Cochlear and lagenar ganglia of the chicken

Evolution and development of the tetrapod auditory system: an organ of Corti-centric perspective

The tetrapod auditory system transmits sound through the outer and middle ear to the organ of Corti or other sound pressure receivers of the inner ear where specialized hair cells translate vibrations of the basilar membrane into electrical potential changes that are conducted by the spiral ganglion neurons to the auditory nuclei. In other systems, notably the vertebrate limb, a detailed connection between the evolutionary variations in adaptive morphology and the underlying alterations in the genetic basis of development has been partially elucidated. In this review, we attempt to correlate evolutionary and partially characterized molecular data into a cohesive perspective of the evolution of the mammalian organ of Corti out of the tetrapod basilar papilla. We propose a stepwise, molecularly partially characterized transformation of the ancestral, vestibular developmental program of the vertebrate ear. This review provides a framework to decipher both discrete steps in development and the evolution of unique functional adaptations of the auditory system. The combined analysis of evolution and development establishes a powerful cross-correlation where conclusions derived from either approach become more meaningful in a larger context which is not possible through exclusively evolution or development centered perspectives. Selection may explain the survival of the fittest auditory system, but only developmental genetics can explain the arrival of the fittest auditory system. [Modified after (Wagner 2011)].

Inner-ear morphology of the New Zealand kiwi (Apteryx mantelli) suggests high-frequency specialization

The sensory systems of the New Zealand kiwi appear to be uniquely adapted to occupy a nocturnal ground-dwelling niche. In addition to well-developed tactile and olfactory systems, the auditory system shows specializations of the ear, which are maintained along the central nervous system. Here, we provide a detailed description of the auditory nerve, hair cells, and stereovillar bundle orientation of the hair cells in the North Island brown kiwi. The auditory nerve of the kiwi contained about 8,000 fibers. Using the number of hair cells and innervating nerve fibers to calculate a ratio of average innervation density showed that the afferent innervation ratio in kiwi was denser than in most other birds examined. The average diameters of cochlear afferent axons in kiwi showed the typical gradient across the tonotopic axis. The kiwi basilar papilla showed a clear differentiation of tall and short hair cells. The proportion of short hair cells was higher than in the emu and likely reflects a bias towards higher frequencies represented on the kiwi basilar papilla. The orientation of the stereovillar bundles in the kiwi basilar papilla showed a pattern similar to that in most other birds but was most similar to that of the emu. Overall, many features of the auditory nerve, hair cells, and stereovilli bundle orientation in the kiwi are typical of most birds examined. Some features of the kiwi auditory system do, however, support a high-frequency specialization, specifically the innervation density and generally small size of hair-cell somata, whereas others showed the presumed ancestral condition similar to that found in the emu.

Slits and Robos in the developing chicken inner ear

Mechanosensory hair cells in the chick inner ear synapse onto afferent neurons of the statoacoustic ganglion (SAG). During development, these neurons extend a central process to the brain and a peripheral process into one of eight sensory organs. A combination of cues, including chemoattractants and chemorepellents, direct otic axons to their peripheral targets. As a first step in evaluating the role of known axon guidance molecules, Slits and Robos, we examined expression of their transcripts in the chick inner ear from embryonic day 2-11 (Hamburger and Hamilton stages 14-37). Robo2 and slit2 are in migrating neuroblasts and the SAG, while both slits and robos are in the otic epithelium. We speculate that this family of signaling molecules may be involved in repulsion, first of otic neuroblasts and then of otic axons. Later our expression data revealed a potentially novel role for these molecules in maintaining sensory/nonsensory boundaries.

Interaural timing difference circuits in the auditory brainstem of the emu (Dromaius novaehollandiae)

In the auditory system, precise encoding of temporal information is critical for sound localization, a task with direct behavioral relevance. Interaural timing differences (ITDs) are computed using axonal delay lines and cellular coincidence detectors in nucleus laminaris (NL). We present morphological and physiological data on the timing circuits in the emu, Dromaius novaehollandiae, and compare these results with those from the barn owl (Tyto alba) and the domestic chick (Gallus gallus). Emu NL was composed of a compact monolayer of bitufted neurons whose two thick primary dendrites were oriented dorsoventrally. They showed a gradient in dendritic length along the presumed tonotopic axis. The NL and nucleus magnocellularis (NM) neurons were strongly immunoreactive for parvalbumin, a calcium-binding protein. Antibodies against synaptic vesicle protein 2 and glutamic acid decarboxlyase revealed that excitatory synapses terminated heavily on the dendritic tufts, while inhibitory terminals were distributed more uniformly. Physiological recordings from brainstem slices demonstrated contralateral delay lines from NM to NL. During whole-cell patch-clamp recordings, NM and NL neurons fired single spikes and were doubly rectifying. NL and NM neurons had input resistances of 30.0 +/- 19.9 Momega and 49.0 +/- 25.6 Momega, respectively, and membrane time constants of 12.8 +/- 3.8 ms and 3.9 +/- 0.2 ms. These results provide further support for the Jeffress model for sound localization in birds. The emu timing circuits showed the ancestral (plesiomorphic) pattern in their anatomy and physiology, while differences in dendritic structure compared to chick and owl may indicate specialization for encoding ITDs at low best frequencies.

Clonal analysis of the relationships between mechanosensory cells and the neurons that innervate them in the chicken ear

In vertebrates, hair-cell-bearing mechanosensory organs and the neurons that innervate them share a common placodal origin. In the inner ear, the peripheral neurons for both auditory and vestibular systems emigrate from the otic placode as neuroblasts, and divide, differentiate and innervate only one of six to eight distinct sensory organs. How these neurons find their correct target is unknown, although one suggestion is that they synapse with clonally related cells. To test this idea for both the middle and inner ears of chicken embryos, lineage analysis was initiated at the time of neuroblast delamination by labeling progenitors with replication-defective retroviruses. The vast majority (89%) of clones were restricted to a single anatomical subdivision of the sensory periphery or its associated ganglia, indicating limited clonal dispersion. Among the remaining clones, we found evidence of a shared neurosensory lineage in the middle ear. Likewise, in the inner ear, neurons could be related to cells of the otic epithelium, although the latter cells were not widely distributed. Rather, they were restricted to a region in or near the utricular macula. None of the other seven sensory organs was related to the ganglion neurons, suggesting that a common lineage between neurons and their targets is not a general mechanism of establishing synaptic connections in the inner ear. This conclusion is further strengthened by finding a shared lineage between the vestibular and acoustic ganglia, revealing the presence of a common progenitor for the two functional classes of neurons.

Primary innervation of the avian and mammalian cochlear nucleus

The auditory nerve of birds and mammals exhibits differences and similarities, but given the millions of years since the two classes diverged from a common ancestor, the similarities are much more impressive than the differences. The avian nerve is simpler than that of mammals, but share many fundamental features including principles of development, structure, and physiological properties. Moreover, the available evidence shows that the human auditory nerve follows this same general organizational plan. Equally impressive are reports that homologous genes in worms, flies, and mice exert the same heredity influences in man. The clear implication is that animal studies will produce knowledge that has a direct bearing on the human condition.

Cholinergic innervation of the chick basilar papilla

Antibodies directed against choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine (ACh) and a specific marker of cholinergic neurons, were used to label axons and nerve terminals of efferent fibers that innervate the chick basilar papilla (BP). Two morphologically distinct populations of cholinergic fibers were labeled and classified according to the region of the BP they innervated. The inferior efferent system was composed of thick fibers that coursed radially across the basilar membrane in small fascicles, gave off small branches that innervated short hair cells with large cup-like endings, and continued past the inferior edge of the BP to ramify extensively in the hyaline cell area. The superior efferent system was made up of a group of thin fibers that remained in the superior half of the epithelium and innervated tall hair cells with bouton endings. Both inferior and superior efferent fibers richly innervated the basal two thirds of the BP. However, the apical quarter of the chick BP was virtually devoid of efferent innervation except for a few fibers that gave off bouton endings around the peripheral edges. The distribution of ChAT-positive efferent endings appeared very similar to the population of efferent endings that labeled with synapsin antisera. Double labeling with ChAT and synapsin antibodies showed that the two markers colocalized in all nerve terminals that were identified in BP whole-mounts and frozen sections. These results strongly suggest that all of the efferent fibers that innervate the chick BP are cholinergic.

Ultrastructure and blood supply of the tegmentum vasculosum in the cochlea of the duckling

The tegmentum vasculosum of the duckling consists of a highly folded epithelium which extends over the dorsal and lateral walls of the cochlear duct, separating the scala media from the scala vestibuli. This epithelium consists of two distinct cell types, dark cells and light cells, and is well vascularized. The surface of the epithelium is formed by a mosaic of alternating dark and light cells. The goblet-shaped dark cells have an electron-dense, organelle-rich cytoplasm, and are expanded basally by extensive basolateral plasma membrane infoldings, within which are numerous mitochondria. Dark cells are isolated from each other and from the capillaries within the epithelium by intervening light cells. In contrast, columnar light cells exhibit an electron-lucent, organelle-poor cytoplasm and may extend from the underlying capillaries to the endolymphatic surface. Light cells contain abundant, coated endocytic vesicles on their apical surfaces and are bound, apically, to other light cells or to dark cells by tight junctions and desmosomes. Laterally, light cells are linked to each other either by complex, fluid-filled membrane interdigitations or by extensive gap junctions. Plasma membrane interdigitations and obvious, fluid-filled intercellular spaces characterize the lateral borders between light and dark cells. Vascular corrosion casting reveals the three-dimensional anatomy of the cochlear vasculature. A continuous arteriolar loop fed by anterior and posterior cochlear arterioles encircles the cochlear duct. The rich capillary beds of the tegmentum vasculosum are supplied by arching arterioles arising from this loop. These capillaries are the continuous type and are situated primarily within the core of the epithelium or along its border with the scala vestibuli. The structure and blood supply of the tegmentum vasculosum are characteristic of an epithelium involved in active transport.

Topological and developmental gradients of calbindin expression in the chick's inner ear

Mobile intracellular calcium buffers play an important role in regulating calcium flux into mechanosensory hair cells and calbindin D-28k is expressed at high levels in the chick's basilar papilla. We have used RT-PCR, in situ hybridization, and immunohistology to demonstrate that calbindin expression varies systematically according to hair cell position and developmental age. RT-PCR using microdissected quarters of the posthatch basilar papilla showed that mRNA levels were lowest in the (low frequency) apex and higher in basal quadrants. In situ hybridization revealed calbindin mRNA in posthatch hair cells and supporting cells, with more intense labeling of hair cells from basal (high frequency) positions. A similar topology was obtained with calbindin antibodies. Neither calbindin riboprobe nor calbindin antibody labeled cochlear neurons. In contrast, a subset of large vestibular neurons and their calyciform endings onto Type I vestibu lar hair cells were strongly labeled by the calbindin antibody, while vestibular hair cells were negative for calbindin immunoreactivity. Likewise, calbindin in situ hybridization was negative for vestibular hair cells but positive in a subset of larger vestibular neurons. Calbindin mRNA was detected in hair cells of the basal half of the papilla at embryonic day 10 (E10) and calbindin immunoreactivity was detected at E12. Hair cells in the apical half of the papilla had equivalent calbindin expression two days later. Immunoreactivity appeared in abneural supporting cells days later than in hair cells, and not until E20 in neurally located supporting cells. These results demonstrate that calbindin message and protein levels are greater in high-frequency hair cells. This "tonotopic" gradient may result from the stabilization of a basal-to-apical developmental gradient and could be related at least in part to calcium channel expression along this axis.

Primordial rhythmic bursting in embryonic cochlear ganglion cells

This study examined the nature of spontaneous discharge patterns in cochlear ganglion cells in embryonic day 13 (E13) to early E17 chicken embryos (stages 39-43). Neural recordings were made with glass micropipettes. No sound-driven activity was seen for the youngest embryos (maximum intensity 107 dB sound pressure level). Ganglion cells were labeled with biotinylated dextran amine in four embryos. In two animals, primary afferents projected to hair cells in the middle region along the length of the basilar papilla in which, in one cell, the terminals occupied a neural transverse position and, in the other, a more abneural location. Statoacoustic ganglion cells showing no spontaneous activity were seen for the first time in the chicken. The proportion of "silent" cells was largest at the youngest stages (stage 39, 67%). In active cells, mean spontaneous discharge rates [9.4 +/- 10.4 spikes (Sp)/sec; n = 44] were lower than rates for older embryos (19 +/- 17 Sp/sec) (Jones and Jones, 2000). Embryos at stages 39-41 evidenced even lower rates (4.2 +/- 5.0 Sp/sec). The most salient feature of spontaneous activity for stages 39-43 was a bursting discharge pattern in >75% of active neurons (33 of 44). Moreover, in 55% of these cells, there was a clear, slow, rhythmic bursting pattern. The proportion of cells showing rhythmic bursting was greatest at the youngest stages (39-42) and decreased to <30% at stage 43. Rate of bursting ranged from 1 to 54 bursts per minute. The presence of rhythmic bursting in cochlear ganglion cells at E13-E17 provides an explanation for the existence of such patterns in central auditory relays. The bursting patterns may serve as a patterning signal for central synaptic refinements in the auditory system during development.

Efferent axons in the avian auditory nerve

The sensory hair cells of the inner ear receive both afferent and efferent innervation. The efferent supply to the auditory organ has evolved in birds and mammals into a separate complex system, with several types of neurons of largely unknown function. In this study, the efferent axons in four different species of birds (chicken, starling, barn owl and emu) were examined anatomically. Total numbers of efferents supplying the cochlear duct (auditory basilar papilla and the vestibular lagenar macula) were determined; separate estimates of the efferents to the lagenar macula only were also derived and subtracted. The numbers for auditory efferents thus varied between 120 (chicken) and 1068 (barn owl). Considering the much larger numbers of hair cells in the basilar papilla, each efferent is predicted to branch extensively. However, pronounced species-specific differences as well as regional differences along the tonotopic gradient of the basilar papilla were documented. Myelinated and unmyelinated axons were found, with mean diameters of about 1 microm and about 0.5 microm, respectively. This suggests two basic populations of efferents, however, they did not appear to be distinguished sharply. Evidence is presented that some efferents lose their myelination at the transition from central oligodendrocyte to peripheral Schwann cell myelin. Finally, a comparison of the four bird species evaluated suggests that the efferent population with smaller, unmyelinated axons is the phylogenetically more primitive one. A new population probably arose in parallel with the evolution and differentiation of the specialized hair-cell type it innervates, the short hair cell.

A quantitative analysis of the nerve fibers in the VIIIth nerve of Belgian Waterslager canaries with a hereditary sensorineural hearing loss

The number of auditory nerve fibers was determined for non-Belgian Waterslager canaries (non-BWS) and Belgian Waterslager canaries (BWS) that are affected by a sensorineural high frequency hearing loss and a 30% reduction in the number of auditory hair cells. Counts were obtained from semithin cross sections of the Durcupan-embedded auditory nerve at the level of the internal auditory meatus. In addition, the number of lagenar fibers was determined from cross sections near the apical end of the cochlear duct in order to separate them from the total number of auditory nerve fibers. The mean number of auditory nerve fibers was 6076 in non-BWS and 5363 in BWS canaries, representing a 12% reduction in BWS. This small reduction in the number of auditory nerve fibers, as compared to the larger reduction in hair cell number, might be explained by a predominant loss of abneural hair cells in BWS, since it has been shown for other species that a large proportion of abneural hair cells are devoid of afferent innervation. In addition, we observed that despite the prominent hair cell pathologies documented for BWS canaries, the mean diameter of auditory nerve fibers from non-BWS canaries (2.22+/-0.81 microm) did not differ from those of BWS canaries (2.21+/-0.96 microm).

Contrasting molecular composition and channel properties of AMPA receptors on chick auditory and brainstem motor neurons

1. Neurons in the brainstem auditory pathway exhibit a number of specializations for transmitting signals reliably at high rates, notably synaptic AMPA receptors with very rapid kinetics. Previous work has not revealed a common structural pattern shared by the AMPA receptors of auditory neurons that could account for their distinct functional properties. 2. We have used whole-cell patch-clamp recordings, mRNA analysis, immunofluorescence, Western blots and agonist-evoked cobalt uptake to compare AMPA receptors on the first-, second- and third-order neurons in the chick ascending auditory pathway with those on brainstem motor neurons of the glossopharyngeal/vagal nucleus, which have been shown to have very slow desensitization kinetics. 3. The results indicate that the AMPA receptors of the cochlear ganglion, nucleus magnocellularis and nucleus laminaris share a number of structural and functional properties that distinguish them from the AMPA receptors of brainstem motor neurons, namely a lower relative abundance of glutamate receptor (GluR)2 transcript and much lower levels of GluR2 immunoreactivity, higher relative levels of GluR3 flop and GluR4 flop, lower relative abundance of the C-terminal splice variants GluR4c and 4d, less R/G editing of GluR2 and 3, greater permeability to calcium, predominantly inwardly rectifying I-V relationships, and greater susceptibility to block by Joro spider toxin. 4. We conclude that the AMPA receptors of auditory neurons acquire rapid kinetics from their high content of GluR3 flop and GluR4 flop subunits and their high permeability to Ca2+ from selective post-transcriptional suppression of GluR2 expression.

A quantitative study of cochlear afferent axons in birds

This paper is a comparative study of auditory-nerve morphology in birds. The chicken (Gallus gallus), the emu (Dromaius novaehollandiae) and the starling (Sturnus vulgaris) were chosen as unspecialised birds that have already been used in auditory research. The data are discussed in comparison to a similar earlier study on the barn owl, a bird with highly specialised hearing, in an attempt to separate general avian patterns from species specialisations. Average numbers of afferent fibres from 8775 (starling) to 12¿ omitted¿406 (chicken) were counted, excluding fibres to the lagenar macula. The number of fibres representing different frequency ranges showed broad maxima in the chicken and emu, corresponding to hearing ranges of best sensitivity and/or particular behavioural relevance. Mean axon diameters were around 2 microm in the chicken and starling, and around 3 microm in the emu. Virtually all auditory afferents were myelinated. The mean thickness of the myelin sheaths was between 0.33 microm (starling) and 0.4 microm (emu). There was a consistent pattern in the diameters of axons deriving from different regions. Axons from very basal, i.e. highest-frequency, parts of the basilar papilla were always the smallest. In the emu and the chicken, axons from the middle papillar regions were, in addition, larger than axons innervating apical regions.

Distribution of rate-intensity function types in chick cochlear nerve after exposure to intense sound

Intense sound exposure to the chick ear produces cochlear damage and losses in auditory function. At twelve days post exposure there is considerable structural repair, although a defect on the sensory epithelium remains in the form of an incompletely healed 'patch' lesion. Auditory function significantly recovers 12 days after the exposure, but it, too, is incomplete. In this paper we describe the relationship between stimulus intensity and cochlear nerve discharge rate (the rate-intensity function) in two groups of chicks. One is exposed to damaging sound levels but allowed 12 days to recover, while the other is a group of non-exposed and age-matched control animals. Three different types of rate-intensity functions were identified; saturating, sloping, and straight. The percentage of saturating and sloping functions was compared across all characteristic frequencies in both groups of animals. A significant change was observed in the distribution of these types for recovered units with characteristic frequencies within the region of the patch lesion. In addition, the rate-intensity functions of these units exhibited a steeper slope and a higher maximum response. The distribution of rate-intensity function types and their slope and maximum responses, for units with characteristic frequencies outside of the patch lesion, was similar to those found in control ears. The changes in the cochlear nerve response in exposed chicks may be due to alterations in cochlear mechanics, hair cell or synaptic membrane properties, hair cell innervation, or the loss of a tonic suppression of afferent activity exerted by the damaged short hair cells.

Macrophage secretory products influence the survival of statoacoustic neurons

Prior studies have shown that macrophages are recruited to sites of injury or infection in the sensory organs of the inner ear, but the effects of macrophages and their cytokine secretory products on the sensory structures of the ear are not known. In the present study, cultures of dissociated statoacoustic neurons were incubated with selected macrophage secretary products and the numbers of surviving neurons after 48 h in vitro were quantified. Results indicate that two macrophage secretary products, interleukin 1 and fibroblast growth factor 2, can enhance the survival of statoacoustic neurons, while another cytokine, tumour necrosis factor-alpha can diminish the survival of those neurons. Also, numerous macrophages were present in both cytokine-treated and control cultures. The findings suggest that macrophages may influence the survival of the sensory neurons of the inner ear.

Frequency tuning and spontaneous activity in the auditory nerve and cochlear nucleus magnocellularis of the barn owl Tyto alba

Single-unit recordings were obtained from the brain stem of the barn owl at the level of entrance of the auditory nerve. Auditory nerve and nucleus magnocellularis units were distinguished by physiological criteria, with the use of the response latency to clicks, the spontaneous discharge rate, and the pattern of characteristic frequencies encountered along an electrode track. The response latency to click stimulation decreased in a logarithmic fashion with increasing characteristic frequency for both auditory nerve and nucleus magnocellularis units. The average difference between these populations was 0.4-0.55 ms. The average most sensitive thresholds were approximately 0 dB SPL and varied little between 0.5 and 9 kHz. Frequency-threshold curves showed the simple V shape that is typical for birds, with no indication of a low-frequency tail. Frequency selectivity increased in a gradual, power-law fashion with increasing characteristic frequency. There was no reflection of the unusual and greatly expanded mapping of higher frequencies on the basilar papilla of the owl. This observation is contrary to the equal-distance hypothesis that relates frequency selectivity to the spatial representation in the cochlea. On the basis of spontaneous rates and/or sensitivity there was no evidence for distinct subpopulations of auditory nerve fibers, such as the well-known type I afferent response classes in mammals. On the whole, barn owl auditory nerve physiology conformed entirely to the typical patterns seen in other bird species. The only exception was a remarkably small spread of thresholds at any one frequency, this being only 10-15 dB in individual owls. Average spontaneous rate was 72.2 spikes/s in the auditory nerve and 219.4 spikes/s for nucleus magnocellularis. This large difference, together with the known properties of endbulb-of-Held synapses, suggests a convergence of approximately 2-4 auditory nerve fibers onto one nucleus magnocellularis neuron. Some auditory nerve fibers as well as nucleus magnocellularis units showed a quasiperiodic spontaneous discharge with preferred intervals in the time-interval histogram. This phenomenon was observed at frequencies as high as 4.7 kHz.

Distribution of nerve fibers in the basilar papilla of normal and sound-damaged chick cochleae

Epifluorescent light microscopy and confocal laser scanning microscopy were employed to visualize the distribution of nerve fibers in whole-mount preparations of normal and sound-damaged chick basilar papillae (BP). In normal cochleae, we identified a consistent pattern of nerve processes that ran transversely across the BP. The transverse processes increase in number from the proximal to the distal ends of the epithelium. However, when the processes are separated into populations of thin fibers and thick bundles, the thin fibers are more prevalent in distal regions whereas thick bundles are more extensive in proximal regions. Furthermore, the thick bundles form an elaborate longitudinal network in the border cell and hyaline cell region. Based on these data and no other previous studies, the thin fibers appear to be afferent nerves and the thick bundles represent efferent nerves. When birds are exposed to acoustic trauma, the normal pattern and number of nerve processes is not altered by levels of sound that produce moderate levels of damage, i.e., damage that leads to hair cell loss and regeneration. However, the nerve pattern is disrupted by severe levels of damage that destroy both hair cells and supporting cells. These findings indicate that the level of sound exposure that induces hair cell regeneration may damage the synaptic endings associated with the lost hair cells, but that the nerve processes that give rise to these endings remain intact within the sensory epithelium. In contrast, severe damage destroys both the hair cells and their associated nerve fibers.

Efferent neurons to the macular lagena in the embryonic chick

The lipophilic dye, DiI, was placed into the macula lagena of paraformaldehyde-fixed embryonic chicks. Retrogradely labeled cells were found bilaterally in the pontine reticular formation (RF) between the dorsal facial nucleus and the abducens nerve root. This location is similar to that of the dorsomedial group of efferent cells that project to the basilar papilla. No lagenar efferent neurons, however, were found near the superior olivary nucleus where the ventrolateral group of cochlear efferents is located. Whether efferent neurons in the pontine RF send collaterals to both the basilar papilla and to the macula lagena has yet to be determined.