Inner hair cell stereocilia are embedded in the tectorial membrane.

Mammalian hearing depends on sound-evoked displacements of the stereocilia of inner hair cells (IHCs), which cause the endogenous mechanoelectrical transducer channels to conduct inward currents of cations including Ca2+. Due to their presumed lack of contacts with the overlaying tectorial membrane (TM), the putative stimulation mechanism for these stereocilia is by means of the viscous drag of the surrounding endolymph. However, despite numerous efforts to characterize the TM by electron microscopy and other techniques, the exact IHC stereocilia-TM relationship remains elusive. Here we show that Ca2+-rich filamentous structures, that we call Ca2+ ducts, connect the TM to the IHC stereocilia to enable mechanical stimulation by the TM while also ensuring the stereocilia access to TM Ca2+. Our results call for a reassessment of the stimulation mechanism for the IHC stereocilia and the TM role in hearing.

Cochlear partition anatomy and motion in humans differ from the classic view of mammals.

Mammals detect sound through mechanosensitive cells of the cochlear organ of Corti that rest on the basilar membrane (BM). Motions of the BM and organ of Corti have been studied at the cochlear base in various laboratory animals, and the assumption has been that the cochleas of all mammals work similarly. In the classic view, the BM attaches to a stationary osseous spiral lamina (OSL), the tectorial membrane (TM) attaches to the limbus above the stationary OSL, and the BM is the major moving element, with a peak displacement near its center. Here, we measured the motion and studied the anatomy of the human cochlear partition (CP) at the cochlear base of fresh human cadaveric specimens. Unlike the classic view, we identified a soft-tissue structure between the BM and OSL in humans, which we name the CP "bridge." We measured CP transverse motion in humans and found that the OSL moved like a plate hinged near the modiolus, with motion increasing from the modiolus to the bridge. The bridge moved almost as much as the BM, with the maximum CP motion near the bridge-BM connection. BM motion accounts for 100% of CP volume displacement in the classic view, but accounts for only 27 to 43% in the base of humans. In humans, the TM-limbus attachment is above the moving bridge, not above a fixed structure. These results challenge long-held assumptions about cochlear mechanics in humans. In addition, animal apical anatomy (in SI Appendix) doesn't always fit the classic view.

Unión cráneo-cervical - anatomía normal y correlación con imágenes / Cranio Cervical Junction - Anatomy and Imaging Correlation

La solicitud de estudios de imagen en pacientes con trauma cervical es muy frecuente en la práctica diaria. Esa patología es causa relativamente frecuente de discapacidad en pacientes jóvenes junto con el trauma encéfalo craneano. En un porcentaje no despreciable de los casos, las lesiones traumáticas comprometen la unión cráneo- cervical y en esos pacientes, la morbi-mortalidad es más significativa. La transición entre el cráneo y el raquis se basa en un conjunto de estructuras óseas relacionadas por articulaciones muy móviles y estabilizadas por un grupo de ligamentos y músculos que le brindan al mismo tiempo gran solidez. Para una correcta interpretación de los estudios de imagen de uso corriente en la clínica, es fundamental un sólido conocimiento anatómico de la unión cráneo-cervical y sus componentes. Es el objetivo de esta revisión, sistematizar la anatomía de la unión cráneo-cervical con especial énfasis en sus ligamentos, analizar la fisiología de sus movimientos y el concepto de estabilidad para luego realizar una correlación con tomografía computada multi-detector y resonancia magnética.

The request of imaging techniques in patients with cervical spine trauma is very common in clinical practice. Cervical trauma is a relatively common cause of disability in young patients. In a significant percentage of cases traumatic injuries compromise the cranio-cervical junction with more important morbidity and mortality in this group of patients. The transition between the skull and the spine is based on a set of bony structures, high mobility joints, and stabilization mechanism formed by a group of ligaments and muscles. A solid anatomical knowledge of the cranio-cervical junction and its components is essential for a correct interpretation of current high resolution imaging studies. The goal of this review is highlight the anatomy of the cranio-cervical junction with special emphasis on the ligaments, analyze the biomechanics of their movements and the concept of stability. At last but not leastwe will establish a correlation with multidetector computed tomography and high-resolutionmagnetic resonance imaging.

Spontaneous Otoacoustic Emissions in TectaY1870C/+ Mice Reflect Changes in Cochlear Amplification and How It Is Controlled by the Tectorial Membrane.

Spontaneous otoacoustic emissions (SOAEs) recorded from the ear canal in the absence of sound reflect cochlear amplification, an outer hair cell (OHC) process required for the extraordinary sensitivity and frequency selectivity of mammalian hearing. Although wild-type mice rarely emit, those with mutations that influence the tectorial membrane (TM) show an incidence of SOAEs similar to that in humans. In this report, we characterized mice with a missense mutation in Tecta, a gene required for the formation of the striated-sheet matrix within the core of the TM. Mice heterozygous for the Y1870C mutation (TectaY1870C/+ ) are prolific emitters, despite a moderate hearing loss. Additionally, Kimura's membrane, into which the OHC stereocilia insert, separates from the main body of the TM, except at apical cochlear locations. Multimodal SOAEs are also observed in TectaY1870C/+ mice where energy is present at frequencies that are integer multiples of a lower-frequency SOAE (the primary). Second-harmonic SOAEs, at twice the frequency of a lower-frequency primary, are the most frequently observed. These secondary SOAEs are found in spatial regions where stimulus-evoked OAEs are small or in the noise floor. Introduction of high-level suppressors just above the primary SOAE frequency reduce or eliminate both primary and second-harmonic SOAEs. In contrast, second-harmonic SOAEs are not affected by suppressors, either above or below the second-harmonic SOAE frequency, even when they are much larger in amplitude. Hence, second-harmonic SOAEs do not appear to be spatially separated from their primaries, a finding that has implications for cochlear mechanics and the consequences of changes to TM structure.

Molecular organization and fine structure of the human tectorial membrane: is it replenished?

Auditory sensitivity and frequency resolution depend on the physical properties of the basilar membrane in combination with outer hair cell-based amplification in the cochlea. The physiological role of the tectorial membrane (TM) in hair cell transduction has been controversial for decades. New insights into the TM structure and function have been gained from studies of targeted gene disruption. Several missense mutations in genes regulating the human TM structure have been described with phenotypic expressions. Here, we portray the remarkable gradient structure and molecular organization of the human TM. Ultrastructural analysis and confocal immunohistochemistry were performed in freshly fixed human cochleae obtained during surgery. Based on these findings and recent literature, we discuss the role of human TMs in hair cell activation. Moreover, the outcome proposes that the α-tectorin-positive amorphous layer of the human TM is replenished and partly undergoes regeneration during life.

Two-compartment passive frequency domain cochlea model allowing independent fluid coupling to the tectorial and basilar membranes.

The cochlea is a spiral-shaped, liquid-filled organ in the inner ear that converts sound with high frequency selectivity over a wide pressure range to neurological signals that are eventually interpreted by the brain. The cochlear partition, consisting of the organ of Corti supported below by the basilar membrane and attached above to the tectorial membrane, plays a major role in the frequency analysis. In early fluid-structure interaction models of the cochlea, the mechanics of the cochlear partition were approximated by a series of single-degree-of-freedom systems representing the distributed stiffness and mass of the basilar membrane. Recent experiments suggest that the mechanical properties of the tectorial membrane may also be important for the cochlea frequency response and that separate waves may propagate along the basilar and tectorial membranes. Therefore, a two-dimensional two-compartment finite difference model of the cochlea was developed to investigate the independent coupling of the basilar and tectorial membranes to the surrounding liquid. Responses are presented for models using two- or three-degree-of-freedom stiffness, damping, and mass parameters derived from a physiologically based finite element model of the cochlear partition. Effects of changes in membrane and organ of Corti stiffnesses on the individual membrane responses are investigated.

Dual traveling waves in an inner ear model with two degrees of freedom.

We calculate traveling waves in the mammalian cochlea, which transduces acoustic vibrations into neural signals. We use a WKB-based mechanical model with both the tectorial membrane (TM) and basilar membrane (BM) coupled to the fluid to calculate motions along the length of the cochlea. This approach generates two wave numbers that manifest as traveling waves with different modes of motion between the BM and TM. The waves add differently on each mass, producing distinct tuning curves and different characteristic frequencies (CFs) for the TM and the BM. We discuss the effect of TM stiffness and coupling on the waves and tuning curves. We also consider how the differential motions between the masses could influence the cochlear amplifier and how mode conversion could take place in the cochlea.

Development of tonotopy in the auditory periphery.

Acoustic frequency analysis plays an essential role in sound perception, communication and behavior. The auditory systems of most vertebrates that perceive sounds in air are organized based on the separation of complex sounds into component frequencies. This process begins at the level of the auditory sensory epithelium where specific frequencies are distributed along the tonotopic axis of the mammalian cochlea or the avian/reptilian basilar papilla (BP). Mechanical and electrical mechanisms mediate this process, but the relative contribution of each mechanism differs between species. Developmentally, structural and physiological specializations related to the formation of a tonotopic axis form gradually over an extended period of time. While some aspects of tonotopy are evident at early stages of auditory development, mature frequency discrimination is typically not achieved until after the onset of hearing. Despite the importance of tonotopic organization, the factors that specify unique positional identities along the cochlea or basilar papilla are unknown. However, recent studies of developing systems, including the inner ear provide some clues regarding the signalling pathways that may be instructive for the formation of a tonotopic axis.

Tectorial membrane morphological variation: effects upon stimulus frequency otoacoustic emissions.

The tectorial membrane (TM) is widely believed to play an important role in determining the ear's ability to detect and resolve incoming acoustic information. While it is still unclear precisely what that role is, the TM has been hypothesized to help overcome viscous forces and thereby sharpen mechanical tuning of the sensory cells. Lizards present a unique opportunity to further study the role of the TM given the diverse inner-ear morphological differences across species. Furthermore, stimulus-frequency otoacoustic emissions (SFOAEs), sounds emitted by the ear in response to a tone, noninvasively probe the frequency selectivity of the ear. We report estimates of auditory tuning derived from SFOAEs for 12 different species of lizards with widely varying TM morphology. Despite gross anatomical differences across the species examined herein, low-level SFOAEs were readily measurable in all ears tested, even in non-TM species whose basilar papilla contained as few as 50-60 hair cells. Our measurements generally support theoretical predictions: longer delays/sharper tuning features are found in species with a TM relative to those without. However, SFOAEs from at least one non-TM species (Anolis) with long delays suggest there are likely additional micromechanical factors at play that can directly affect tuning. Additionally, in the one species examined with a continuous TM (Aspidoscelis) where cell-to-cell coupling is presumably relatively stronger, delays were intermediate. This observation appears consistent with recent reports that suggest the TM may play a more complex macromechanical role in the mammalian cochlea via longitudinal energy distribution (and thereby affect tuning). Although significant differences exist between reptilian and mammalian auditory biophysics, understanding lizard OAE generation mechanisms yields significant insight into fundamental principles at work in all vertebrate ears.

The tectorial membrane: one slice of a complex cochlear sandwich.

PURPOSE OF REVIEW: The review is both timely and relevant, as recent findings have shown the tectorial membrane plays a more dynamic role in hearing than hitherto suspected, and that many forms of deafness can result from mutations in tectorial membrane proteins. RECENT FINDINGS: Main themes covered are the molecular composition, the structural organization and properties of the tectorial membrane, the role of the tectorial membrane as a second resonator and a structure within which there is significant longitudinal coupling, and how mutations in tectorial membrane proteins cause deafness in mice and men. CONCLUSION: Findings from experimental models imply that the tectorial membrane plays multiple, critical roles in hearing. These include coupling elements along the length of the cochlea, supporting a travelling wave and ensuring the gain and timing of cochlear feedback are optimal. The clinical findings suggest stable, moderate-to-severe forms of hereditary hearing loss may be diagnostic of a mutation in TECTA, a gene encoding one of the major, noncollagenous proteins of the tectorial membrane.

The supraodontoid space or "apical cave" at the craniocervical junction: a microdissection study.

The extradural supraodontoid space lies anteriorly at the craniocervical junction (CCJ) between the alar ligaments and foramen magnum. It occupies the space between the tectorial and atlanto-occipital membranes. A variety of benign and traumatic lesions may result in neurological compression here with harmful effects. Decompression by the transoral surgical approach often provides relief from these effects. Knowledge of the detailed microanatomy of this space is fragmentary. The purpose of this study was to identify the boundaries and contents of this space by microdissection. Twenty-three en bloc preserved adult cadaveric specimens of the CCJ were dissected to identify the boundaries and contents of the supraodontoid space. The posterior bony elements of the CCJ were removed to enable microdissection (Zeiss DXE Microscope 4-40x) from the tectorial membrane (TM) forwards. The cave-like space faced posteriorly. It had a roof which extended into a wall (anterior atlanto-occipital membrane), a floor (superior surface of the alar ligament), and a mouth covered by the TM. The apical ligament and a thin lining membranous fatty layer divided the cave into a pair of symmetrical halves. The contents, from dorsal to ventral, lay deep to a thin subtectorial membrane. These were the superior fasciculus of the cruciate ligament, a fat-ensheathed knot of plexiform veins (which communicated with the surrounding CCJ vertebral venous plexuses), an arterial arcade between the veins, a pair of fat pads, and branches of the sinuvertebral nerves of the CCJ (lying on the floor). No synovial membrane was found. Knowledge of the anatomy of the apical cave may be of some assistance in transoral (extra- and transdural) surgical approaches to the anterior CCJ region.

Temporal bone study of development of the organ of Corti: correlation between auditory function and anatomical structure.

OBJECTIVE: To study the development of the organ of Corti in the human cochlea, and to correlate our findings with the onset of auditory function. MATERIAL AND METHODS: Step sections of 81 human fetal temporal bones were studied, from eight weeks of gestation to full term. RESULTS: By the end of the 10th week, the tectorial membrane primordium could be traced even in the most apical turns. Individual hair cells became identifiable at the basal turn at 14 weeks. At the same time, a small but well formed oval space was observed between the inner and outer hair cells in the basal turn. This does not correspond to the tunnel of Corti, as is erroneously quoted in the literature, as the individual pillar cells develop at later stages. Between 14 and 15 weeks, Hensen's cells were recognised for the first time. Individual pillar cells were identifiable at 17 weeks and the tunnel of Corti opened at 20 weeks. By 25 weeks, the cochlea had reached its adult size, but continued to develop until full term. DISCUSSION AND CONCLUSIONS: A temporal coincidence of different developmental events is responsible for early fetal audition at 20 weeks, including growth of pillar cells, opening of the tunnel of Corti and regression of Kollicker's organ, with the subsequent formation of the inner spiral sulcus and then separation of the tectorial membrane. The fine structures of the organ of Corti continue to develop well after the 25th week, and this may well alter the mechanical properties of the vibrating parts of the cochlea, which may in turn account for the frequency shift observed in preterm infants. These changes will have to be taken into account in the development of prenatal hearing screening tests.

Tectorial membrane stiffness gradients.

The mammalian inner ear processes sound with high sensitivity and fine resolution over a wide frequency range. The underlying mechanism for this remarkable ability is the "cochlear amplifier", which operates by modifying cochlear micromechanics. However, it is largely unknown how the cochlea implements this modification. Although gradual improvements in experimental techniques have yielded ever-better descriptions of gross basilar membrane vibration, the internal workings of the organ of Corti and of the tectorial membrane have resisted exploration. Although measurements of cochlear function in mice with a gene mutation for alpha-tectorin indicate the tectorial membrane's key role in the mechanoelectrical transformation by the inner ear, direct experimental data on the tectorial membrane's physical properties are limited, and only a few direct measurements on tectorial micromechanics are available. Using the hemicochlea, we are able to show that a tectorial membrane stiffness gradient exists along the cochlea, similar to that of the basilar membrane. In artificial perilymph (but with low calcium), the transversal and radial driving point stiffnesses change at a rate of -4.0 dB/mm and -4.9 dB/mm, respectively, along the length of the cochlear spiral. In artificial endolymph, the stiffness gradient for the transversal component was -3.4 dB/mm. Combined with the changes in tectorial membrane dimensions from base to apex, the radial stiffness changes would be able to provide a second frequency-place map in the cochlea. Young's modulus, which was obtained from measurements performed in the transversal direction, decreased by -2.6 dB/mm from base to apex.

The tectorial membrane: anatomical, biomechanical, and histological analysis.

There is minimal information in the literature regarding the tectorial membrane. Further, information in the literature regarding the anatomy and function of this structure is often contradictory. We performed the current study to elucidate further this structure's detailed anatomy, function, and histology. Thirteen adult cadavers underwent dissection of their tectorial membranes and detailed observations and measurements were made of them. Ranges of motion of the craniocervical junction were performed before and after transection of this structure. Histological analysis was performed on all membranes. The tectorial membrane was found to attach much more superiorly than previously described and was found to be firmly adherent to the cranial base and body of the axis but not to the posterior aspect of the odontoid process. The mean thickness of this membrane was found to be 1 mm. Flexion of the head made the tectorial membrane fully taut at 15 degrees and extension made it fully taut at 20 degrees; however, there was a buckling effect (redundant tectorial membrane) noted at the level of the odontoid process in extension. With the alar and transverse ligaments cut and with flexion of the head, the middle portion of this membrane was stretched over the odontoid process, thus acting as a "hammock" that inhibited the odontoid process from moving posteriorly. The tectorial membrane did not limit cervical flexion per se but rather helped to insure that the odontoid process did not impinge into the cervical canal. Lateral flexion was not found to be limited by this structure. Histologically, parallel collagen fibers with spindle-shaped fibrocytes were observed within this membrane and near its attachment to the posterior axis, the collagen fibers were noted to be more homogenous with larger non-spindled fibrocytes. At the cranial attachment of the tectorial membrane, multiple calcified areas were noted that interdigitated with the underlying bone. Also near this cephalic bony attachment, there was an increase in the number of elastic fibers, which were found running parallel with the surrounding Type III collagen fibers. The tectorial membrane was found to attach much more superiorly than previously described. We would propose that the tectorial membrane provides for a second line of defense, preventing the odontoid process from compressing the spinal cord and by doing so, secondarily limits movement of the craniocervical juncture. This hypothesis is strengthened by the finding of many elastic fibers in the tectorial membrane. To our knowledge, our study is the first to perform a detailed histological analysis of the tectorial membrane. We hope that these data are useful to the clinician who investigates this ligament of the craniocervical region.

Does the geometrical arrangement of the outer hair cell stereocilia perform a fluid-mechanical function?

CONCLUSIONS: Our experiments qualitatively show that the geometrical structure of the inner ear may have the features of a micro-pump. To further substantiate this hypothesis, additional experiments, particularly on in vivo preparations, are needed. OBJECTIVE: To introduce some new ideas about the functional purpose of the geometric arrangement of the outer hair cell stereocilia. Analogies to some recently developed valveless micro-pumps are pointed out. To illustrate these points, comparative experiments were performed using a simplified macro model. METHODS: Specific structures of the organ of Corti were simulated in a partially open, partially closed acrylic tank. This rough approximation allows the visualization of fluid flows that are generated as a result of the relative motions between the tectorial membrane and the reticular lamina. RESULTS: It was shown that the arrangement of the cochlear elements not only forces fluid to flow in a one-way direction, but also generates a fluid stream that flows through the "outlets" between each two V or W formations of stereocilia. These fluid streams are directed towards the inner hair cells.

Avaliação radiológica da transição crânio-vertebral / Radiological evaluation of the craniovertebral junction

A transição crânio-vertebral compreende o occiput, o atlas, o axis, suas articulações e suporte ligamentar. Para sua avaliação, além do estudo radiológico convencional, são necessários exames com parâmetros específicos - campo de visão, espessura de corte e deslocamento adequados na tomografia computadorizada e na ressonância magnética de alta resolução -, bem como estudo dinâmico que viabilize a análise da estabilidade da transição crânio-vertebral e da repercussão sobre a transição bulbo-medular. Este trabalho tem como objetivo revisar a técnica utilizada para estudo da transição crânio-vertebral, os parâmetros da semiologia radiológica e avaliação das entidades que mais freqüentemente a comprometem. Revisamos a literatura e exemplificamos com imagens do arquivo didático do Serviço de Radiologia da Med Imagem do Hospital da Beneficência Portuguesa de São Paulo, casos demonstrativos da anatomia normal, principais entidades patológicas congênitas e adquiridas da transição crânio-vertebral, através de radiologia convencional, tomografia computadorizada e ressonância magnética. Na prática diária, afecções da transição crânio-vertebral são detectadas por tomografia computadorizada e ressonância magnética do encéfalo e da coluna cervical. O conhecimento dessas entidades é fundamental para indicação de exames específicos que possibilitem estudo detalhado da transição crânio-vertebral, fornecendo subsídios para programação terapêutica quando indicada.

Motion analysis in the hemicochlea.

Optical flow techniques are often used to estimate velocity fields to represent motion in successive video images. Usually the method is mathematically ill-posed, because the single scalar equation representing the conservation of local intensity contains more than one unknown velocity component. Instead of regularizing the problem using optimization techniques, we formulate a well-posed problem for the gerbil hemicochlea preparation by introducing an in-plane incompressibility constraint, and then show that local brightness is also conserved. We solve the resulting system using a Lagrangian description of the conservation equations. With this approach, the displacement of isointensity contours on sequential images determines the normal component of velocity of an area element, while the tangential component is computed from the local constant area constraint. We have validated our method using pairs of images generated from our calculations of the vibrational deformation in a cross section of the organ of Corti and tectorial membrane in the mammalian cochlea, and quantified the superior performance of our method when complex artificial motion is applied to a noisy image obtained from the hemicochlea preparation. The micromechanics of the organ of Corti and the tectorial membrane is then analyzed by our new method.

Cochlear dimensions obtained in hemicochleae of four different strains of mice: CBA/CaJ, 129/CD1, 129/SvEv and C57BL/6J.

Because homologies between mice and human genomes are well established and hereditary abnormalities are similar in both, mice present a valuable animal model to study hereditary hearing disorders in humans. One of the manifestations of hereditary hearing disorders might be in the structure of cochlear elements, such as the gross morphology of the cochlea. Cochlear dimensions, however, are one factor that determines inner ear mechanics and thus hearing function. Therefore, gross cochlear dimension might be important when different strains of mice are compared regarding their hearing. Although several studies have examined mouse inner ear structures on a sub-cellular level, only few have studied cochlear gross morphology. Moreover, the sparse data available were acquired from fixed and dehydrated tissue. Dehydration, however, produces severe distortion of gel-like cochlear structures such as the tectorial membrane and the basilar membrane hyaline matrix. In this study, the hemicochlea technique, which allows fresh mouse cochlear material to be viewed from a radial perspective, was used to provide an itemized study of the dimensions of gross cochlear structures in four mouse strains (CBA/CaJ, 129/SvEv, 129/CD1 and C57BL/6J). Except for the CBA/CaJ, these strains are known to possess genes for age-related hearing loss. The measurements showed no major differences among the four strains. However, when compared with previous data, the thickness measures of the basilar membrane were up to 10 times larger. Such differences are likely to result from the different techniques used to process the material. The hemicochlea technique eliminates much of the distortion caused by dehydration, which was present in previous experiments.

Structural relationships of the unfixed tectorial membrane.

Although the tectorial membrane has a key role in the function of the organ of Corti, its structural relationship within the cochlear partition is still not fully characterised. Being an acellular structure, the tectorial membrane is not readily stained with dyes and is thus difficult to visualise. We present here detailed observations of the unfixed tectorial membrane in an in vitro preparation of the guinea pig cochlea using confocal microscopy. By perfusing the fluid compartments within the cochlear partition with fluorochrome-conjugated dextran solutions, the tectorial membrane stood out against the bright background. The tectorial membrane was seen as a relatively loose structure as indicated by the dextran molecules being able to diffuse within its entire volume. There were, however, regions showing much less staining, demonstrating a heterogeneous organisation of the membrane. Especially Hensen's stripe and regions facing the outer hair cell bundles appeared more condensed. Whereas no connections between Hensen's stripe and the inner hair cell bundles could be observed, there was clearly a contact zone between the stripe and the reticular lamina inside of the inner hair cell.

Fine structure of the basilar papilla of the emu: implications for the evolution of avian hair-cell types.

The morphology of the basilar papilla of the emu was investigated quantitatively with light and scanning electron microscopical techniques. The emu is a member of the Paleognathae, a group of flightless birds that represent the most primitive living avian species. The comparison of the emu papilla with that of other, more advanced birds provides insights into the evolution of the avian papilla. The morphology of the emu papilla is that of an unspecialised bird, but shows the full range of features previously shown to be typical for the avian basilar papilla. For example, the orientation of the hair cells' sensitive axes varied in characteristic fashion both along and across the papilla. Many of the quantitative details correlate well with the representation of predominantly low frequencies along the papilla. The most distinctive features were an unusually high density of hair cells and an unusual tallness of the hair-cell bodies. This suggests that the evolution of morphologically very short hair cells, which are a hallmark of avian papillae, is a recent development in evolution. The small degree of differentiation in hair-cell size contrasts with the observation that a significant number of hair cells in the emu lack afferent innervation. It is therefore suggested that the development of functionally different hair-cell types in birds preceded the differentiation into morphologically tall and short hair cells.