Cytoskeletal organization of the human inner ear

Anatomical and physiological development of the human inner ear

We describe the development of the human inner ear with the invagination of the otic vesicle at 4 weeks gestation (WG), the growth of the semicircular canals from 5 WG, and the elongation and coiling of the cochlea at 10 WG. As the membranous labyrinth takes shape, there is a concomitant development of the sensory neuroepithelia and their associated structures within. This review details the growth and differentiation of the vestibular and auditory neuroepithelia, including synaptogenesis, the expression of stereocilia and kinocilia, and innervation of hair cells by afferent and efferent nerve fibres. Along with development of essential sensory structures we outline the formation of crucial accessory structures of the vestibular system - the cupula and otolithic membrane and otoconia as well as the three cochlea compartments and the tectorial membrane. Recent molecular studies have elaborated on classical anatomical studies to characterize the development of prosensory and sensory regions of the fetal human cochlea using the transcription factors, PAX2, MAF-B, SOX2, and SOX9. Further advances are being made with recent physiological studies that are beginning to describe when hair cells become functionally active during human gestation. This article is part of a Special Issue entitled <Annual Reviews 2016>.

Changes in cytochemistry of sensory and nonsensory cells in gentamicin-treated cochleas

Effects of a single local dose of gentamicin upon sensory and nonsensory cells throughout the cochlea were assessed by changes in immunostaining patterns for a broad array of functionally important proteins. Cytochemical changes in hair cells, spiral ganglion cells, and cells of the stria vascularis, spiral ligament, and spiral limbus were found beginning 4 days post administration. The extent of changes in immunostaining varied with survival time and with cell type and was not always commensurate with the degree to which individual cell types accumulated gentamicin. Outer hair cells, types I and II fibrocytes of the spiral ligament, and fibrocytes in the spiral limbus showed marked decreases in immunostaining for a number of constituents. In contrast, inner hair cells, type III fibrocytes and root cells of the spiral ligament, cells of the stria vascularis, and interdental cells in the spiral limbus showed less dramatic decreases, and in some cases they showed increases in immunostaining. Results indicate that, in addition to damaging sensory cells, local application of gentamicin results in widespread and disparate disruptions of a variety of cochlear cell types. Only in the case of ganglion cells was it apparent that the changes in nonsensory cells were secondary to loss or damage of hair cells. These results indicate that malfunction of the ear following gentamicin treatment is widespread and far more complex than simple loss of sensory elements. The results have implications for efforts directed toward detecting, preventing, and treating toxic effects of aminoglycosides upon the inner ear.

Cytoskeletal organization of the vestibular sensory epithelia

The cytoskeletal organization of the guinea pig vestibular sensory epithelium was investigated by employing the saponin perfusion method and scanning electron microscopy. The skeletal framework of a a cell was found to consist of actin, intermediate filaments and microtubules. The membrane-bound organelles such as nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, etc. were also well documented. This made it possible to investigate the three-dimensional organization of cytoskeletons as well as their complex interaction with various membrane-bound organelles. The intermediate filaments were demonstrated not only in the supporting cells but also in the sensory cells. They are usually seen surrounding the nucleus and extending through the cytoplasm which connects the nuclear membrane to the plasma membrane, cuticular plate, reticular lamina or other cyto-organelles. The intermediate filaments are also closely related to the desmosomes. These findings suggest that the intermediate filaments provide mechanical support to the cell and its nucleus. In the sensory cells, microtubules are found mainly in the supranuclear portion of the cells, running parallel to the main axis of the cell body, which is directly connected to the cuticular plate. These findings suggest that the microtubules provide the cell with mechanical support and may be closely related to the sensory cell transduction system. Inside the supporting cells, the microtubules are closely related to the secretory granules, Golgi apparatus and intermediate filaments, which supports the idea that the microtubules may control the distribution of intermediate filaments, and also play an important role for transport of the secretory granules.

Neurofilament proteins form an annular superstructure in guinea-pig type I vestibular hair cells

Neurofilaments, the neuron-specific intermediate filaments, are composed of three immunochemically distinct subunits: NF-L, NF-M and NF-H that can be either phosphorylated or unphosphorylated. In mammals, the distribution of these subunits has been described in vestibular ganglion neurons, but there are no reports on the presence of neurofilaments in vestibular hair cells. We investigated, by immunocytochemistry, neurofilaments in vestibular hair cells from rat and guinea-pig using antibodies against the three subunits and to dephosphorylated NF-H (clone SMI 32, recognizes also NF-M on immunoblots), on Vibratome sections of the vestibular end-organs and on isolated hair cells. Various immunostaining protocols were used, as appropriate for the method of observation: laser scanning confocal microscopy (immunofluorescence) and transmission electron microscopy (immunoperoxidase, pre-embedding technique). In rat and guinea-pig cristae and utricles, neurofilament immunoreactivity was observed in axons inside and below the sensory epithelia. In guinea-pig, in addition to this staining, intensely immunoreactive annular structures were found in the basal regions of hair cells. These rings were detected with anti-NF-L, -NF-M and -dephosphorylated NF-H/M antibodies, but not with anti-phosphorylation-independent NF-H. Ring-containing hair cells were present in all regions of the sensory epithelia but were more abundant in the peripheral areas. All levels of observation (Vibratome and thin sections, and isolated hair cells) showed that only the guinea-pig type I hair cells contained a neurofilament ring. High-resolution observations showed that the ring was located below the nucleus, often close to smooth endoplasmic reticulum and the cell membrane.

Cytoskeletal organization of the vestibular supporting cells. Saponin perfusion method for observing intracellular structures by scanning electron microscopy

The cytoskeletal organization of the guinea pig vestibular supporting cells was investigated employing the saponin perfusion method using scanning electron microscopy. The skeletal framework of a cell was composed of actin, intermediate filaments and microtubules. The membrane bound organelles such as nucleus mitochondria, endoplasmic reticulum, Golgi apparatus, etc. were also well documented. This made it possible to investigate the three-dimensional organization of cytoskeletons as well as their complex interaction with various membrane bound organelles. The intermediate filaments were usually seen surrounding the nucleus and extending through the cytoplasm which connected the nuclear membrane to the plasma membrane, reticular lamina or other cytoorganelles. The intermediate filaments also closely related to the desmosomes. These findings suggest that the intermediate filaments provide mechanical support to the cell and its nucleus. The microtubules were closely related to the secretory granules, Golgi apparatus and intermediate filaments, which supports the idea that the microtubules may control the distribution of intermediate filaments, and also play an important role for the transport of the secretory granules.

Cytoskeletal organization of the vestibular sensory epithelia: saponin perfusion method for observing intracellular structures by scanning electron microscopy

The cytoskeletal organization of the guinea pig vestibular sensory epithelial cells were investigated by the use of saponin perfusion method using scanning electron microscopy. The skeletal framework of a cell is composed of thin (actin or intermediate filaments) and thick filaments (microtubules). The membrane bound organelles such as nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, etc. were also well demonstrated. This made it possible to investigate the three-dimensional structures of cytoskeletons as well as their complex interactions with various membranes bound organelles. It is therefore suspected that this technique may provide us further information about distribution, topographic relationships, and the functional role of cytoskeletons.

Cytoskeletal polypeptides: cell-type specific markers useful in investigative otorhinolaryngology

In the last decade, it has been established that eukaryotic cells possess a cytoskeleton, i.e. an integrated cytoplasmic network of microfilaments (MFs), microtubules (MTs) and intermediate filaments (IFs). Moreover, certain cell membrane specializations as well as the inner lamina of the nuclear membrane also participate in the cytoskeletal structure. Although this definition of the cytoskeleton is up to date it is obvious that the future course of cell biology will be reflected in a revised definition. While the bulk of structural polypeptides involved were characterized at regular intervals, surprisingly, the function of the cytoskeleton remained largely speculative and is still less precisely defined. The most widely postulated function concerns mechanical support and integration of diverse cellular activities and thus refers to cellular architecture. Briefly, the mechanical function is thought to involve cell movement, adhesive interaction with the extracellular matrix and neighbouring cells, as well as the stabilization of cell shape. The integrative function refers to intracellular movement, i.e. transport and positioning to the appropriate locations of organelles, intracellular particles, RNA and proteins. It has been established from numerous investigations that (certain) cytoskeletal polypeptides provide significant information about the cellular origin and differentiation state. This consideration constitutes the most prominent reflection underlying this review. Furthermore, this appreciation encourages additional efforts to explore these markers in normal and pathological conditions. The first purpose of this review is briefly to summarize our present comprehension of the molecular components of the cytoskeleton, restricted to the filamentous trinity for practical reasons. The second and main aim is to survey the field with respect to otorhinolaryngology-related issues. To the author's knowledge, this has not been dealt with in the past. In bridging this gap in the literature, I hope to provoke additional interest in one of the fastest moving areas of cell biology. A comprehensive review covering the whole cytoskeleton has been covered by Preston et al. (The Cytoskeleton and Cell Motility. Blackie, Glasgow and London, 1990, pp. 7-69, 188-191). Additional information on the participating substructures is provided in the text, inclusive of last year's reviews.

Glial fibrillary acidic protein-like immunoreactivity in the human fetal inner ear

The distribution of glial fibrillary acidic protein (GFAP)-like immunoreactivity (IR) was analyzed in the human fetal inner ear using immunohistochemical techniques. In the 11-week-old human fetal cochlea, nerve fibers labelled with GFAP had reached the basilar membrane, but innervation stained by antisera against GFAP to the hair cells reached only as far as the basal coil. In the 11-week-old human fetal vestibular organs, little GFAP-IR was present in the epithelia. In 14- and 15-week-old human fetal cochleae, rich immunoreactive neural networks were observed, including the inner spiral bundle. Many immunoreactive sites were found below inner hair cells in all coils. Outer spiral bundles in the first, second and third rows in the basal coil were labelled, but the outer spiral bundle in the third row in the apical coil was not stained by the antisera. In the macula utriculi, many heavily stained sites and a rich immunoreactive network in the sensory epithelium were labelled, while considerably fewer positive nerve fibers and sites were present in the sensory epithelia in the macula utriculi and cristae ampullares. These results suggest that GFAP-IR is a very useful marker of differentiation of Schwann cells in the peripheral nerve in the human inner ear.

Postnatal development of the rat organ of Corti. II. Hair cell receptors and their supporting elements

The development of cochlear receptor cells and their supporting elements was studied by means of semi-thin and ultra-thin sections during the first postnatal weeks in the rat. The temporal and spatial patterns of the receptor cell development were investigated between the 4th and 24th days after birth. At approx. ten equidistant positions along the entire cochlear duct length of inner and outer hair cells, width of outer hair cell triad and stereocilia-length of the outer hair cells were quantitatively analyzed. Striking maturational changes take place before the 12th day after birth, that is, when the onset of hearing occurs. These changes are the formation of the tunnel of Corti, of the Nuel spaces, the appearance of filaments within the supporting elements and the change in cell shape of the hair cells. Between 4 days and 20 days after birth the maturation of outer hair cells is characterized by a decrease of organelles in the cytoplasm and establishment of the subsurface cistern. The quantitative analysis revealed a unique developmental pattern of the length of the outer hair cells, the width of the outer hair cell triad and the stereocilia length of the outer hair cells. Shortly after birth these structures have an almost constant size along the whole cochlear duct, but with increasing age the structures shorten at the cochlear base and enlarge at the apex. This pattern results in the establishment of a baso-apical gradient of the above mentioned structures. We assume that this baso-apical gradient is of central importance for the frequency representation.

Regional variations in the expression of cytokeratin proteins in the adult human cochlea

In the adult human cochlea, a cytokeratin (Ck) network exists along the entire surface of the organ of Corti, enclosing it like a shell. Only the surfaces of the outer and inner hair cells are not integrated in this network. In temporal bone specimens, Ck filaments in Hensen's cells were found to be arranged parallel with and closely apposed to the plasma membrane. In the stria vascularis, Cks were identified only in the marginal cells. Cells in Reissner's membrane and spiral prominence showed varying degrees of immunoreactivity to different monoclonal antibodies directed against Cks. A distinct positivity for Cks was found in most spiral ganglion cells, indicating their presence in all cells. The principal pattern of immunoreactivity was the same in the organ of Corti of the entire cochlea. However, a quantitative gradient in the expression of Cks was observed, with more Cks at the apex than at the base. This was correlated to a difference in the number of Hensen's cells between the two regions. The distinct shell configuration of the Ck network in Corti's organ gives it a tonotopically related difference in rigidity which must be of considerable importance for the perception of sound in the cochlea. The absence of Cks in inner and outer sulcus cells gives them cytoskeletal characteristics of mesenchymal cells with a possible regenerative potential.

Transient expression of neurofilament protein during hair cell development in the mouse cochlea

A polyclonal antiserum raised against the 145 kDa neurofilament protein (NFM) has been used to study the distribution of neurofilaments both in organotypic cultures of the early postnatal mouse cochlea and during development of the mouse cochlea in vivo. In the cultures, both the inner hair cells and the outer hair cells are stained by the antibodies, as well as the innervating afferent fibres from the spiral ganglion. In cultures denervated at the time of preparation, neurofilament positive hair cells can still be detected after 7 days in vitro. NFM can also be detected by immunoblotting in such denervated cultures. Characteristic 10 nm diameter, cytoplasmic filaments can also be observed in cultured hair cells using transmission electron microscopy. Immunostaining of cryosections prepared from cochleas at embryonic days 17 and 19, and days 1, 2, 5, 10 and 21 post-partum reveals that hair cells transiently express NFM during their development in vivo. Expression of NFM in hair cells is first detected at embryonic day 19 in the basal region of the cochlea and, by 2 days post-partum, neurofilament positive hair cells are found throughout the entire length of the cochlea. By 10 days post-partum, staining of hair cells begins to diminish and, by 21 days post-partum, NFM can no longer be detected in hair cells.

Neurofilament immunoreactivity in vestibular ganglion neurons of the adult rat

Immunocytochemical methods were used to study the distribution of neurofilament (NF) proteins in vestibular ganglion neurons of the adult rat. Monoclonal antibodies against the three triplet proteins were used. By indirect immunofluorescence and the peroxidase-antiperoxidase method, two populations of neurons were distinguished. One population with large perikarya showed strong NF immunoreactivity. A second population of neurons presented only slight or no immunoreactivity. The strong NF immunoreactivity in the perikarya of certain neurons seems to be a general feature of many sensory ganglia.

Actin-associated proteins and fibronectin in the fetal human inner ear

The distribution of alpha-actinin, vinculin, alpha-spectrin, beta-spectrin and fibronectin was analyzed in 14- to 21-week-old fetal human inner ears using immunofluorescence microscopy. Staining for alpha-actinin was fairly evenly distributed at the epithelial surfaces of all five vestibular organs, whereas in the cochlea it was mainly at the surface of the receding greater epithelial ridge and in some foci apically at the lesser epithelial ridge. Fluorescence for vinculin was observed mainly at the surface of vestibular organs, but was lacking in the LER. Intense fluorescence for alpha-spectrin was found at the apical surface of individual cells of the cristae and maculae. Antibodies against beta-spectrin mainly stained the endothelial cells of blood vessels, but faint staining of the epithelial cell surfaces of the vestibular organs was also detected. The fluorescence pattern of the actin-associated proteins is indicative of structural differences between cochlear and vestibular hair cells. Fibronectin was identified only between mesenchymal cells and its functional importance in the mature inner ear epithelia can be discounted.

Intermediate filaments in the cochleas of normal and mutant (w/wv, sl/sld) mice

Using monoclonal and polyclonal antibodies to intermediate filaments in the cochleas of normal and mutant mice, we were able to distinguish between different cell types of the stria vascularis. Immunostaining for vimentin was found in the intermediate and basal cells of the stria vascularis of the normal mice. In contrast, vimentin was seen to stain only the basal cells in the mutant mice, confirming our previous findings of the absence of intermediate cells in these cases of hereditary deafness. Immunostaining for cytokeratin was identical in both the mutant and control mice.

Developmental stage-dependent pattern of inner ear expression of intermediate filaments

The expression of vimentin, cytokeratins (CKs) and neurofilament (NF) proteins was analysed (using monoclonal antibodies) in the mouse inner ear at the otocyst stage (13th gestational day), when organogenesis was largely completed (16th gestational day) and at birth (21st gestational day). Co-expression of vimentin and CKs occurred at the otocyst stage. On the 16th gestational day, most epithelial cells lacked immunoreactivity for vimentin and considerable variation in CK positivity was found between different regions of the epithelial lining. At birth, CK positivity was lacking in the developing organ of Corti but was present in other types of epithelium lining the scala media. In the vestibular half of the labyrinth, positivity for CKs was found at the apical surfaces of both sensory cells and supporting cells and in epithelia lining the membranous labyrinth. Vimentin positivity occurred in the greater epithelial ridge of the differentiating organ of Corti. Even at this stage the statoacoustic ganglion comprised two subpopulations of ganglion cells: those staining for NF proteins and those lacking this immunoreactivity. Thus, as the inner ear matures, a pattern of cytoskeletal reorganization occurs that is dependent on developmental stage.