the tissue was post-fixed in 1% osmium tetroxide and then processed exactly as in procedure A. Here, unless the Epon and osmium was remoyed, there was no staining of the outer hair cell cytoplasm. However, after Epon removal there was diffuse, grainy appearance of the outer hair cell cytoplasm which we considered to be due to glycogen although diastase confirmation was not possible. We have concluded that osmium tetroxide (1) inhibits PAS or PATCO staining, (2) prevents diastase digestion, and (3) prevents the appearance by light microscopy of distinct granules of glycogen.