Intracellular distribution of actin in cells of the organ of Corti: a structural basis for cell shape and motility

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

Pronounced infracuticular endocytosis in mammalian outer hair cells

Endocytosis in cochlear hair cells was investigated by staining with the vital fluorescent dye FM 1-43, that partitions reversibly into membranes and is trapped in vesicles during endocytosis. The temporal development and spatial distribution of FM 1-43 induced fluorescence was investigated using confocal laser-scanning microscopy. FM 1-43 rapidly and intensely stained cochlear hair cells, leaving the supporting cells unstained. For short application (0.2-30 s), only the infracuticular region of outer hair cells (OHCs) was labeled, whereas for long application (30-60 s), the OHCs were also labeled in the infranuclear zone and along a central strand extending from the infracuticular zone down to the nucleus, as well as along the entire cell membrane. Except for the cell membrane, the infracuticular zone, directly below the cuticular plate, showed the most rapid and intense staining, and in most cases staining was spherically shaped with a diameter of 3-7 microm. Localization and size of this infracuticular staining coincided with Hensen's body, a specialized variant of the endoplasmic reticulum. In contrast to the OHCs, apical fluorescence of inner hair cells presented a homogeneous distribution. When OHCs were incubated in FM 1-43 for longer than 1 min, many points of contact between the central strand, the infracuticular zone and the lateral cell membrane were observed. Since Hensen's bodies are a specialty of OHCs and the fluorescent staining pattern of these cells was unique, it is proposed that Hensen's body is involved in the turnover of OHC-specific proteins, such as those involved in the molecular machinery of the motor action of the plasma membrane.

Pathology of the cochlea following a spontaneous mutation in DBA/2 mice

The DBA/2 strain of mice usually presents with noise-induced epileptic seizures and hearing disorders. After a spontaneous mutation a strain with early hearing loss and circling behaviour was produced. This strain presents with clinical symptoms found in diseases connected to inner ear disorders. These animals do not suffer from periodical disorders, however, but have functional disturbances continuously and can therefore serve as an animal model for diseases originating from both parts of the inner ear. The genetic inheritance appears to be autosomal recessive. Offspring showed circling behaviour and severe pathology in the vestibular part of the inner ear. In the present study pathology of the cochlear part of the inner ear was visualized using conventional microscopical techniques. The content of actin and fodrin was labelled immunohistochemically, and hearing was assessed with auditory brainstem recordings. After 1 month the animals showed deterioration of the cochlear part of the inner ear. At 6 months no organ of Corti remained and the animals were deaf. Transmission and scanning electron microscopy revealed severe apical hair cell changes. The content of alpha-actinin and fodrin in the DBA/2 mouse was already fainter than that in age-matched CBA control mice at the age of 1 month. Labelling of antibodies against fodrin increased in the supporting cells of the older animals, probably owing to the replacement of hair cells.

Localization of actin in basal cells of stria vascularis

The distribution of actin in the lateral wall of the cochlear duct was investigated in the gerbil, rat, mouse and hamster. A monoclonal antibody specific for muscle alpha and gamma actins, and a polyclonal antiserum reactive with smooth muscle gamma and non-muscle beta actins yielded strong immunostaining of basal cells in the stria vascularis and of smooth muscle cells in lateral wall blood vessels. Both cell types stained in all four genera. Diffuse cytosolic staining was observed along the full-length of the basal cell layer including the blunt cell processes which they extend toward strial marginal cells. Immunoreactive basal cells appeared continuous with morphologically similar cells investing vessels penetrating the stria from the spiral ligament. The basal cells failed to bind antibodies to smooth muscle alpha actin and sarcomeric actin. By electron microscopic immunocytochemistry, gold labeling for actin was observed on dense, fine fibrils in the cytoplasm of the basal cells. In paraffin sections adjacent to those stained for actin, antibody to vimentin stained intermediate and basal cells in the stria vascularis whereas antibody to isoform 1 of the facilitated glucose transporter protein family (GLUT1) labeled only the non-overlapping apical and basal plasmalemma of basal cells. Content of vimentin, GLUT1 and muscle gamma actin supports the derivation of basal cells from mesoderm. The presence of stress fibers containing muscle gamma actin points to a possible contractile activity of basal cells which conceivably could be related to transport of K+ to and within the intrastrial compartment or regulation of blood flow in the stria vascularis.

The rosette complex in gerbil Deiters cells contains gamma actin

The relationship of selected cytoskeletal elements with the rosette complex of Deiters cells was examined immunocytochemically in the gerbil cochlea. By light microscopy, the staining pattern for actin in the apical portion of Deiters cells corresponded with the location of the rosette complex. At the ultrastructural level, the actin antibodies bound selectively at the periphery of the dense trabeculae in the center of the complex. Comparative staining with a battery of polyclonal and isoform-specific monoclonal antibodies revealed selective presence of the gamma muscle actin isoform in this location. The loose meshwork at the periphery of the rosette complex stained selectively with a monoclonal antibody to vimentin. beta-tubulin was not associated with the rosette complex but occurred in abundance in the microtubule-rich stalk. Actin and vimentin were not detected in the apical compartment of Deiters cells at the extreme base of the cochlea, thus confirming their association with the rosette complex which is not present in regions of the gerbil cochlea tuned to frequencies of 20 kHz or higher (Spicer and Schulte, 1993, 1994). The cytoarchitecture of the rosette complex and its preferential distribution along the place-frequency map promote speculation that Deiters cells may play a role in regulating ion homeostasis and/or micromechanical response properties of the organ of Corti.

Fimbrin expression in the developing rat cochlea

The expression of fimbrin in the developing rat cochlea was analyzed using an immunohistochemical technique with fimbrin antibody. The cochlea displayed temporal and lateral-longitudinal gradients for fimbrin expression during development. Fimbrin immunoreactivity first appeared in the inner hair cell stereocilia of the basal turn on the first gestational day studied (day 18). At birth, both inner (IHC) and outer hair cell (OHC) stereocilia of the basal turn showed positive labeling with fimbrin antibody. The progression of appearance was always from IHCs to OHCs and fimbrin immunostaining appeared in the apical hair cells by postnatal day 6. Immunostaining was restricted to stereocilia and the cuticular plate, and no immunoreactivity was observed in neighboring structures of the epithelium. Double labeling using both fimbrin antibody and phalloidin binding revealed similar chronological expression from the earliest stage studied. Increasing fimbrin immunoreactivity was observed in hair cells until late postnatal and adult stages. This study suggests that fimbrin is expressed with F-actin during development and fimbrin together with actin may constitute the two basic molecules that participate in stereocilia formation. We speculate that fimbrin may help maintain the parallel growth of actin filaments within the stereocilia. These data additionally support previous findings that hair cell maturation occurs from the base to the apex and from IHCs to OHCs.

Cytoskeletal proteins in human hair cells

Human cochlear and vestibular hair cells (HCs) were analysed for the three major cytoskeletal types of structures: microfilaments, microtubules, and intermediate filaments and several of their associated proteins. Specific immunomorphological features were identified: the apical surface, the infracuticular rod of F-actin, the cytoplasm and the HC bases and their interactions with adjacent supporting cells. There is morphological evidence for at least three functionally interacting units: the OHC as a cytoskeletal string interacting with the Deiter's cell, being a part of the rigid pancellular protein shell of the organ of Corti; compartmentalization of HCs (surrounded by rigid intermediate filament proteins in the supporting cells); and a calcium-guided short-loop feed-back control of type I HCs. The cytoskeletal composition of all four types of human HCs and their adjacent structures is similar to, although not identical with, corresponding compositions in other mammalian species.

Vestibular morphology in relation to age and circling behavior

The DBA/2 strain of mice have genetically induced vestibular dysfunction that presents grossly at an early age as circling behavior and abnormal righting reflexes. The vestibular morphology of this strain has not previously been examined. DBA/2 mice of different ages that showed circling behavior were sacrificed and then had their inner ears immediately removed and fixed in glutaraldehyde. The specimens were prepared for light, transmission and scanning electron microscopy. Additional specimens at 10 months of age were fixed with paraformaldehyde for immunohistochemical investigation and labelling of alpha-actinin. Non-circling litter mates served as controls. The morphology and immunohistochemistry of the vestibular end-organs were evaluated as a function of age and circling behavior. The sensory epithelium of the ampulla and utricle in the circling mice showed softening of the cuticle, hair cell cytoplasmic herniation, expelled cellular debris, fused stereocilia and giant hair cells that progressively increased in severity with age. The non-circling litter mates showed similar but less severe pathology of the vestibular sensory epithelium. The immunohistochemical analysis showed no differences at a magnification of 400 x.

The cuticular plate of the hair cell in relation to morphological gradients of the chicken basilar papilla

The aim of the present study was to provide details on the diversity and morphological gradients in the anatomy of the cuticular plate of hair cells in the chicken basilar papilla. The structure of the cuticular plate, which is mainly made up of a network of actin filaments, may be related to differences in the mechanical demands on the anchorage of the stereovillar bundle. We describe the morphological gradients in the cuticular plates as seen in transverse section for four positions along the basilar papilla. Three different shapes of cuticular plate could be distinguished. In general, cuticular plates in neurally-lying hair cells have their main mass on the neural side of the cells; for abneural cells, the converse is true. The shape of the plates changes gradually across the papilla; symmetrical forms exist. The hair-cell bundle orientation (and thus the preferred direction of stimulation of the bundle), as measured using scanning EM preparations, does not correlate with the shape of the plate in transverse section. The present data confirm the notion developed from other studies that (1) there are no distinct populations of hair cells, (2) there are no linear or monotonic morphological gradients, and (3) the gradients on the papilla are species- and position-specific.

Effects of endolymphatic and perilymphatic application of salicylate in the pigeon. II: Fine structure of auditory hair cells

Large doses of salicylate are known to cause reversible ototoxic effects including fine structural alterations of the auditory hair cells in mammals. To investigate possible fine structural correlates of salicylate effects on pigeon auditory hair cells, the basilar papillae following perilymphatic or endolymphatic application of salicylate were fixed and processed for transmission electron microscopy. The pigeon auditory hair cells possessed organelles typically described in avians. A single or multi-layered array of cisternae along the cytoplasmic side of the lateral plasma membrane, i.e. subsurface cisternae that are characteristic for mammalian outer hair cells, was not seen. The most prominent fine structural alterations of hair cells after salicylate application were an increase in the luminal width of smooth and rough endoplasmic reticulum as well as the frequent occurrence of prominent single-membrane-bound vesicles filled with electron-dense bodies. Based on the assumption that subsurface cisternae represent a specialized form of endoplasmic reticulum, the present findings indicate that the structural correlates of salicylate toxicity are similar in mammalian and avian auditory hair cells.

Expression of actin isoforms in the guinea pig organ of Corti: muscle isoforms are not detected

The specificity of antibodies to actin was assayed by use of immunoblots and histological sections of control tissues enriched for each of six different isoforms. On immunoblots, all antibodies stained at most one band of protein in most of the control materials, with a molecular weight of approximately 43 kDa. Their pattern of staining of muscle and nonmuscle tissues indicated their isoform specificity. On tissue sections, immunocytochemical staining demonstrated cellular and subcellular localization of the different isoforms. Once characterized with regard to specificity, these antibodies were used to probe actin in the guinea pig organ of Corti. None of the four muscle isoforms of actin were found in either immunoblots or tissue sections of the organ of Corti. Both beta- and gamma-cytoplasmic isoforms of actin were present in hair cells and supporting cells. This leaves open to investigation the role which cytoplasmic actins play in these cells of the organ of Corti.

Distribution of actin in developing outer hair cells in the gerbil

During the two weeks following the onset of cochlear function in the gerbil, active cochlear processes appear to mature. The active cochlear processes likely involve outer hair cells with their specialized lateral wall structures, including the subsurface cisternae and associated cytoskeletal elements. We have previously demonstrated that the subsurface cisternae mature gradually during the time that active cochlear processes mature in the gerbil. In the study reported here, we used postembedding immunocytochemical electron microscopy to investigate whether actin labelling associated with the cortical cytoskeleton of the gerbil outer hair cell increased concomitantly. In contrast to the gradual development of the subsurface cisternae, actin labelling in the region of the cortical cytoskeleton significantly increased during the onset of cochlear function and maintained this level during the time that active cochlear processes mature. Thus, it appears that increased actin adjacent to the lateral plasma membrane of the outer hair cell is related to the onset of cochlear function rather than to the maturation of active cochlear processes.

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.

Immunohistochemical localization of intracellular Ca-ATPase in outer hair cells, neurons and fibrocytes in the adult and developing inner ear

Intracellular isoforms of the enzyme Ca-ATPase were identified in the inner ear by immunostaining paraffin sections with a polyclonal antiserum against rabbit cardiac muscle Ca-ATPase. In the adult cochlea, intense staining was present at the lateral border of outer hair cells in regions corresponding with the distribution of the subsurface cisternal system. Other cell types containing high levels of Ca-ATPase were skeletal muscle fibers in the tensor tympani, vascular smooth muscle, spiral ganglion neurons and subpopulations of fibrocytes in the limbus, spiral ligament and underlying vestibular neurosensory epithelium. In neonatal gerbils, staining of tensor tympani muscle fibers was observed at 4 days after birth and approached adult levels by 8 days after birth. Ca-ATPase was first detected in other cell types between postnatal days 12 and 14 but immunostaining still remained well below the intensity seen in adults at 20 days after birth. The demonstration of abundant calcium pumps in the subsurface cisternae confirms the role of this organelle as an intracellular reservoir for Ca2+ in outer hair cells. The presence of high levels of Ca-ATPase in spiral ganglion neurons and in fibrocytes specialized for ion transport points to a role for the enzyme in regulating the activity of other cell types of importance to normal hearing.

Ontogenesis of F-actin in hair cells

This report describes the ontogenesis of cochlear stereocilia using scanning electron microscopy for analysis of cilia appearance, and fluorescence microscopy of phalloidin, a label for F-actin, to determine the maturation of the cilia framework. Surface and frozen-sectioned preparations of the otic capsule were obtained from several stages of rat pup development beginning at the 16th gestational day and at various stages until adulthood. In the earliest stage investigated, strong fluorescence labeling was visible on the apical part of Kölliker's organ, revealing a reticular outline of cell junctions. Hair cells started to differentiate at the 18th day of gestation from cells within the primordial receptor area. Phalloidin labeling revealed a sequential appearance of F-actin as the hair cells differentiated from the cells with the Kölliker's organ. The differentiation of receptor cells occurred first with the appearance of a junctional complex between the hair cell and the surrounding cells. Then a cuticular plate appeared followed by the progressive emergence of stereocilia. The F-actin labeling also revealed a progressive differentiation of receptor cells from the cochlear base to its apex. There was also an inner to outer hair cell developmental gradient of label. Inner hair cells developed stereocilia before outer hair cells. The third row of outer hair cells was the last to acquire stereocilia. The adult pattern of stereocilia was reached around the 6th postnatal day. We conclude that the appearance of actin filaments in developing receptor cells and the emergence of stereocilia can be regraded as markers for correlating function and other structural differentiation.

Actin distribution along the lateral wall of gerbil outer hair cells

Outer hair cells can contract parallel to their long axis, and it has been hypothesized that actin may play a role in this contraction. In this study, actin distribution was examined in the gerbil organ of Corti using postembedment immunoelectron microscopy. In addition to regions typically labelled by actin antibodies and observed by epifluorescence--the cuticular plate, stereocilia, and supporting cell processes--these procedures preserved the ultrastructure of the cell and allowed us to demonstrate actin reactivity along the lateral wall of the outer hair cells between the subsurface cisterns and the plasma membrane. This region is the location of structures (pillars and cortical cytoskeleton) though to be associated with contraction of the outer hair cells.

Characterization of inner ear sensory hair cells after rapid-freezing and freeze-substitution

In order to determine the ultrastructural organization of normal cells and to understand better the anatomical substrates for outer hair cell motility, cryofixation was performed on the sensory epithelium of the inner ear of guinea pigs prior to substitution of frozen water with organic solvents containing chemical fixatives. In this way cells would not be altered by the direct application of the chemicals commonly used for preservation, which are also known to cause fixation-induced shape changes in outer hair cells. Following rapid freezing and freeze-substitution, preservation of cells within the isolated sensory epithelium containing the organ of Corti was similar to that seen in conventionally fixed cells. However, in rapidly frozen and freeze-substituted outer hair cells the cytoplasm and the cellular membranes differed from those seen in conventionally fixed preparations. The cytoplasmic matrix was densely packed with filaments and stained homogeneously, suggesting better preservation of the cytoskeleton and less extraction of the soluble ground substance. Cell membranes were smooth, indicating that fixation-induced shape changes and shrinkage had been avoided. The subsurface cisternal system of intracellular membranes lining the lateral wall of the outer hair cells was composed of continuous, tightly packed, parallel rows of membranous lamellae. Thus rapid-freezing and freeze-substitution are important techniques by which structural alterations correlated with outer hair cell motility can be separated from fixation-induced cell shape changes.

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.

Concomitant salicylate-induced alterations of outer hair cell subsurface cisternae and electromotility

Isolated cochlear outer hair cells undergo rapid, force-generating length changes in response to electrical stimulation. The cellular mechanism responsible for electromotility and its structural substrate is not yet known. Salicylates reduce and block electromotility in vitro. Therefore, we exposed isolated outer hair cells from the guinea pig cochlea to various doses of sodium salicylate and evaluated both ultrastructural changes and responses to electrical stimulation. Following salicylate superfusion, the subsurface cisternae showed dilatation, vesiculation and a deviation from their normal, unfenestrated, axial orientation below the plasma membrane. These changes were time and dose dependent and reversible over a time course of about 30 min. Electromotility was blocked and showed recovery following the same time course as the salicylate-induced reversible structural changes. These results indicate that intact, unfenestrated subsurface cisternae are required for the optimal generation of electrically-induced motility in mammalian outer hair cells.