The 3-dimensional organization of the sensory epithelium of the cochlea in birds

Using authors' original approach to studying histoarchitecture of cellular layers three-dimensional structure of bird helix sensory epithelium was studied theoretically and experimentally. The composition of its elementary morphofunctional unit, AB3 (one sensory cell per three supporting cells) was established. Three dimensional model of this unit was worked out that described shape and arrangement of cells at apical, basal and intermediate levels of the layer. The presence of its translation symmetry was demonstrated. Comparison of theoretical model and results of morphological research of sensory epithelium allowed to conclude that the model corresponds with the real tissue. The model assists in three-dimensional reconstruction of sensory epithelium. And allows to deal with minimum number of sections and also gives an opportunity to forecast its developmental changes.     

Phosphatase and tensin homolog deleted on chromosome 10 regulates sensory cell proliferation and differentiation of hair bundles in the mammalian cochlea

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is a tumor suppressor gene that regulates cell proliferation, differentiation and growth. It regulates neural and glioma stem/progenitor cell renewal and PTEN deletion can drive expansion of epithelial progenitors in the lung, enhancing their capacity for regeneration. Because it is expressed at relatively high levels in developing mammalian auditory hair cells we have analyzed the phenotype of the auditory epithelium in PTEN knock-out mice. PTEN^+/− heterozygous littermates have only one functional copy of the gene and show clear evidence for haploinsufficiency in the organ of Corti. Auditory sensory epithelial progenitors withdraw from the cell cycle later than in wild-type animals and this is associated with increases in the numbers of both inner and outer hair cells. The cytoskeletal differentiation of hair cells was also affected. While many hair bundles on the hair cells appeared to develop normally, others were structurally disorganized and a number were missing, apparently lost after they had been formed. The results show that PTEN plays a novel role in regulating cell proliferation and differentiation of hair bundles in auditory sensory epithelial cells and suggest that PTEN signaling pathways may provide therapeutic targets for auditory sensory regeneration     

Where hearing starts: the development of the mammalian cochlea

The mammalian cochlea is a remarkable sensory organ, capable of perceiving sound over a range of 10¹² in pressure, and discriminating both infrasonic and ultrasonic frequencies in different species. The sensory hair cells of the mammalian cochlea are exquisitely sensitive, responding to atomic‐level deflections at speeds on the order of tens of microseconds. The number and placement of hair cells are precisely determined during inner ear development, and a large number of developmental processes sculpt the shape, size and morphology of these cells along the length of the cochlear duct to make them optimally responsive to different sound frequencies. In this review, we briefly discuss the evolutionary origins of the mammalian cochlea, and then describe the successive developmental processes that lead to its induction, cell cycle exit, cellular patterning and the establishment of topologically distinct frequency responses along its length.     

Mechanics of the mammalian cochlea.

In mammals, environmental sounds stimulate the auditory receptor, the cochlea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated and spirally wound basilar membrane (BM). As they travel, waves grow in amplitude, reaching a maximum and then dying out. The location of maximum BM motion is a function of stimulus frequency, with high-frequency waves being localized to the "base" of the cochlea (near the stapes) and low-frequency waves approaching the "apex" of the cochlea. Thus each cochlear site has a characteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduction channels leading to the generation of hair cell receptor potentials and the excitation of afferent auditory nerve fibers. At the base of the cochlea, BM motion exhibits a CF-specific and level-dependent compressive nonlinearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive and poorly tuned. The high sensitivity and sharp-frequency tuning, as well as compression and other nonlinearities (two-tone suppression and intermodulation distortion), are highly labile, indicating the presence in normal cochleae of a positive feedback from the organ of Corti, the "cochlear amplifier." This mechanism involves forces generated by the outer hair cells and controlled, directly or indirectly, by their transduction currents. At the apex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in determining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency-specific properties of the responses to sound of auditory nerve fibers.     

Expression patterns of FGF receptors in the developing mammalian cochlea

Many studies have shown the importance of the fibroblast growth factor (FGF) family of factors in the development of the mammalian cochlea. There are four fibroblast growth factor receptors (FGFR1–4) and all four are expressed in the cochlea during development. While there are examples in the literature of expression patterns of some of the receptors at specific stages of cochlear development there has been no systematic study. We have assembled a full analysis of the patterns of receptor expression during cochlear development for all four Fgfrs using in situ hybridization. We have analyzed the expression patterns from embryonic day 13.5 through postnatal ages. We find that Fgfr1, 2, and 3 are expressed in the epithelium of the cochlear duct and Fgfr4 is limited in its expression to the mesenchyme surrounding the duct. We compare the receptor expression pattern to markers of the sensory domain (p27^kip1) and the early hair cells (math1). Developmental Dynamics 239:1019–1026, 2010. © 2010 Wiley‐Liss, Inc.     

Specification of the mammalian cochlea is dependent on Sonic hedgehog

Organization of the inner ear into auditory and vestibular components is dependent on localized patterns of gene expression within the otic vesicle. Surrounding tissues are known to influence compartmentalization of the otic vesicle, yet the participating signals remain unclear. This study identifies Sonic hedgehog (Shh) secreted by the notochord and/or floor plate as a primary regulator of auditory cell fates within the mouse inner ear. Whereas otic induction proceeds normally in Shh(-/-) embryos, morphogenesis of the inner ear is greatly perturbed by midgestation. Ventral otic derivatives including the cochlear duct and cochleovestibular ganglia failed to develop in the absence of Shh. The origin of the inner ear defects in Shh(-/-) embryos could be traced back to alterations in the expression of a number of genes involved in cell fate specification including Pax2, Otx1, Otx2, Tbx1, and Ngn1. We further show that several of these genes are targets of Shh signaling given their ectopic activation in transgenic mice that misexpress Shh in the inner ear. Taken together, our data support a model whereby auditory cell fates in the otic vesicle are established by the direct action of Shh.     

Ca2+ current-driven nonlinear amplification by the mammalian cochlea in vitro

An active process in the inner ear expends energy to enhance the sensitivity and frequency selectivity of hearing. Two mechanisms have been proposed to underlie this process in the mammalian cochlea: receptor potential-based electromotility and Ca(2+)-driven active hair-bundle motility. To link the phenomenology of the cochlear amplifier with these cellular mechanisms, we developed an in vitro cochlear preparation from Meriones unguiculatus that affords optical access to the sensory epithelium while mimicking its in vivo environment. Acoustic and electrical stimulation elicited microphonic potentials and electrically evoked hair-bundle movement, demonstrating intact forward and reverse mechanotransduction. The mechanical responses of hair bundles from inner hair cells revealed a characteristic resonance and a compressive nonlinearity diagnostic of the active process. Blocking transduction with amiloride abolished nonlinear amplification, whereas eliminating all but the Ca(2+) component of the transduction current did not. These results suggest that the Ca(2+) current drives the cochlear active process, and they support the hypothesis that active hair-bundle motility underlies cochlear amplification.     

The development of innervation patterns in the avian cochlea

The sequence of developmental events leading to the innervation of the cochlea and the differentiation of its receptor cells has been studied in chick embryos with Golgi methods. We describe the morphogenesis of cochlear ganglion cell peripheral processes from their appearance in early embryos to the formation of their mature endings on hair cells in the basilar papilla (organ of Corti) of prehatching chicks. In the stage of peripheral fiber outgrowth, embryonic days 3-5, the fibers emerge from the ganglion cell bodies and grow, in a uniform fashion, toward the undifferentiated receptor epithelium of the otocyst. In the stage of the invasion of the otocyst by the peripheral fibers, embryonic days 6-7, some fibers enter the epithelium directly after reaching it, others enter after traveling some distance longitudinally beneath its basal lamina. The invading fibers appear to encounter resistance at the basal lamina, but, once within the epithelium, at embryonic days 8-9, they form a surfeit of branches in columnar zones oriented radially toward the surface. In early synaptogenesis (embryonic days 8-9) hair cells first become apparent. They differentiate from primitive epithelial cells. These cells withdraw their basal processes, which appear to accompany the growing fibers into the superficial epithelium. At embryonic days 11-13, the stage of mid-synaptogenesis, the fibers develop large, bulbous, preterminal and terminal swellings, which are located below the bases of the hair cells; the surplus branches atrophy or withdraw. Efferent axons are first seen in the epithelium at this time. In late synaptogenesis (embryonic days 14-17), the preterminal swellings disappear and the endings transform into mature foot-shapes at the bases of the hair cells. These morphological changes during the development of the peripheral endings are comparable to those of cochlear axons in nucleus magnocellularis (cochlear nucleus). During mid-synaptogenesis, when the ganglion cells develop swellings in the periphery, their central axons ramify extensively. Late in synaptogenesis, while the peripheral swellings disappear, there is a corresponding condensation of the central terminals to form the end-bulbs of Held. Thus, specific connections of the cochlear ganglion cells and their target cells in the ear and brain may result from two sequential developmental phases: (1) loosely organized and overabundant initial growth of branches from the fibers entering their target tissue; (2) reorganization of these fibers with the disappearance or resorption of the surplus branches during the transformation of their endings into mature synaptic arrangements.(ABSTRACT TRUNCATED AT 400 WORDS)     

Synaptogene sis in the vestibular sensory epithelium of the chick embryo

Summary The formation of synapses between sensory cells and the terminals of afferent axons has been examined in the embryonic chick labyrinth. Neurites initially cross the otocyst basal lamina and ramify among the undifferentiated epithelial cells by stage 25 of Hamburger and Hamilton. At the same time granular vesicles, with diameters averaging 130 nm, appear in the basal cytoplasm of a few of the epithelial cells. These vesicles often exist in groups at sites of contact with neuntes. By stages 27–28, non membrane-bound densities are frequently found in association with groups of granular vesicles at the plasma membrane. Smaller, clear synaptic vesicles are also a prominent component of these arrangements in presumptive hair cells. Presynaptic ribbons opposite postsynaptic densites are identifiable at about stage 28, and their number increases during subsequent embryonic stages. Specialized appositions, including adherent, postsynaptic and possibly gap junctional contacts, join epithelial cells and nerve terminals throughout this period. The distribution of these junctions is variable, and is not necessarily correlated with the sites of formation of presynaptic ribbons. By stage 32, well-developed chemical synapses consisting of presynaptic ribbons with vesicle halos and postsynaptic densities are common features of hair cell-afferent nerve terminal contact regions. In addition, possible sites of gap junctional contact between adjacent intra-epithelial nerve endings found at stage 32 presage those found in the cristae and maculae of pre-hatch (stage 45) embryos and adults.     

Neuropeptides in physiologically identified mammalian sensory neurones

In cats, intracellular dye injection of single sensory neurones of known fibre type and sensory modality has been combined with peptide immunohistochemistry. There was no clear relationship between the sensory function of a neurone and the presence of the neuropeptides substance P, somatostatin, cholecystokinin and vasoactive intestinal polypeptide, in its cytoplasm. In particular, substance P was not detected in many nociceptive sensory neurons even though it could be demonstrated with the same technique in many sensory neurones which did not have cutaneous receptive fields. These results mean that the role, if any, of these neuropeptides in the transmission of pain, must be regarded as complex.     

Dll3 is expressed in developing hair cells in the mammalian cochlea.

Notch mediates the process of lateral inhibition that controls the production of hair cells in the inner ear. Hair cells are known to express Notch ligands Dll1 and Jag2, which signal through Notch1 in adjacent supporting cells. However, recent genetic and pharmacological studies indicate that the level of Notch-mediated lateral inhibition is greater than can be accounted for by Dll1 and Jag2. Here, we report that another Notch ligand, Dll3, is expressed in developing hair cells, in a pattern that overlaps that of Dll1 and Jag2. We analyzed the cochleae of Dll3(pu) mutant mice, but did not detect any abnormalities. However, earlier studies have demonstrated that there is functional redundancy among Notch ligands in cochlear development and loss of one ligand can be at least partially compensated for by another. Thus Dll3 may play a role in lateral inhibition similar to that of Dll1 and Jag2.     

IL-13 regulates cilia loss and foxj1 expression in human airway epithelium

Mucociliary clearance is essential to the defense mechanisms of the respiratory system. Loss of normal mucociliary clearance contributes to the pathogenesis of genetic and acquired lung diseases. Treatment of cultured differentiated human airway epithelial tissue with IL-13 resulted in a loss of ciliated epithelial cells and an increase in mucus-secreting cells. The loss of ciliated cells was characterized by mislocation of basal bodies and loss of ezrin from the apical cell compartment. In addition to the loss of ciliated cells and increase in mucous cells after IL-13 treatment, cells with characteristics of both ciliated and mucous cells were observed in the airway epithelium. In association with the decrease in ciliated cells after IL-13 treatment, there was noted a decrease in foxj1 expression in the airway epithelium, characterized by a decrease in the number of foxj1-expressing cells. Within the foxj1 promoter, a STAT-binding element was identified and inhibition of foxj1 expression by STAT-6 and IL-13 was demonstrated. These findings suggest molecular and cellular mechanisms for cilia loss in pulmonary disease. Inhibition of foxj1 expression results in loss of apical localization of ezrin and basal bodies with subsequent loss of axonemal structures. These findings have important implications for the pathogenesis and treatment of airway diseases.     

Stereocilia and tectorial membrane development in the rat cochlea

Summary Maturation of the rat cochlea, from postnatal days 2 to 60, was studied using scanning electron microscopyt (SEM), with emphasis on stereocilia and tectorial membrane (TM). Two days after birth, the organ of Corti was very immature. An adult appearance of its surface was observed by day 16 in the basal turn, and by the end of the 3rd postnatal week in the apex. Stereocilia started their development first on inner hair cells. By contrast, the apical pole of outer hair cells ended its maturation before that of inner hair cells. Top-links were detected very early in inner hair cell stereociliary development (postnatal day 2). Marginal pillars temporarily attached the TM to the organ of Corti; they disappeared first in the apical region. This transient attachment seems to play a role in the coupling of outer hair cells to the TM, as prints of their longest stereocilia appeared at the undersurface of the TM by the same time. Moreover, these prints were more clear and regular at the base than at the apex of the cochlea. Results are discussed in relation to ultrastructural and functional data on rat cochlea maturation.