Macula Neglecta Research Articles

Vestibular physiology and function in zebrafish.

The vestibular system of the inner ear provides information about head motion and spatial orientation relative to gravity to ensure gaze stability, balance, and postural control. Zebrafish, like humans, have five sensory patches per ear that serve as peripheral vestibular organs, with the addition of the lagena and macula neglecta. The zebrafish inner ear can be easily studied due to its accessible location, the transparent tissue of larval fish, and the early development of vestibular behaviors. Thus, zebrafish are an excellent model for studying the development, physiology, and function of the vestibular system. Recent work has made great strides to elucidate vestibular neural circuitry in fish, tracing sensory transmission from receptors in the periphery to central computational circuits driving vestibular reflexes. Here we highlight recent work that illuminates the functional organization of vestibular sensory epithelia, innervating first-order afferent neurons, and second-order neuronal targets in the hindbrain. Using a combination of genetic, anatomical, electrophysiological, and optical techniques, these studies have probed the roles of vestibular sensory signals in fish gaze, postural, and swimming behaviors. We discuss remaining questions in vestibular development and organization that are tractable in the zebrafish model.

Interspecific Variation in the Inner Ear Maculae of Sharks.

There is well-documented diversity in the organization of inner ear hair cells in fishes; this variation is thought to reflect the differing functional requirements of species across a range of ecological niches. However, relatively little is known about interspecific variation (and its potential ecological implications) in the number and density of inner ear hair cells in elasmobranchs (sharks, skates, and rays). In this study, we quantified inner ear hair cells in the saccule, lagena, utricle, and macula neglecta of 9 taxonomically and ecologically distinct shark species. Using phylogenetically informed comparative approaches, sharks that feed in the water column had significantly greater hair cell density and total number of hair cells in the lagena and macula neglecta (i.e., vertically oriented maculae) compared to species that feed primarily on the seafloor. In addition, sharks within Carcharhinidae seemingly possess a specialized macula neglecta compared to other shark species. Overall, findings suggest that, similar to bony fishes, there is considerable variation in hair cell organization of shark inner ears, which may be tied to variation in ecology and/or specialized behaviors between different species.

Ontogenetic development of inner ear hair cell organization in the New Zealand carpet shark Cephaloscyllium isabellum

IntroductionThe inner ear hair cells of fishes can provide insight into the early evolution of vertebrate inner ear structure. Fishes represent some of the first vertebrates to evolve auditory capacity, and the same basic structure, the sensory hair cell, provides the fundament for auditory and vestibular function in jawed vertebrates. Despite holding critical basal position in the evolutionary tree of gnathostomes, relatively little is known about inner ear hair cells in elasmobranchs. Specifically, the extent of plasticity in hair cell organization throughout ontogeny among different sensory epithelia and the degree of variation between species is unknown.MethodsIn this study, we characterized the inner ear hair cells of the New Zealand carpet shark Cephaloscyllium isabellum throughout ontogeny by quantifying macular area, number of hair cells, hair cell density, and hair cell orientations in the inner ear maculae from a range of body sizes.ResultsSimilar to other elasmobranchs and bony fishes, macular area and the number of hair cells increased throughout ontogeny in the otolith organs. The orientations of hair cells within each maculae also was consistent with the limited data on other elasmobranchs. However, contrary to expectation, the macula neglecta did not increase in area or hair cell number throughout ontogeny, and hair cell density did not change with body size in any maculae.DiscussionThese findings suggest there may be variation between elasmobranch species in ontogenetic development of hair cell organization that may be related to hearing capabilities throughout life.

Ontogeny of the inner ear maculae in school sharks (Galeorhinus galeus)

Studies on the auditory system of fishes can provide fundamental information about the early evolution of vertebrate hearing. While there are limited data available on the auditory system of bony fishes, comparatively far less is known about auditory structures in elasmobranchs, despite their critical basal position within vertebrate evolution. Specifically, while there is a high degree of plasticity in the nervous system, little is known about how the different sensory epithelia within the inner ear vary throughout life in elasmobranchs. Using a combination of immunohistochemistry and fluorescence microscopy, we quantified macular area, number of sensory hair cells, hair cell density, and hair cell orientations in the saccule, lagena, utricle, and macula neglecta of school sharks (Galeorhinus galeus) of varying body size. In all maculae, macular area and the number of hair cells increased significantly throughout ontogeny, while hair cell density displayed a concurrent ontogenetic decrease (excluding the utricle). There were also significant differences in macular area, hair cell number, and hair cell density between the four maculae. However, hair cell orientation patterns did not vary between individuals and did not change with body growth. These findings represent one of the first comprehensive characterisations of the inner ear sensory epithelia in an elasmobranch, and reveal changes in morphology that may have implications for auditory capabilities through ontogeny.

Cyp1B1 expression patterns in the developing chick inner ear.

Retinoic acid (RA) plays an important role in organogenesis as a paracrine signal through transcriptional regulation of an increasing number of known downstream target genes, regulating cell proliferation, and differentiation. During the development of the inner ear, RA directly governs the morphogenesis and specification processes mainly by means of RA-synthesizing retinaldehyde dehydrogenase (RALDH) enzymes. Interestingly, CYP1B1, a cytochrome P450 enzyme, is able to mediate the oxidative metabolisms also leading to RA generation, its expression patterns being associated with many known sites of RA activity. This study describes for the first time the presence of CYP1B1 in the developing chick inner ear as a RALDH-independent RA-signaling mechanism. In our in situ hybridization analysis, Cyp1B1 expression was first observed in a domain located in the ventromedial wall of the otic anlagen, being included within the rostralmost aspect of an Fgf10-positive pan-sensory domain. As development proceeds, all identified Fgf10-positive areas were Cyp1B1 stained, with all sensory patches being Cyp1B1 positive at stage HH34, except the macula neglecta. Cyp1B1 expression suggested a possible contribution of CYP1B1 action in the specification of the lateral-to-medial and dorsal-to-ventral axes of the developing chick inner ear.

Fgf3 and Fgf16 expression patterns define spatial and temporal domains in the developing chick inner ear.

The inner ear is a morphologically complex sensory structure with auditory and vestibular functions. The developing otic epithelium gives rise to neurosensory and non-sensory elements of the adult membranous labyrinth. Extrinsic and intrinsic signals manage the patterning and cell specification of the developing otic epithelium by establishing lineage-restricted compartments defined in turn by differential expression of regulatory genes. FGF3 and FGF16 are excellent candidates to govern these developmental events. Using the chick inner ear, we show that Fgf3 expression is present in the borders of all developing cristae. Strong Fgf16 expression was detected in a portion of the developing vertical and horizontal pouches, whereas the cristae show weaker or undetected Fgf16 expression at different developmental stages. Concerning the rest of the vestibular sensory elements, both the utricular and saccular maculae were Fgf3 positive. Interestingly, strong Fgf16 expression delimited these Fgf16-negative sensory patches. The Fgf3-negative macula neglecta and the Fgf3-positive macula lagena were included within weakly Fgf16-expressing areas. Therefore, different FGF-mediated mechanisms might regulate the specification of the anterior (utricular and saccular) and posterior (neglecta and lagena) maculae. In the developing cochlear duct, dynamic Fgf3 and Fgf16 expression suggests their cooperation in the early specification and later cell differentiation in the hearing system. The requirement of Fgf3 and Fgf16 genes in endolymphatic apparatus development and neurogenesis are discussed. Based on these observations, FGF3 and FGF16 seem to be key signaling pathways that control the inner ear plan by defining epithelial identities within the developing otic epithelium.

Fgf10 expression patterns in the developing chick inner ear

The inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. It originates from the otic placode, which invaginates, forming the otic vesicle; the latter gives rise to neurosensory and nonsensory elements of the adult membranous labyrinth. A hypothesis based on descriptive and experimental evidence suggests that the acquisition of discrete sensory patches during evolution of this primordium may be related to subdivision of an early pansensory domain. In order to gain insight into this developmental mechanism, we carried out a detailed analysis of the spatial and temporal expression pattern of the gene Fgf10, by comparing different markers of otic patterning and hair cell differentiation. Fgf10 expression labels a sensory-competent domain included in a Serrate-positive territory from which most of the sensory epithelia arise. Our data show that Fgf10 transcripts are present initially in a narrow ventromedial band of the rudimentary otocyst, extending between its rostral and caudal poles. During development, this Fgf10-expressing area splits repetitively into several separate subareas, creating six of the eight sensory organs present in birds. Only the lateral crista and the macula neglecta were initially Fgf10 negative, although they activated Fgf10 expression after their specification as sensory elements. These results allowed us to determine a timetable of sensory specification in the developing chick inner ear. The comparison of the expression pattern of Fgf10 with those of other markers of sensory differentiation contributes to our understanding of the mechanism by which vertebrate inner ear prosensory domains have arisen during evolution.

The polarization of hair cells from the inner ear of the lesser spotted dogfish Scyliorhinus canicula

The polarization of hair cells from the end organs of the inner ear from the lesser spotted dogfish Scyliorhinus canicula from the order Carcharhiniforms (ground sharks) was studied using a scanning electron microscope (SEM), revealing arrays of hair cells with diverse orientations on each of the sensory epithelia. The greatest numbers of hair cells were found on the utricular epithelium and orientated on the horizontal plane, whilst the smallest number were found on the anterior macula neglecta and orientated on the vertical plane. Examination of the posterior macula neglecta revealed a dense profusion of c. 500 000 individual elongate structures, each resembling long microvilli, a finding not previously described in elasmobranch auditory physiology.

The directional hearing abilities of two species of bamboo sharks

Auditory evoked potentials (AEPs) were used to measure the directional hearing thresholds of the white-spotted bamboo shark Chiloscyllium plagiosum and the brown-banded bamboo shark Chiloscyllium punctatum at four frequencies and seven directions, using a shaker table designed to mimic the particle motion component of sound. Over most directions and frequencies there were no significant differences in acceleration thresholds, suggesting that the sharks have omni-directional hearing abilities. Goldfish Carassius auratus were used as a baseline to compare a species with specialized hearing adaptations versus sharks with no known adaptations, and were found to have more sensitive directional responses than the sharks. Composite audiograms of the sharks were created from the average of all of the directions at each frequency and were compared with an audiogram obtained for C. plagiosum using a dipole stimulus. The dipole stimulus audiograms were significantly lower at 50 and 200 Hz compared to the shaker audiograms in terms of particle acceleration. This difference is hypothesized to be a result of the dipole stimulating the macula neglecta, which would not be stimulated by the shaker table.

Fgf19 expression patterns in the developing chick inner ear

The inner ear is a complex sensorial structure with hearing and balance functions. A key aim of developmental biology is to understand the molecular and cellular mechanisms involved in the induction, patterning and innervation of the vertebrate inner ear. These developmental events could be mediated by the expression of regulating genes, such as the members of the family of Fibroblast Growth Factors (Fgfs). This work reports the detailed spatial and temporal patterns of Fgf19 expression in the developing inner ear from otic cup (stage 14) to 8 embryonic days (stage 34). In the earliest stages, Fgf19 and Fgf8 expressions determine two subdomains within the Fgf10-positive proneural-sensory territory. We show that, from the earliest stages, the Fgf19 expression was detected in the acoustic-vestibular ganglion and the macula utriculi. The Fgf19 gene was also strongly, but transiently, expressed in the macula lagena, whereas the macula neglecta never expressed this gene in the period analysed. The Fgf19 expression was also clearly observed in some borders of various sensory elements. These results could be useful from further investigations into the role of FGF19 in otic patterning.

Structural variation in the inner ears of four deep‐sea elopomorph fishes

Deep-sea fishes have evolved in dark or dimly lit environments devoid of the visual cues available to shallow-water species. Because of the limited opportunity for visual scene analysis by deep-sea fishes, it is reasonable to hypothesize that the inner ears of at least some such species may have evolved structural adaptations to enhance hearing capabilities in lieu of vision. As an initial test of this hypothesis, scanning electron microscopy was used to examine the structure of the inner ears of four deep-sea elopomorph species inhabiting different depths: Synaphobranchus kaupii, Synaphobranchus bathybius, Polyacanthonotus challengeri, and Halosauropsis macrochir. The shape of the sensory epithelia and hair cell ciliary bundle orientation of the saccule, lagena, and utricle, the three otolithic organs associated with audition and vestibular function, are described. The saccules of all four species have a common, alternating ciliary bundle orientation pattern. In contrast, the lagena exhibits more interspecific diversity in shape and ciliary bundle orientation, suggesting that it has special adaptations in these species. The macula neglecta, a sensory epithelium of unknown function, is present in all four species.

The inner ear of the lungfish Protopterus.

The sensory end organs of the inner ear of the lungfish, Protopterus, were examined using scanning and transmission electron microscopy. The utricle has a structure and hair cell orientation pattern that are typical for vertebrates, although the hair cells are unusually large. There are the typical three semicircular canals extending from the utricle, with the typical hair cell orientations, but the lateral canal sensory crista looks like the "hemicrista" of some amphibians and amniotes, lacking a saddle-shaped flare on one wall of the ampulla. Unlike most vertebrates that have the saccule and lagena as two separate pouches ventral to the utricle, the lungfish has a single large ventral pouch that contains a single large pasty otoconial mass. This mass covers two hair cell patches, each like a striola with prominent hair cell ciliary bundles, that are presumed to represent saccular and lagenar maculae. However, these two major sensory patches are not completely separate maculae because they lie within a less densely populated field of smaller hair cells, which forms an extrastriolar region that surrounds and fills the region between the two striolae of higher hair cell density. The more caudal lagenar striola is a vertically elongated stripe with hair cell orientation vectors facing antiparallel on either side of a midline drawn vertically along the macula, resembling the macula lagena of some bony fishes but not of tetrapods. The more rostral saccular striola is a curving band with hair cell orientation vectors facing away from its midline, but because this macula curves in three dimensions, the vectors at the rostral end of this striola are oriented mediolaterally, whereas the vectors on the caudal half of this striola are oriented dorsoventrally. The presence of a macula neglecta was confirmed near the posterior canal as a tiny single patch of a few dozen hair cells with all the cell orientations directed caudally. The ciliary bundles on the cells in the striolar-like regions of all of three otolithic organs average over 80 cilia, a number far greater than for any other fish studied to date. The features of the single sacculolagenar pouch with separate striolar-like regions, the cellular orientation in the otolith organs, and the large cells and ciliary bundles in Protopterus also were observed in specimens of the other extant lungfish genera, Lepidosiren and Neoceratodus.

The Hearing Sensitivity of the Little Skate, Raja erinacea: A Comparison of Two Methods

We determined the hearing sensitivity of the little skate, Raja erinacea using two methods: Behavioral conditioning and the auditory brainstem response (ABR). This marks the first time that the hearing in any member of the Rajiformes has been examined and the first time that the ABR method has been used with an elasmobranch. We obtained audiograms of R. erinacea using each method and were found to be statistically similar. The best hearing sensitivity for R. erinacea was between 100 and 300 Hz. We compared the audiograms to audiograms obtained from other species of elasmobranchs. This analysis showed that R. erinacea, a bottom-dwelling elasmobranch, has less sensitive hearing than the lemon shark, Negaprion brevirostris, and the bull shark, Carcharhinus leucas, a free-swimming, raptorial elasmobranch. However, R. erinacea showed sensitivity comparable to that of the horn shark, Heterodontus francisi, another bottom-dwelling elasmobranch; both species feed primarily on benthic prey. These findings are in agreement with Corwin's hypothesis (1978) that hearing sensitivity is correlated with feeding behavior. An examination of the macula neglecta of R. erinacea found a total count of 10 000 hair cells, which is within the range of other bottom-dwelling elasmobranchs.

Morphology and cell type heterogeneities of the inner ear epithelia in adult and juvenile zebrafish (Danio rerio).

Although the zebrafish has become an important model for genetic analysis of the vertebrate auditory system, a comprehensive description of the zebrafish ear has been provided for embryonic and larval development only (Haddon and Lewis [1996] J. Comp. Neurol. 365:113). Here we describe the development of sensory maculae in juvenile fish and the morphology of the adult zebrafish ear. This description was obtained via three-dimensional reconstruction of serial sections and confocal microscopy of immunolabeled preparations and includes the Weberian ossicles and fluid spaces. Phalloidin staining, which labels actin filaments of stereocilia, was used to delineate the sensory epithelia, to visualize the distribution of hair cells, to estimate their density in different areas of the maculae, and to perform hair cell counts. Morphology of ciliary bundles in different regions of the lagena, saccule, utricle, macula neglecta, and cristae was characterized with an anti-acetylated tubulin antibody and by phalloidin staining. We have identified two antibodies characterized by region-specific staining patterns in the inner ear epithelia. Zn-1 antibody staining largely correlates with the presence of short-bundle hair cells in the peripheral regions of sensory epithelia. Zn-4 antibody, on the other hand, labels a zone of epithelial cells surrounding the sensory maculae. These analyses extend previous observations of cell-type heterogeneity in both sensory and nonsensory epithelia of the fish ear.

Sensory Organ Generation in the Chick Inner Ear

There are a total of eight sensory organs in the chick inner ear. Each sensory organ has a distinct structure tailored for its function, and its morphology is well characterized. However, the origin of these sensory organs and the lineage relationships among them are largely unknown. In this report, we show that BMP4 (bone morphogenetic protein), a secreted protein of the TGF-beta gene family, is the earliest sensory marker identified to date for the chick inner ear. In addition to BMP4, we show that Msx-1 is a sensory marker for the three cristae, the lagena, and macula neglecta. P75NGFR (nerve growth factor receptor) is a marker for the three cristae only. Based on the expression pattern of these three genes-BMP4, Msx-1, and p75NGFR-it is estimated that the first sensory organs to be generated were the superior and posterior cristae at stage 19, followed by the macula sacculi at stage 20, the lateral crista at stage 22, the basilar papilla and lagena at stage 23, the macula utriculi at stage 24, and the macula neglecta at stage 29. The age of generation of each sensory organ as defined by the first appearance of these molecular markers is well in advance of the histological differentiation. In addition, the differential gene expressions in each presumptive sensory organ may contribute to the distinct structure of the mature organ.

Organization of Inner Ear Endorgan Projections in the Goldfish, <i>Carassius auratus</i>

Cytoarchitectural analysis of the octavolateralis area of the goldfish, Carassius auratus, reveals that as in other teleosts, five first-order octaval nuclei are present: the anterior octaval, magnocellular, descending, tangential, and posterior octaval nuclei. The descending nucleus appears to be anatomically specialized relative to that of the halecomorph Amia calva and many teleosts in that a large dorsomedial subpopulation of the nucleus lies medial to nucleus medialis, a first-order lateral line nucleus. In addition to this dorsomedial zone, the descending nucleus is made up of an intermediate and a ventral zone. Application of horseradish peroxidase (HRP) to individual inner ear endorgans reveals that the distribution of these afferents to the octaval nuclei is generally similar to that in another otophysan, Ictalurus punctatus [McCormick and Braford, 1993]. Nucleus magnocellularis receives a diffuse projection from all of the endorgans. The semicircular canals project heavily to the nucleus tangentialis, the entire ventral zone and portions of the intermediate zone of the descending nucleus, the ventral portion of the caudal anterior nucleus, and the bulk of the rostral anterior nucleus. The macula neglecta projects to the intermediate zone of the descending nucleus and to ventral locations within the dorsal half of the caudal anterior nucleus. The otolithic endorgans--the saccule, lagena, and utricle--project, in an overlapping manner, to the dorsal half of the caudal anterior nucleus and minimally to the rostral anterior nucleus. The inputs of the otolithic endorgans to the intermediate zone of the descending nucleus are more segregated, though a given region is sometimes supplied by more than one endorgan. The projections of the saccule tend to be concentrated more medially than those of the other two endorgans. The dorsomedial zone of the descending nucleus receives the majority of its primary input from the saccule, and a much smaller input from the lagena, over most of its rostrocaudal extent. At caudal-most levels of the dorsomedial zone, afferents from the three otolithic endorgans overlap.

Development of the inner ear of the brown trout (Salmo trutta fario): I. Gross morphology and sensory cell proliferation.

The gross development of the trout inner ear between embryonic and juvenile stages was studied by light microscopy. The otocyst has already formed in 3-4 mm embryos. The semicircular canals begin to separate from the utriculo-saccular cavity in 6 mm embryos, the anterior canal first, then the posterior and the horizontal canal later. The formation of the saccular cavity begins in 7 mm embryos, whereas that of the lagena occurs in 18 mm fry. The first macular primordia appear before the separation of cavities. The anterior and horizontal crests arise from the primordium of the utricular macula, and the posterior crest, macula lagena, and macula neglecta arise from that of the saccular macula. The macula lagena and macula neglecta appear later. The sensory areas of the labyrinth and the number of receptor cells grow continuously between the embryonic and juvenile stages. © 1993 Wiley-Liss, Inc.

The Primary Octaval Nuclei and Inner Ear Afferent Projections in the Otophysan <i>Ictalurus punctatus</i>

Physical coupling between the swimbladder and the inner ear is believed to contribute to the enhanced hearing abilities of otophysans relative to those of teleosts lacking comparable otic specializations. We hypothesized that the auditory circuitry of otophysans might also exhibit derived features [McCormick and Braford, 1988]. As a first test of this hypothesis, we examined the normal anatomy and inner ear inputs of the primary octaval nuclei in the catfish Ictalurus punctatus. From Nissl-stained sections we conclude that Ictalurus, like other teleosts, has five first-order octaval nuclei: the anterior, magnocellular, tangential, descending, and posterior octaval nuclei. The overall projection pattern of the seven inner ear endorgans, determined using the exsanguinated horseradish peroxidase procedure [McCormick and Braford, 1984], is very similar to that hypothesized as primitive for gnathostome fishes. The otolithic endorgans project to dorsal areas of the descending and anterior nuclei, whereas the macula neglecta and cristae of the semicircular canals project more ventrally. Three zones are recognized within the descending nucleus-the dorsomedial, intermediate, and ventral zones. The dorsomedial zone, which is the main terminus of saccular fibers, has specialized morphological features: it extends far dorsally and contacts the medial portion of the cerebellar crest, and it is partitioned into lateral and medial portions by entering facial nerve fibers. The caudal anterior nucleus is likewise partitioned into lateral and medial portions by the facial nerve. Hypotheses addressing these derived features are presented.

The membranous labyrinth and its innervation inChaetodon trifasciatus (Pisces, Teleostei, Chaetodontidae)

In the butterflyfishChaetodon trifasciatus, the labyrinth is characterized by its elevated form and especially the size of the vertical canals, the almost circular form of the horizontal canal and its posterior opening not directly in the utriculus but in the common pillar of the two vertical canals. There is an almost complete separation between utriculus and sacculus which are only linked by a virtual pore. The lagena, which is medially situated to the posterior part of the sacculus, is separated from it by an incomplete vertical wall. There are two maculae neglectae, the anterior macula being situated in the pore separating utriculus from sacculus and filling this pore, the posterior in a gutter of the floor of the utriculus. A long and narrow endolymphatic canal, originating from the sacculus close to the communication with the utriculus, follows the common pillar of the two vertical canals and widens into an endolymphatic sac at the top of the membranous labyrinth. The innervation of the labyrinth is made by the acoustic ganglion, which is connected to the brain by two roots and elongated into three parts: the anterior part innervates the anterior and horizontal cristae and the utricular and saccular maculae; the middle part innervates the macula sacculae and the macula neglecta 1; the posterior part innervates the macula neglecta II, the macula lagenae and the posterior crista. The important size of the vertical canals and the almost circular form of the horizontal canal may reflect very precise locomotory aptitudes.

Functional anatomy of the auditory system in sharks and rays

This article proposes that elasmobranch ears may be divided into two general categories based on differences in the development of an ensemble of anatomical features that are believed to be associated with specialization for sound detection. The ear in Carcharhinus is typical of one category. It contains the largest sensory epithelia in any vertebrate ear. In the wild, carcharhinid sharks can be attracted by pulsed low-frequency sounds, such as those that are produced by struggling wounded fish. They can hear frequencies up to at least 1 kHz and can localize the direction to a source of sound from at least 250 m. Responses to sound have been recorded from the non-otolithic macula neglecta and the otolithic sacculus in carcharhinid sharks. Those sensory epithelia and the structures associated with a dorsal sound channel that leads to the macula neglecta are much more developed in this type of ear than in the ears of other elasmobranchs. It is proposed that auditory specialization may provide a particular selective advantage to carcharhinid sharks and other shark species that feed by raptorial predation. The auditory pathways of the brain have been studied in batoid elasmobranchs where the ear does not appear to be particularly specialized for sound detection. The neurons that contact the sensory epithelia of the ear project to a series of first-order relay nuclei that form the octaval column of the medulla oblongata. The cells in that column project to the vestibular system and to centers in a lemniscal pathway where physiological responses to sound have been recorded. The organization of the identified acoustic centers in elasmobranch brains resembles that found in tetrapods, suggesting that a basic auditory lemniscal pathway may have evolved in a common ancestor of those groups more than 400 million years ago.