The saccule may be the transducer for directional hearing of nonostariophysine teleosts

The hearing of fishes is transduced by the otolithic end-organs of the eighth nerve. In several nonostariophysine fish, the nerve innervation and hair cell orientation in the saccule, one otolithic organ, suggest that directionality is encoded by a set of mutually perpendicular sensory epithelia. The anterior saccular branch innervates only the hair cell groups oriented along the rostrocaudal body axis which are located at the anterior of the saccule. The posterior saccular branches innervate the hair cell groups oriented along the dorsoventral body axis and are found at the posterior of the saccule.     

Scanning electron microscopic study of the sacculus and lagena in several deep-sea fishes

The ultrastructure of the sacculus and lagena, two otolithic organs involved in audition, was studied in seven species of bathypelagic and mesopelagic fishes representing a taxonomically diverse sampling of Division-III teleost fishes. The saccular macula in each species had hair cells oriented in four directions, with cells on the rostral part of the macula oriented anteriorly and posteriorly, and those on the caudal end of the macula oriented dorsally and ventrally. The most significant variation from this pattern was in a Gadiform fish, Bregmaceros sp., which also had additional groups of horizontally oriented cells on the posterior end of the macula. The lagenar macula in each species had very similar hair cell orientation patterns, there being dorsally oriented cells on the anterior side of the macula and ventrally oriented cells on the posterior side. The only exception to this pattern was in Ectreposebastes, in which the two groups of cells were oriented towards one another. Significantly, the predominant ciliary bundles on the sensory hair cells of the saccular maculae, and to a slightly lesser degree on the lagenar maculae, were quite similar in almost all species. The bundles had a single long kinocilium and graded stereocilia, the longest of which were almost as long as the kinocilium. This pattern is far less frequently found in shallow water fishes. These data further demonstrate that the hair cells oriented in four directions on the saccular macula may be ubiquitous among all teleost fishes other than the Ostariophysi. The data also lead to the suggestion that the elongate ciliary bundle may be adaptive to certain features of life in deep water.     

Lectin from Griffonia simplicifolia identifies an immature-appearing subpopulation of sensory hair cells in the avian utricle

The sensory hair cells of the inner ear are coated with a variety of glycoproteins and glycolipids which can be identified by the binding of specific lectins. The present study examined the binding patterns of three lectins–Wheat Germ Agglutinin, Peanut Agglutinin, and lectin from Griffonia simplicifolia (Isoform B₄)–in the avian utricle. Each of the lectins exhibited a distinct pattern of hair cell labeling. Wheat Germ Agglutinin (WGA) appeared to label the ciliary bundles of all sensory hair cells. In contrast, the binding of Peanut Agglutinin (PNA) was mainly confined to the ciliary bundles of extrastriolar hair cells. Finally, lectin from Griffonia simplicifolia (GS-IB₄) labeled a subpopulation of hair cells in all regions of the chick utricle. Those bundles were much smaller than the majority of ciliary bundles labeled by either WGA or PNA, and the density of GS-IB₄-labeled bundles in the normal mature utricle was relatively low. Increased densities of GS-IB₄-labeled hair cells were observed in the embryonic utricle and during the process of hair cell regeneration. The observations suggest that GS-IB₄ labels a glycoprotein that is expressed preferentially on the ciliary bundles of immature hair cells.     

A transmission and scanning electron microscopic study of the saccule in five species of catfishes

The sacculi of five species of catfishes were studied by transmission and scanning electron microscopy. In four species, the sagitta exhibited a multifluted anterior part and a tapered posterior part; in Corydoras aeneus, however, the fluted part was absent, and a vertical component extended dorsally to terminate near the opening of the transverse canal. In all species, the otoliths had a laminar structure. An otolithic membrane was present, and hair cell bundles projected into cavities on the macular surface of the membrane. Attachments of the otolithic membrane to the neuroepithelium included short extensions of the membrane to the tallest components of the hair cell bundles of the peripheral cells and more delicate connections to the kinocilium and taller stereocilia of central cells; in addition, attachments to the microvilli of supporting cells were present. In both hair cells and supporting cells single microtubules and bundles of microtubules were present; the bundles had an orderly arrangement and were associated with cytoplasmic densities surrounding the desmosomes. The hair cells were innervated by both afferent and efferent nerve endings. Studies of the polarization of the hair cells in all species (except C. aeneus) showed that there was a single longitudinal axis that divided dorsally polarized cells from those oriented ventrally. In Doras spinosissimus and Bunocephalus bicolor, an additional line of polarization was evident in a small area in the anterior part of the macula; therefore, in these forms there was a double bipolar orientation.     

Structure of the chicken's inner ear: SEM and TEM study

The inner ears of 35 adult chickens were studied by TEM, SEM and light microscopy. Two well differentiated hair cell/nerve ending units were present: tall hair cells with small vesiculated nerve endings were located on the attached part of the basilar membrane; short hair cells with large vesiculated nerve endings were located on the free basilar membrane except for the distal tip. In this respect the chicken ear is similar to that of the pigeon. The chickens examined did have some unique features. Sensory cells of lenticular and hemispheric shape were also present at the proximal end. Bundles of long dense tubules were seen frequently within the sensory cell cytoplasm. Kinocilia were absent from the hair bundles of many of the sensory cells. The internal structure of the kinocilia which were present was atypical and consisted of a variable number of doublets. Eight peripheral plus one central doublet were found most frequently.     

Progenitor Cells from the Adult Human Inner Ear

Loss of inner ear hair cells leads to incurable balance and hearing disorders because these sensory cells do not effectively regenerate in humans. A potential starting point for therapy would be the stimulation of quiescent progenitor cells within the damaged inner ear. Inner ear progenitor/stem cells, which have been described in rodent inner ears, would be principal candidates for such an approach. Despite the identification of progenitor cell populations in the human fetal cochlea and in the adult human spiral ganglion, no proliferative cell populations with the capacity to generate hair cells have been reported in vestibular and cochlear tissues of adult humans. The present study aimed at filling this gap by isolating colony-forming progenitor cells from surgery- and autopsy-derived adult human temporal bones in order to generate inner ear cell types in vitro. Sphere-forming and mitogen-responding progenitor cells were isolated from vestibular and cochlear tissues. Clonal spheres grown from adult human utricle and cochlear duct were propagated for a limited number of generations. When differentiated in absence of mitogens, the utricle-derived spheres robustly gave rise to hair cell-like cells, as well as to cells expressing supporting cell-, neuron-, and glial markers, indicating that the adult human utricle harbors multipotent progenitor cells. Spheres derived from the adult human cochlear duct did not give rise to hair cell-like or neuronal cell types, which is an indication that human cochlear cells have limited proliferative potential but lack the ability to differentiate into major inner ear cell types. Anat Rec, 303:461–470, 2020. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.     

Ionic selectivity of the mechano-electrical transduction channels in the hair cells of the frog sacculus

In the isolated saccular macula of Ratio, esculenta extracellular hair cell receptor currents evoked by mechanical stimulation of the otolithic membrane were recorded under transepithelial voltage clamp conditions. The ionic selectivity of the mechano-electrical transduction channels of the hair cells was determined by examining the effects of different concentrations of Ca²⁺ and K⁺ in the apical solution on the transepithelial voltage at which the extracellular receptor current was zero (V_rev). Changing the concentration of Ca²⁺ from 0.26 mM to 0.026 and to 2.6 mM at a constant K⁺ concentration caused changes in V_rev of – 15 + 7 mV (mean±SD; n= 9) and 20±6mV (n= 13), respectively. The relative ionic permeabilities of the transduction channels were estimated from a modified Goldman, Hodgkin and Katz equation, assuming that 80% of the transepithelial resistance is located in the apical membranes of the hair cells. The permeability of the transduction channels for Ca²⁺ was found to be two orders of magnitude larger than that for K⁺. The measured effects on V_rev of changing the concentration of K⁺ at constant ionic strength and at different constant Ca²⁺ concentrations were well predicted by the same equation. These results indicate that the transduction channels of the frog saccular hair cells are highly selective to Ca²⁺.     

Hair cells in the inner ear of the pirouette and shaker 2 mutant mice

The shaker 2 (sh2) and pirouette (pi) mouse mutants display severe inner ear dysfunction that involves both auditory and vestibular manifestation. Pathology of the stereocilia of hair cells has been found in both mutants. This study was designed to further our knowledge of the pathological characteristics of the inner ear sensory epithelia in both the sh2 and pi strains. Measurements of auditory brainstem responses indicated that both mutants were profoundly deaf. The morphological assays were specifically designed to characterize a pathological actin bundle that is found in both the inner hair cells and the vestibular hair cells in all five vestibular organs in these two mutants. Using light microscope analysis of phalloidin-stained specimens, these actin bundles could first be detected on postnatal day 3. As the cochleae matured, each inner hair cell and type I vestibular hair cell contained a bundle that spans from the region of the cuticular plate to the basal end of the cell, then extends along with cytoplasm and membrane, towards the basement membrane. Abnormal contact with the basement membrane was found in vestibular hair cells. Based on the shape of the cellular extension and the actin bundle that supports it, we propose to name these extensions cytocauds. The data suggest that the cytocauds in type I vestibular hair cells and inner hair cells are associated with a failure to differentiate and detach from the basement membrane.     

Cochlear apical morphology in toothed whales: Using the pairing hair cell—Deiters' cell as a marker to detect lesions

The apex or apical region of the cochlear spiral within the inner ear encodes for low-frequency sounds. The disposition of sensory hair cells on the organ of Corti is largely variable in the apical region of mammals, and it does not necessarily follow the typical three-row pattern of outer hair cells (OHCs). As most underwater noise sources contain low-frequency components, we expect to find most lesions in the apical region of the cochlea of toothed whales, in cases of permanent noise-induced hearing loss. To further understand how man-made noise might affect cetacean hearing, there is a need to describe normal morphological features of the apex and document interspecific anatomic variations in cetaceans. However, distinguishing between apical normal variability and hair cell death is challenging. We describe anatomical features of the organ of Corti of the apex in 23 ears from five species of toothed whales (harbor porpoise Phocoena phocoena, spinner dolphin Stenella longirostris, pantropical spotted dolphin Stenella attenuata, pygmy sperm whale Kogia breviceps, and beluga whale Delphinapterus leucas) by scanning electron microscopy and immunofluorescence. Our results showed an initial region where the lowest frequencies are encoded with two or three rows of OHCs, followed by the typical configuration of three OHC rows and three rows of supporting Deiters' cells. Whenever two rows of OHCs were detected, there were usually only two corresponding rows of supporting Deiters' cells, suggesting that the number of rows of Deiters' cells is a good indicator to distinguish between normal and pathological features.     

Foxg1 is required for proper separation and formation of sensory cristae during inner ear development

The vestibular portion of the inner ear, the three semicircular canals and their sensory cristae, is responsible for detecting angular head movements. It was proposed that sensory cristae induce formation of their non‐sensory components, the semicircular canals. Here, we analyzed the inner ears of Foxg1^?/? mouse mutants, which display vestibular defects that are in conflict with the above model. In Foxg1^?/? ears, the lateral canal is present without the lateral ampulla, which houses the lateral crista. Our gene expression analyses indicate that at the time when canal specification is thought to occur, the prospective lateral crista is present, which could have induced lateral canal formation prior to its demise. Our genetic fate‐mapping analyses indicate an improper separation between anterior and lateral cristae in Foxg1^?/? mutants. Our data further suggest that a function of Foxg1 in the inner ear is to restrict sensory fate. Developmental Dynamics 238:2725–2734, 2009. © 2009 Wiley‐Liss, Inc.     

Growth factor regulation of the cell cycle in developing and mature inner ear sensory epithelia

Growth factors and other extracellular signals regulate cell division in many tissues. Consequently, growth factors may have therapeutic uses to stimulate the production of replacement sensory hair cells in damaged human inner ears, thereby assisting in alleviating hearing loss and vestibular dysfunction. Assessment of the ability of growth factors to stimulate cell proliferation in inner ear sensory epithelia is at an early stage. This paper provides a brief account of what we know regarding growth factor regulation of cell proliferation in developing and mature inner ear sensory epithelia.     

Gap junctions between sensory and supporting cells of the utricular and saccular maculae in Anolis carolinensis examined by transmission electron microscopy

Gap junctions, related to projections of sensory cells into supporting cells, occur from supranuclear to basal levels on hair cells, in both saccular and utricular maculae of the lizard, Anolis carolinensis. The larger numbers of junctional projections observed at some levels may indicate a zonular distribution within the neuroepithelia, but only on supracalyceal parts of hair cells in the utricular striola was a series of gap junctions found to be closely associated with the reticular lamina. The dimensions of the junctions, as characterized by the distance between opposite extremities of the arciform membrane appositions, averaged 0.14, 0.19, and 0.28 μm, respectively, on non-calyceal utricular and saccular hair cells, and calyceal cells of the utricular striola. A notable number of the junctional profiles were adjacent or contiguous to open, coated, putatively endocytotic vesicles. These findings are discussed in respect to phylogenetic, developmental, and functional considerations.     

The sensory hairs and tectorial membrane of the basilar papilla in the lizardCalotes versicolor

Summary The hearing organ in the lizard, the basilar papilla, is an oblong organ situated in the central opening of the surrounding limbus. The hair cells of the basilar papilla inCalotes versicolor consist of two different types. The type A sensory cells have short hair bundles whose arrangement resembles that of organ pipes, and are situated in the ventral part of the organ. The type B sensory cells have tall, whisk-like hair bundles and are situated in the dorsal part of the basilar papilla. The type A sensory cells are unidirectionally orientated, whereas the type B cells are orientated towards the central sulcus in the papilla. Between the stereocilia, quite close to their base, there is a thin network of interconnecting fibres. Another type of connection is found between the kinocilium and the five adjacent stereocilia. These fibres, however, are situated close to the tips of the relevant cilia. The ventral part of the basilar papilla, i.e., the type A cell population, is covered by a tectorial membrane. Between the microvilli of the supporting cells and the tectorial membrane a network of thin interconnecting filaments is seen. This totally encloses the hair bundles, thus causing them to stand in tubular formations between the sensory epithelium and the tectorial membrane.     

Atoh1 null mice show directed afferent fiber growth to undifferentiated ear sensory epithelia followed by incomplete fiber retention.

Inner ear hair cells have been suggested as attractors for growing afferent fibers, possibly through the release of the neurotrophin brain-derived neurotrophic factor (BDNF). Atoh1 null mice never fully differentiate hair cells and supporting cells and, therefore, may show aberrations in the growth and/or retention of their innervation. We investigated the distribution of cells positive for Atoh1- or Bdnf-mediated beta-galactosidase expression in Atoh1 null and Atoh1 heterozygotic mice and correlated the distribution of these cells with their innervation. Embryonic day (E) 18.5 Atoh1 null and heterozygotic littermates show Atoh1- and BDNF-beta-galactosidase-positive cells in comparable distributions in the canal cristae and the cochlea apex. Atoh1-beta-galactosidase-positive but only occasional Bdnf-beta-galactosidase-positive cells are found in the utricle, saccule, and cochlea base of Atoh1 null mutant mice. Absence of Bdnf-beta-galactosidase expression in the utricle and saccule of Atoh1 null mice is first noted at E12.5, a time when Atoh1-beta-galactosidase expression is also first detected in these epithelia. These data suggest that expression of Bdnf is dependent on ATOH1 protein in some but does not require ATOH1 protein in other inner ear cells. Overall, the undifferentiated Atoh1- and Bdnf-beta-galactosidase-positive cells show a distribution reminiscent of that in the six sensory epithelia in control mice, suggesting that ear patterning processes can form discrete patches of Atoh1 and Bdnf expression in the absence of ATOH1 protein. The almost normal growth of afferent and efferent fibers in younger embryos suggests that neither fully differentiated hair cells nor BDNF are necessary for the initial targeted growth of fibers. E18.5 Atoh1 null mice have many afferent fibers to the apex of the cochlea, the anterior and the posterior crista, all areas with numerous Bdnf-beta-galactosidase-positive cells. Few fibers remain to the saccule, utricle, and the base of the cochlea, all areas with few or no Bdnf-beta-galactosidase-positive cells. Thus, retention of fibers is possible with BDNF, even in the absence of differentiated hair cells.