Structural variation in ciliary bundles of the posterior semicircular canal. Quantitative anatomy and computational analysis

Dimensions of a Living Cochlear Hair Bundle

The hair bundle is the mechanosensory organelle of hair cells that detects mechanical stimuli caused by sounds, head motions, and fluid flows. Each hair bundle is an assembly of cellular-protrusions called stereocilia, which differ in height to form a staircase. Stereocilia have different heights, widths, and separations in different species, sensory organs, positions within an organ, hair-cell types, and even within a single hair bundle. The dimensions of the stereociliary assembly dictate how the hair bundle responds to stimuli. These hair-bundle properties have been measured previously only to a limited degree. In particular, mammalian data are either incomplete, lack control for age or position within an organ, or have artifacts owing to fixation or dehydration. Here, we provide a complete set of measurements for postnatal day (P) 11 C57BL/6J mouse apical inner hair cells (IHCs) obtained from living tissue, tissue mildly-fixed for fluorescent imaging, or tissue strongly fixed and dehydrated for scanning electronic microscopy (SEM). We found that hair bundles mildly-fixed for fluorescence had the same dimensions as living hair bundles, whereas SEM-prepared hair bundles shrank uniformly in stereociliary heights, widths, and separations. By determining the shrinkage factors, we imputed live dimensions from SEM that were too small to observe optically. Accordingly, we created the first complete blueprint of a living IHC hair bundle. We show that SEM-prepared measurements strongly affect calculations of a bundle’s mechanical properties – overestimating stereociliary deflection stiffness and underestimating the fluid coupling between stereocilia. The methods of measurement, the data, and the consequences we describe illustrate the high levels of accuracy and precision required to understand hair-bundle mechanotransduction.

Development and evolution of the vestibular apparatuses of the inner ear

The vertebrate inner ear is a labyrinthine sensory organ responsible for perceiving sound and body motion. While a great deal of research has been invested in understanding the auditory system, a growing body of work has begun to delineate the complex developmental program behind the apparatuses of the inner ear involved with vestibular function. These animal studies have helped identify genes involved in inner ear development and model syndromes known to include vestibular dysfunction, paving the way for generating treatments for people suffering from these disorders. This review will provide an overview of known inner ear anatomy and function and summarize the exciting discoveries behind inner ear development and the evolution of its vestibular apparatuses.

Multiscale modeling of mechanotransduction in the utricle

We review recent progress in using numerical models to relate utricular hair bundle and otoconial membrane (OM) structure to the functional requirements imposed by natural behavior in turtles. The head movements section reviews the evolution of experimental attempts to understand vestibular system function with emphasis on turtles, including data showing that accelerations occurring during natural head movements achieve higher magnitudes and frequencies than previously assumed. The structure section reviews quantitative anatomical data documenting topographical variation in the structures underlying macromechanical and micromechanical responses of the turtle utricle to head movement: hair bundles, OM, and bundle-OM coupling. The macromechanics section reviews macromechanical models that incorporate realistic anatomical and mechanical parameters and reveal that the system is significantly underdamped, contrary to previous assumptions. The micromechanics: hair bundle motion and met currents section reviews work based on micromechanical models, which demonstrates that topographical variation in the structure of hair bundles and OM, and their mode of coupling, result in regional specializations for signaling of low frequency (or static) head position and high frequency head accelerations. We conclude that computational models based on empirical data are especially promising for investigating mechanotransduction in this challenging sensorimotor system.

The genetics of hair-cell function in zebrafish

Our ears are remarkable sensory organs, providing the important senses of balance and hearing. The complex structure of the inner ear, or 'labyrinth', along with the assorted neuroepithelia, have evolved to detect head movements and sounds with impressive sensitivity. The rub is that the inner ear is highly vulnerable to genetic lesions and environmental insults. According to National Institute of Health estimates, hearing loss is one of the most commonly inherited or acquired sensorineural diseases. To understand the causes of deafness and balance disorders, it is imperative to understand the underlying biology of the inner ear, especially the inner workings of the sensory receptors. These receptors, which are termed hair cells, are particularly susceptible to genetic mutations - more than two dozen genes are associated with defects in this cell type in humans. Over the past decade, a substantial amount of progress has been made in working out the molecular basis of hair-cell function using vertebrate animal models. Given the transparency of the inner ear and the genetic tools that are available, zebrafish have become an increasingly popular animal model for the study of deafness and vestibular dysfunction. Mutagenesis screens for larval defects in hearing and balance have been fruitful in finding key components, many of which have been implicated in human deafness. This review will focus on the genes that are required for hair-cell function in zebrafish, with a particular emphasis on mechanotransduction. In addition, the generation of new tools available for the characterization of zebrafish hair-cell mutants will be discussed.

Large basolateral processes on type II hair cells are novel processing units in mammalian vestibular organs

Sensory receptors in the vestibular system (hair cells) encode head movements and drive central motor reflexes that control gaze, body movements, and body orientation. In mammals, type I and II vestibular hair cells are defined by their shape, contacts with vestibular afferent nerves, and membrane conductance. Here we describe unique morphological features of type II vestibular hair cells in mature rodents (mice and gerbils) and bats. These features are cytoplasmic processes that extend laterally from the hair cell base and project under type I hair cells. Closer analysis of adult mouse utricles demonstrated that the basolateral processes of type II hair cells vary in shape, size, and branching, with the longest processes extending three to four hair cell widths. The hair cell basolateral processes synapse upon vestibular afferent nerves and receive inputs from vestibular efferent nerves. Furthermore, some basolateral processes make physical contacts with the processes of other type II hair cells, forming some sort of network among type II hair cells. Basolateral processes are rare in perinatal mice and do not attain their mature form until 3-6 weeks of age. These observations demonstrate that basolateral processes are significant signaling regions of type II vestibular hair cells and suggest that type II hair cells may directly communicate with each other, which has not been described in vertebrates.

Vestibular hair cells and afferents: two channels for head motion signals

Vestibular epithelia of the inner ear detect head motions over a wide range of amplitudes and frequencies. In mammals, afferent nerve fibers from central and peripheral zones of vestibular epithelia form distinct populations with different response dynamics and spike timing. Central-zone afferents are large, fast conduits for phasic signals encoded in irregular spike trains. The finer afferents from peripheral zones conduct more slowly and encode more tonic, linear signals in highly regular spike trains. The hair cells are also of two types, I and II, but the two types do not correspond directly to the two afferent populations. Zonal differences in afferent response dynamics may arise at multiple stages, including mechanoelectrical transduction, voltage-gated channels in hair cells and afferents, afferent transmission at calyceal and bouton synapses, and spike generation in regular and irregular afferents. In contrast, zonal differences in spike timing may depend more simply on the selective expression of low-voltage-activated ion channels by irregular afferents.

Dynamic displacement of normal and detached semicircular canal cupula

The dynamic displacement of the semicircular canal cupula and modulation of afferent nerve discharge were measured simultaneously in response to physiological stimuli in vivo. The adaptation time constant(s) of normal cupulae in response to step stimuli averaged 36 s, corresponding to a mechanical lower corner frequency for sinusoidal stimuli of 0.0044 Hz. For stimuli equivalent to 40–200 deg/s of angular head velocity, the displacement gain of the central region of the cupula averaged 53 nm per deg/s. Afferents adapted more rapidly than the cupula, demonstrating the presence of a relaxation process that contributes significantly to the neural representation of angular head motions by the discharge patterns of canal afferent neurons. We also investigated changes in time constants of the cupula and afferents following detachment of the cupula at its apex—mechanical detachment that occurs in response to excessive transcupular endolymph pressure. Detached cupulae exhibited sharply reduced adaptation time constants (300 ms–3 s, n = 3) and can be explained by endolymph flowing rapidly over the apex of the cupula. Partially detached cupulae reattached and normal afferent discharge patterns were recovered 5–7 h following detachment. This regeneration process may have relevance to the recovery of semicircular canal function following head trauma.

Autocorrelation Analysis of Hair Bundle Structure in the Utricle

The ability of hair bundles to signal head movements and sounds depends significantly on their structure, but a quantitative picture of bundle structure has proved elusive. The problem is acute for vestibular organs because their hair bundles exhibit complex morphologies that vary with endorgan, hair cell type, and epithelial locus. Here we use autocorrelation analysis to quantify stereociliary arrays (the number, spacing, and distribution of stereocilia) on hair cells of the turtle utricle. Our first goal was to characterize zonal variation across the macula, from medial extrastriola, through striola, to lateral extrastriola. This is important because it may help explain zonal variation in response dynamics of utricular hair cells and afferents. We also use known differences in type I and II bundles to estimate array characteristics of these two hair cell types. Our second goal was to quantify variation in array orientation at single macular loci and use this to estimate directional tuning in utricular afferents. Our major findings are that, of the features measured, array width is the most distinctive feature of striolar bundles, and within the striola there are significant, negatively correlated gradients in stereocilia number and spacing that parallel gradients in bundle heights. Together with previous results on stereocilia number and bundle heights, our results support the hypothesis that striolar hair cells are specialized to signal high-frequency/acceleration head movements. Finally, there is substantial variation in bundle orientation at single macular loci that may help explain why utricular afferents respond to stimuli orthogonal to their preferred directions.

Mechanical properties and consequences of stereocilia and extracellular links in vestibular hair bundles

Although knowledge of the fine structure of vestibular hair bundles is increasing, the mechanical properties and functional significance of those structures remain unclear. In 2004, Bashtanov and colleagues reported the contribution of different extracellular links to bundle stiffness. We simulated Bashtanov's experimental protocol using a three-dimensional finite element bundle model with geometry measured from a typical striolar hair cell. Unlike any previous models, we separately consider two types of horizontal links: shaft links and upper lateral links. Our most important results are as follows. First, we identified the material properties required to match Bashtanov's experiment: stereocilia Young's modulus of 0.74 GPa, tip link assembly (gating spring) stiffness of 5,300 pN/microm, and the combined stiffness of shaft links binding two adjacent stereocilia of 750 approximately 2,250 pN/microm. Second, we conclude that upper lateral links are likely to have nonlinear mechanical properties: they have minimal stiffness during small bundle deformations but stiffen as the bundle deflects further. Third, we estimated the stiffness of the gating spring based on our realistic three-dimensional bundle model rather than a conventional model relying on the parallel arrangement assumption. Our predicted stiffness of the gating spring was greater than the previous estimation.

Computational models of hair cell bundle mechanics: II. Simplified bundle models

Simplified versions of hair cell bundles are mechanically modeled. The influence of various geometric and material combinations on bundle stiffness, link tensions and deformation shape are examined. Three models are analyzed within this paper: two stereocilia connected by one link, two stereocilia connected by a biologically realistic set of links, and a column of stereocilia connected by realistic links. Stereocilia are modeled using a distributed parameter model [J. Biomech. Eng. 122, 44]. Some fundamental rules for linking bundles emerge from these tests: (1) Links must have a threshold stiffness value for the bundle to deform as a whole. Beyond this value, the stereocilia are perfectly linked and variations in link stiffness do not significantly effect the bundle stiffness or link tension. (2) Decreasing the relative heights of successive stereocilia may increase link tension while decreasing bundle stiffness. (3) When lateral links exist, the top most lateral links carry the majority of tension. Lower links in single column model appear mechanically insignificant. (4) Extending the length of the bundle in a column does not increase the stiffness once the column reaches a certain length.

Computational models of hair cell bundle mechanics: III. 3-D utricular bundles

Six utricular hair bundles from a red-eared turtle are modeled using 3-D finite element analysis. The mechanical model includes shear deformable stereocilia, realignment of all forces during force load increments, and tip and lateral link inter-stereocilia connections. Results show that there are two distinct bundle types that can be separated by mechanical bundle stiffness. The more compliant group has fewer total stereocilia and short stereocilia relative to kinocilium height; these cells are located in the medial and lateral extrastriola. The stiff group are located in the striola. They have more stereocilia and long stereocilia relative to kinocilia heights. Tip link tensions show parallel behavior in peripheral columns of the bundle and serial behavior in central columns when the tip link modulus is near or above that of collagen (1x10(9) N/m(2)). This analysis shows that lumped parameter models of single stereocilia columns can show some aspects of bundle mechanics; however, a distributed, 3-D model is needed to explore overall bundle behavior.

Differences between stereocilia numbers on type I and type II vestibular hair cells

A major outstanding goal of vestibular neuroscience is to understand the distinctive functional roles of type I and type II hair cells. One important question is whether these two hair cell types differ in bundle structure. To address this, we have developed methods to characterize stereocilia numbers on identified type I and type II hair cells in the utricle of a turtle, Trachemys scripta. Our data indicate that type I hair cells, which occur only in the striola, average 95.9 +/-16.73 (SD) stereocilia per bundle. In contrast, striolar type II hair cells have 59.9 +/- 8.98 stereocilia, and type II hair cells in the adjacent extrastriola average 44.8 +/- 10.82 stereocilia. Thus type I hair cells have the highest stereocilia counts in the utricle. These results provide the first direct evidence that type I hair cells have significantly more stereocilia than type II hair cells, and they suggest that the two hair cell types may differ in bundle mechanics and peak mechanoelectric transduction currents.

Quantitative analysis of stereociliary arrays on vestibular hair cells

We have developed a method for quantifying the number, spacing, and distribution of stereocilia on the apical surface of hair cells using spatial autocorrelation analysis and statistics for directional data. Here, we illustrate the method using idealized hair bundles, and we apply it to scanning micrographs of turtle hair cells from the utricle and posterior canal, and to freeze-fracture preparations of bullfrog saccule. The analysis suggests three common features of stereociliary bundles. First, bundle geometries form a continuum from 'loose' to 'tight' rather than two distinct groups. Second, interciliary spacing along the three hexagon axes is not equal; spacing is usually widest along the hexagon axis closest to the bundle's axis of bilateral symmetry (the presumptive excitatory axis). Third, spacing between stereocilia changes with distance from the kinocilium. All three features will influence predictions of the tip link tensions that accompany bundle deflection.

Structure and growth of the utricular macula in the inner ear of the slider turtle Trachemys scripta

In general, postembryonic production of inner ear vestibular hair cells (HCs) is believed to occur in all nonmammalian vertebrates. However, no study on this topic has been published on reptiles and, consequently, it is not known whether this also applies to these vertebrates. Therefore, the present study applied stereological methods in order to estimate the total number of HCs in turtles of varying sizes. The findings are that in prehatchlings the utricular macula (UM) contains approximately 4000 HCs as compared to approximately 5000 in juveniles, approximately 8000 in medium-sized turtles, and approximately 12,000 in large, sexually mature turtles. Scanning electron microscopy (SEM) reveals that presumably newly generated HCs with small surface areas and thin stereovilli are found in all regions of the UM. Furthermore, it reveals that utricular HCs can be classified as belonging to a specific region from the morphology of their apical structure. Striolar HCs have a large free oval-to-ovoid surface, a hair bundle with numerous stereovilli, and a short kinocilium. Rampary and cotillary HCs have smaller and slimmer free surfaces, comparatively fewer stereovilli, but much longer kinocilia. In conclusion, the current study demonstrates that postembryonic production of HCs does occur in reptiles and thereby supports the general view that this is a common trait in all nonmammalian vertebrates.

Hair bundle profiles along the chick basilar papilla

Cochlear hair cells play a central role in the transduction of sound into neural output. Anatomical descriptions of these cells, and their protruding hair bundles, are of fundamental interest since hair cell transduction is dependent on hair bundle micromechanics and hair bundle micromechanics depends on hair bundle morphology. In this paper, we describe quantitatively changes in the staircase profile of the hair bundle along the apical portion of the chick's basilar papilla. Images of hair cells from 8 discretely dissected segments of the apical 3rd of the basilar papilla were archived, and the profile contour outlined by the tips of the stereocilia was digitised and curves were fitted by linear and power equations. The hair bundles of tall hair cells exhibited both linear and curvilinear profiles, which were equally distributed along the papilla. All short hair cells in our sample had straight contours. The differences in hair bundle shape among the tall hair cells may lead to differential susceptibility to injury and some variance in the current-displacement transduction curves due to differences in the translation of forces throughout the hair bundle.

Structure and function in the saccule of the goldfish (Carassius auratus): a model of diversity in the non-amniote ear

The vertebrate inner ear is comprised of a remarkable diversity of cell types, including several types of sensory hair cells. In amniotes (reptiles, birds, and mammals), the morphological and physiological characteristics that distinguish these cell types have been well documented, while cellular variation in the ears of non-amniotes (all other vertebrate groups) has remained underrecognized. Since non-amniotes have become increasingly popular models for developmental and genetic research, a more comprehensive understanding of structure and function in the inner ears of these species is warranted. This paper first reviews the large body of data describing the morphology and physiology of hair cells and afferent neurons in the inner ear of the goldfish (Carassius auratus). In particular, we examine the structure of the goldfish saccule, an endorgan that has been the subject of numerous investigations on audition. New data on the structural variation of synaptic bodies in saccular hair cells are also presented, and the functional implications of these data are discussed. Finally, we conclude that hair cell structure varies along the length of the goldfish saccule in a manner consistent with known physiological characteristics of the endorgan. The saccule provides an excellent model for investigating structure-function relationships in the vertebrate inner ear, as well as the development of auditory and vestibular sensory epithelia.

A finite element method for mechanical response of hair cell ciliary bundles

This paper describes the development of a methodology for performing a mechanical analysis of hair cell ciliary bundles. The cilia were modeled as shear deformable beams, and interconnections were modeled as two-force members. These models were incorporated into software, which performs a finite element analysis of a user-defined bundle. The algorithm incorporates aspects of the bundle such as geometric realignment and buckling of compressed side links. A sample bundle is introduced and results of modeling it are presented.

Distal separation of chick cochlear hair cell stereocilia: analysis of contact-constraint models

One model often used in the study of hair bundle micromechanics assumes simple geometric relationships between hair displacements, constrained by contact between neighboring hairs at their distal tips. Recent observations of hair bundle motion provided the opportunity to evaluate the contact-constraint model against measured displacements for the tallest and shortest sensory hairs. A contact-constraint model was developed based on the geometry of a single column of stereocilia. The model used morphological data from chick hair bundles for which displacement data in the excitatory and inhibitory directions were also available. For each hair bundle, a unique sensory hair radius was determined so that the calculated resting bundle morphology matched the measured values. The model was then evaluated against the displacement data for each hair bundle. In each case, the model underestimated the excitatory displacement of the shortest hairs. Failure of the model to accurately predict bundle motion raises the possibility of a distal separation between the hairs at rest. It is suggested that tip links pull the hairs through this separation during excitatory deflections. Perhaps at damaging levels of displacement, the hairs suddenly come into contact, tip-link tension dramatically increases, and the tip-links are fractured.

Postnatal development of type I and type II hair cells in the mouse utricle: acquisition of voltage-gated conductances and differentiated morphology

The type I and type II hair cells of mature amniote vestibular organs have been classified according to their afferent nerve terminals: calyx and bouton, respectively. Mature type I and type II cells also have different complements of voltage-gated channels. Type I cells alone express a delayed rectifier, gK,L, that is activated at resting potential. We report that in mouse utricles this electrophysiological differentiation occurs during the first postnatal week. Whole-cell currents were recorded from hair cells in denervated organotypic cultures and in acutely excised epithelia. From postnatal day 1 (P1) to P3, most hair cells expressed a delayed rectifier that activated positive to resting potential and a fast inward rectifier, gK1. Between P4 and P8, many cells acquired the type I-specific conductance gK,L and/or a slow inward rectifier, gh. By P8, the percentages of cells expressing gK,L and gh were at mature levels. To investigate whether the electrophysiological differentiation correlated with morphological changes, we fixed utricles at different times between P0 and P28. Ultrastructural criteria were developed to classify cells when calyces were not present, as in cultures and neonatal organs. The morphological and electrophysiological differentiation followed different time courses, converging by P28. At P0, when no hair cells expressed gK,L, 33% were classified as type I by ultrastructural criteria. By P28, approximately 60% of hair cells in acute preparations received calyx terminals and expressed gK,L. Data from the denervated cultures showed that neither electrophysiological nor morphological differentiation depended on ongoing innervation.

Are There Parallel Channels in the Vestibular Nerve?

A popular concept in neurobiology is that sensory information is transmitted to the central nervous system over parallel channels of neurons that play different functional roles. But alternative organizing schemes are possible, and it is useful to ask whether some other framework might better account for the diversity of vestibular primary afferents.