Morphology of the monotreme organ of Corti and macula lagena

Development and evolution of the vestibular apparatuses of the inner ear

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

Development of the cochlea

The cochlea, a coiled structure located in the ventral region of the inner ear, acts as the primary structure for the perception of sound. Along the length of the cochlear spiral is the organ of Corti, a highly derived and rigorously patterned sensory epithelium that acts to convert auditory stimuli into neural impulses. The development of the organ of Corti requires a series of inductive events that specify unique cellular characteristics and axial identities along its three major axes. Here, we review recent studies of the cellular and molecular processes regulating several aspects of cochlear development, such as axial patterning, cochlear outgrowth and cellular differentiation. We highlight how the precise coordination of multiple signaling pathways is required for the successful formation of a complete organ of Corti.

The common marmoset as suitable nonhuman alternative for the analysis of primate cochlear development

Cochlear development is a complex process with precise spatiotemporal patterns. A detailed understanding of this process is important for studies of congenital hearing loss and regenerative medicine. However, much of our understanding of cochlear development is based on rodent models. Animal models that bridge the gap between humans and rodents are needed. In this study, we investigated the development of hearing organs in a small New World monkey species, the common marmoset (Callithrix jacchus). We describe the general stages of cochlear development in comparison with those of humans and mice. Moreover, we examined more than 25 proteins involved in cochlear development and found that expression patterns were generally conserved between rodents and primates. However, several proteins involved in supporting cell processes and neuronal development exhibited interspecific expression differences. Human fetal samples for studies of primate‐specific cochlear development are extremely rare, especially for late developmental stages. Our results support the use of the common marmoset as an effective alternative for analyses of primate cochlear development.

Inner Ear Genes Underwent Positive Selection and Adaptation in the Mammalian Lineage

The mammalian inner ear possesses functional and morphological innovations that contribute to its unique hearing capacities. The genetic bases underlying the evolution of this mammalian landmark are poorly understood. We propose that the emergence of morphological and functional innovations in the mammalian inner ear could have been driven by adaptive molecular evolution. In this work, we performed a meta-analysis of available inner ear gene expression data sets in order to identify genes that show signatures of adaptive evolution in the mammalian lineage. We analyzed ∼1,300 inner ear expressed genes and found that 13% show signatures of positive selection in the mammalian lineage. Several of these genes are known to play an important function in the inner ear. In addition, we identified that a significant proportion of genes showing signatures of adaptive evolution in mammals have not been previously reported to participate in inner ear development and/or physiology. We focused our analysis in two of these genes: STRIP2 and ABLIM2 by generating null mutant mice and analyzed their auditory function. We found that mice lacking Strip2 displayed a decrease in neural response amplitudes. In addition, we observed a reduction in the number of afferent synapses, suggesting a potential cochlear neuropathy. Thus, this study shows the usefulness of pursuing a high-throughput evolutionary approach followed by functional studies to track down genes that are important for inner ear function. Moreover, this approach sheds light on the genetic bases underlying the evolution of the mammalian inner ear.

Petrosal morphology and cochlear function in Mesozoic stem therians

Here we describe the bony anatomy of the inner ear and surrounding structures seen in three plesiomorphic crown mammalian petrosal specimens. Our study sample includes the triconodont Priacodon fruitaensis from the Upper Jurassic of North America, and two isolated stem therian petrosal specimens colloquially known as the Höövör petrosals, recovered from Aptian-Albian sediments in Mongolia. The second Höövör petrosal is here described at length for the first time. All three of these petrosals and a comparative sample of extant mammalian taxa have been imaged using micro-CT, allowing for detailed anatomical descriptions of the osteological correlates of functionally significant neurovascular features, especially along the abneural wall of the cochlear canal. The high resolution imaging provided here clarifies several hypotheses regarding the mosaic evolution of features of the cochlear endocast in early mammals. In particular, these images demonstrate that the membranous cochlear duct adhered to the bony cochlear canal abneurally to a secondary bony lamina before the appearance of an opposing primary bony lamina or tractus foraminosus. Additionally, while corroborating the general trend of reduction of venous sinuses and plexuses within the pars cochlearis seen in crownward mammaliaforms generally, the Höövör petrosals show the localized enlargement of a portion of the intrapetrosal venous plexus. This new vascular feature is here interpreted as the bony accommodation for the vein of cochlear aqueduct, a structure that is solely, or predominantly, responsible for the venous drainage of the cochlear apparatus in extant therians. Given that our fossil stem therian inner ear specimens appear to have very limited high-frequency capabilities, the development of these modern vascular features of the cochlear endocast suggest that neither the initiation or enlargement of the stria vascularis (a unique mammalian organ) was originally associated with the capacity for high-frequency hearing or precise sound-source localization.

A Functional Perspective on the Evolution of the Cochlea

This review summarizes paleontological data as well as studies on the morphology, function, and molecular evolution of the cochlea of living mammals (monotremes, marsupials, and placentals). The most parsimonious scenario is an early evolution of the characteristic organ of Corti, with inner and outer hair cells and nascent electromotility. Most remaining unique features, such as loss of the lagenar macula, coiling of the cochlea, and bony laminae supporting the basilar membrane, arose later, after the separation of the monotreme lineage, but before marsupial and placental mammals diverged. The question of when hearing sensitivity first extended into the ultrasonic range (defined here as >20 kHz) remains speculative, not least because of the late appearance of the definitive mammalian middle ear. The last significant change was optimizing the operating voltage range of prestin, and thus the efficiency of the outer hair cells' amplifying action, in the placental lineage only.

Evolutionary and Developmental Biology Provide Insights Into the Regeneration of Organ of Corti Hair Cells

We review the evolution and development of organ of Corti hair cells with a focus on their molecular differences from vestibular hair cells. Such information is needed to therapeutically guide organ of Corti hair cell development in flat epithelia and generate the correct arrangement of different hair cell types, orientation of stereocilia, and the delayed loss of the kinocilium that are all essential for hearing, while avoiding driving hair cells toward a vestibular fate. Highlighting the differences from vestibular organs and defining what is known about the regulation of these differences will help focus future research directions toward successful restoration of an organ of Corti following long-term hair cell loss.

Understanding Molecular Evolution and Development of the Organ of Corti Can Provide Clues for Hearing Restoration

The mammalian hearing organ is a stereotyped cellular assembly with orderly innervation: two types of spiral ganglion neurons (SGNs) innervate two types of differentially distributed hair cells (HCs). HCs and SGNs evolved from single neurosensory cells through gene multiplication and diversification. Independent regulation of HCs and neuronal differentiation through expression of basic helix-loop-helix transcription factors (bHLH TFs: Atoh1, Neurog1, Neurod1) led to the evolution of vestibular HC assembly and their unique type of innervation. In ancestral mammals, a vestibular organ was transformed into the organ of Corti (OC) containing a single row of inner HC (IHC), three rows of outer HCs (OHCs), several unique supporting cell types, and a peculiar innervation distribution. Restoring the OC following long-term hearing loss is complicated by the fact that the entire organ is replaced by a flat epithelium and requires reconstructing the organ from uniform undifferentiated cell types, recapitulating both evolution and development. Finding the right sequence of gene activation during development that is useful for regeneration could benefit from an understanding of the OC evolution. Toward this end, we report on Foxg1 and Lmx1a mutants that radically alter the OC cell assembly and its innervation when mutated and may have driven the evolutionary reorganization of the basilar papilla into an OC in ancestral Therapsids. Furthermore, genetically manipulating the level of bHLH TFs changes HC type and distribution and allows inference how transformation of HCs might have happened evolutionarily. We report on how bHLH TFs regulate OHC/IHC and how misexpression (Atoh1-Cre; Atoh1f/kiNeurog1) alters HC fate and supporting cell development. Using mice with altered HC types and distribution, we demonstrate innervation changes driven by HC patterning. Using these insights, we speculate on necessary steps needed to convert a random mixture of post-mitotic precursors into the orderly OC through spatially and temporally regulated critical bHLH genes in the context of other TFs to restore normal innervation patterns.

Structure, Function, and Development of the Tectorial Membrane: An Extracellular Matrix Essential for Hearing

The tectorial membrane is an extracellular matrix that lies over the apical surface of the auditory epithelia in the inner ears of reptiles, birds, and mammals. Recent studies have shown it is composed of a small set of proteins, some of which are only produced at high levels in the ear and many of which are the products of genes that, when mutated, cause nonsyndromic forms of human hereditary deafness. Quite how the proteins of the tectorial membrane are assembled within the lumen of the inner ear to form a structure that is precisely regulated in its size and physical properties along the length of a tonotopically organized hearing organ is a question that remains to be fully answered. In this brief review we will summarize what is known thus far about the structure, protein composition, and function of the tectorial membrane in birds and mammals, describe how the tectorial membrane develops, and discuss major events that have occurred during the evolution of this extracellular matrix.

Shaping of inner ear sensory organs through antagonistic interactions between Notch signalling and Lmx1a

The mechanisms of formation of the distinct sensory organs of the inner ear and the non-sensory domains that separate them are still unclear. Here, we show that several sensory patches arise by progressive segregation from a common prosensory domain in the embryonic chicken and mouse otocyst. This process is regulated by mutually antagonistic signals: Notch signalling and Lmx1a. Notch-mediated lateral induction promotes prosensory fate. Some of the early Notch-active cells, however, are normally diverted from this fate and increasing lateral induction produces misshapen or fused sensory organs in the chick. Conversely Lmx1a (or cLmx1b in the chick) allows sensory organ segregation by antagonizing lateral induction and promoting commitment to the non-sensory fate. Our findings highlight the dynamic nature of sensory patch formation and the labile character of the sensory-competent progenitors, which could have facilitated the emergence of new inner ear organs and their functional diversification in the course of evolution.

Gene, cell, and organ multiplication drives inner ear evolution

We review the development and evolution of the ear neurosensory cells, the aggregation of neurosensory cells into an otic placode, the evolution of novel neurosensory structures dedicated to hearing and the evolution of novel nuclei in the brain and their input dedicated to processing those novel auditory stimuli. The evolution of the apparently novel auditory system lies in duplication and diversification of cell fate transcription regulation that allows variation at the cellular level [transforming a single neurosensory cell into a sensory cell connected to its targets by a sensory neuron as well as diversifying hair cells], organ level [duplication of organ development followed by diversification and novel stimulus acquisition] and brain nuclear level [multiplication of transcription factors to regulate various neuron and neuron aggregate fate to transform the spinal cord into the unique hindbrain organization]. Tying cell fate changes driven by bHLH and other transcription factors into cell and organ changes is at the moment tentative as not all relevant factors are known and their gene regulatory network is only rudimentary understood. Future research can use the blueprint proposed here to provide both the deeper molecular evolutionary understanding as well as a more detailed appreciation of developmental networks. This understanding can reveal how an auditory system evolved through transformation of existing cell fate determining networks and thus how neurosensory evolution occurred through molecular changes affecting cell fate decision processes. Appreciating the evolutionary cascade of developmental program changes could allow identifying essential steps needed to restore cells and organs in the future.

The mammalian Cretaceous cochlear revolution

The hearing organs of amniote vertebrates show large differences in their size and structure between the species' groups. In spite of this, their performance in terms of hearing sensitivity and the frequency selectivity of auditory-nerve units shows unexpectedly small differences. The only substantial difference is that therian, defined as live-bearing, mammalian groups are able to hear ultrasonic frequencies (above 15-20 kHz), whereas in contrast monotreme (egg laying) mammals and all non-mammalian amniotes cannot. This review compares the structure and physiology of the cochleae of the main groups and asks the question as to why the many structural differences seen in therian mammals arose, yet did not result in greater differences in physiology. The likely answers to this question are found in the history of the mammals during the Cretaceous period that ended 65 million years ago. During that period, the therian cochlea lost its lagenar macula, leading to a fall in endolymph calcium levels. This likely resulted in a small revolution and an auditory crisis that was compensated for by a subsequent series of structural and physiological adaptations. The end result was a system of equivalent performance to that independently evolved in other amniotes but with the additional - and of course "unforeseen" - advantage that ultrasonic-frequency responses became an available option. That option was not always availed of, but in most groups of therian mammals it did evolve and is used for communication and orientation based on improved sound localization, with micro-bats and toothed whales relying on it for prey capture.

Comparative Auditory Neuroscience: Understanding the Evolution and Function of Ears

Comparative auditory studies make it possible both to understand the origins of modern ears and the factors underlying the similarities and differences in their performance. After all lineages of land vertebrates had independently evolved tympanic middle ears in the early Mesozoic era, the subsequent tens of millions of years led to the hearing organ of lizards, birds, and mammals becoming larger and their upper frequency limits higher. In extant species, lizard papillae remained relatively small (<2 mm), but avian papillae attained a maximum length of 11 mm, with the highest frequencies in both groups near 12 kHz. Hearing-organ sizes in modern mammals vary more than tenfold, up to >70 mm (made possible by coiling), as do their upper frequency limits (from 12 to >200 kHz). The auditory organs of the three amniote groups differ characteristically in their cellular structure, but their hearing sensitivity and frequency selectivity within their respective hearing ranges hardly differ. In the immediate primate ancestors of humans, the cochlea became larger and lowered its upper frequency limit. Modern humans show an unusual trend in frequency selectivity as a function of frequency. It is conceivable that the frequency selectivity patterns in humans were influenced in their evolution by the development of speech.

Vestibular animal models: contributions to understanding physiology and disease

Our knowledge of the vestibular sensory system, its functional significance for gaze and posture stabilization, and its capability to ensure accurate spatial orientation perception and spatial navigation has greatly benefitted from experimental approaches using a variety of vertebrate species. This review summarizes the attempts to establish the roles of semicircular canal and otolith endorgans in these functions followed by an overview of the most relevant fields of vestibular research including major findings that have advanced our understanding of how this system exerts its influence on reflexive and cognitive challenges encountered during daily life. In particular, we highlight the contributions of different animal models and the advantage of using a comparative research approach. Cross-species comparisons have established that the morpho-physiological properties underlying vestibular signal processing are evolutionarily inherent, thereby disclosing general principles. Based on the documented success of this approach, we suggest that future research employing a balanced spectrum of standard animal models such as fish/frog, mouse and primate will optimize our progress in understanding vestibular processing in health and disease. Moreover, we propose that this should be further supplemented by research employing more “exotic” species that offer unique experimental access and/or have specific vestibular adaptations due to unusual locomotor capabilities or lifestyles. Taken together this strategy will expedite our understanding of the basic principles underlying vestibular computations to reveal relevant translational aspects. Accordingly, studies employing animal models are indispensible and even mandatory for the development of new treatments, medication and technical aids (implants) for patients with vestibular pathologies.

Evolution of mammalian sound localization circuits: A developmental perspective

Localization of sound sources is a central aspect of auditory processing. A unique feature of mammals is the smooth, tonotopically organized extension of the hearing range to high frequencies (HF) above 10kHz, which likely induced positive selection for novel mechanisms of sound localization. How this change in the auditory periphery is accompanied by changes in the central auditory system is unresolved. I will argue that the major VGlut2(+) excitatory projection neurons of sound localization circuits (dorsal cochlear nucleus (DCN), lateral and medial superior olive (LSO and MSO)) represent serial homologs with modifications, thus being paramorphs. This assumption is based on common embryonic origin from an Atoh1(+)/Wnt1(+) cell lineage in the rhombic lip of r5, same cell birth, a fusiform cell morphology, shared genetic components such as Lhx2 and Lhx9 transcription factors, and similar projection patterns. Such a parsimonious evolutionary mechanism likely accelerated the emergence of neurons for sound localization in all three dimensions. Genetic analyses indicate that auditory nuclei in fish, birds, and mammals receive contributions from the same progenitor lineages. Anatomical and physiological differences and the independent evolution of tympanic ears in vertebrate groups, however, argue for convergent evolution of sound localization circuits in tetrapods (amphibians, reptiles, birds, and mammals). These disparate findings are discussed in the context of the genetic architecture of the developing hindbrain, which facilitates convergent evolution. Yet, it will be critical to decipher the gene regulatory networks underlying development of auditory neurons across vertebrates to explore the possibility of homologous neuronal populations.

Where hearing starts: the development of the mammalian cochlea

The mammalian cochlea is a remarkable sensory organ, capable of perceiving sound over a range of 10(12) in pressure, and discriminating both infrasonic and ultrasonic frequencies in different species. The sensory hair cells of the mammalian cochlea are exquisitely sensitive, responding to atomic-level deflections at speeds on the order of tens of microseconds. The number and placement of hair cells are precisely determined during inner ear development, and a large number of developmental processes sculpt the shape, size and morphology of these cells along the length of the cochlear duct to make them optimally responsive to different sound frequencies. In this review, we briefly discuss the evolutionary origins of the mammalian cochlea, and then describe the successive developmental processes that lead to its induction, cell cycle exit, cellular patterning and the establishment of topologically distinct frequency responses along its length.

The quest for restoring hearing: Understanding ear development more completely

Neurosensory hearing loss is a growing problem of super-aged societies. Cochlear implants can restore some hearing, but rebuilding a lost hearing organ would be superior. Research has discovered many cellular and molecular steps to develop a hearing organ but translating those insights into hearing organ restoration remains unclear. For example, we cannot make various hair cell types and arrange them into their specific patterns surrounded by the right type of supporting cells in the right numbers. Our overview of the topologically highly organized and functionally diversified cellular mosaic of the mammalian hearing organ highlights what is known and unknown about its development. Following this analysis, we suggest critical steps to guide future attempts toward restoration of a functional organ of Corti. We argue that generating mutant mouse lines that mimic human pathology to fine-tune attempts toward long-term functional restoration are needed to go beyond the hope generated by restoring single hair cells in postnatal sensory epithelia.

Inner ear development: building a spiral ganglion and an organ of Corti out of unspecified ectoderm

The mammalian inner ear develops from a placodal thickening into a complex labyrinth of ducts with five sensory organs specialized to detect position and movement in space. The mammalian ear also develops a spiraled cochlear duct containing the auditory organ, the organ of Corti (OC), specialized to translate sound into hearing. Development of the OC from a uniform sheet of ectoderm requires unparalleled precision in the topological developmental engineering of four different general cell types, namely sensory neurons, hair cells, supporting cells, and general otic epithelium, into a mosaic of ten distinctly recognizable cell types in and around the OC, each with a unique distribution. Moreover, the OC receives unique innervation by ear-derived spiral ganglion afferents and brainstem-derived motor neurons as efferents and requires neural-crest-derived Schwann cells to form myelin and neural-crest-derived cells to induce the stria vascularis. This transformation of a sheet of cells into a complicated interdigitating set of cells necessitates the orchestrated expression of multiple transcription factors that enable the cellular transformation from ectoderm into neurosensory cells forming the spiral ganglion neurons (SGNs), while simultaneously transforming the flat epithelium into a tube, the cochlear duct, housing the OC. In addition to the cellular and conformational changes forming the cochlear duct with the OC, changes in the surrounding periotic mesenchyme form passageways for sound to stimulate the OC. We review molecular developmental data, generated predominantly in mice, in order to integrate the well-described expression changes of transcription factors and their actions, as revealed in mutants, in the formation of SGNs and OC in the correct position and orientation with suitable innervation. Understanding the molecular basis of these developmental changes leading to the formation of the mammalian OC and highlighting the gaps in our knowledge might guide in vivo attempts to regenerate this most complicated cellular mosaic of the mammalian body for the reconstitution of hearing in a rapidly growing population of aging people suffering from hearing loss.

Current concepts of hair cell differentiation and planar cell polarity in inner ear sensory organs

Phylogenetically and ontogenetically, vertebrate development led to the generation of several inner ear sensory organs. During embryogenesis, cell fate specification determines whether each progenitor cell differentiates into a sensory hair cell or a supporting cell within the common sensory primordium. Finally, all sensory epithelia of the inner ear consist of a hair cell/supporting cell mosaic, albeit with anatomical differences depending on the sensory organ type. Hair cells develop a polarized bundle of stereovilli that is of functional importance for mechanotransduction. After initiating stereovillar development, hair cells align their bundles in a coordinated fashion, generating a characteristic hair cell orientation pattern, a process referred to as planar cell polarity (PCP). The pathway that controls PCP in the inner ear needs both to establish the development of a polarized morphology of the stereovillar bundle of the hair cell and to organize a systematic hair cell alignment. Because the hair cell orientation patterns of the various inner ear organs and vertebrate species differ fundamentally, it becomes apparent that in vertebrates, different aspects of PCP need to be independently controlled. In spite of important progress recently gained in the field of PCP research, we still need to identify the mechanisms (1) that initiate molecular asymmetries in cells, (2) that guide the transmission of polarity information from cell to cell, and (3) that consistently translate such polarity information into morphological asymmetries of hair cells.

Brain and behaviour of living and extinct echidnas

The Tachyglossidae (long- and short-beaked echidnas) are a family of monotremes, confined to Australia and New Guinea, that exhibit striking trigeminal, olfactory and cortical specialisations. Several species of long-beaked echidna (Zaglossus robusta, Zaglossus hacketti, Megalibgwilia ramsayi) were part of the large-bodied (10 kg or more) fauna of Pleistocene Australasia, but only the diminutive (2-7 kg) Tachyglossus aculeatus is widespread today on the Australian mainland. We used high-resolution CT scanning and other osteological techniques to determine whether the remarkable neurological specialisations of modern echidnas were also present in Pleistocene forms or have undergone modification as the Australian climate changed in the transition from the Pleistocene to the Holocene. All the living and extinct echidnas studied have a similar pattern of cortical gyrification that suggests comparable functional topography to the modern short-beaked form. Osteological features related to olfactory, trigeminal, auditory and vestibular specialisation (e.g., foramina and cribriform plate area, osseous labyrinth topography) are also similar in living and extinct species. Our findings indicate that despite differences in diet, habitat and body size, the suite of neurological specialisations in the Tachyglossidae has been remarkably constant: encephalisation, sensory anatomy and specialisation (olfactory, trigeminal, auditory and vestibular), hypoglossal nerve size and cortical topography have all been stable neurological features of the group for at least 300,000 years.

Central projections of lagenar primary neurons in the chick

Perception of linear acceleration and head position is the function of the utricle and saccule in mammals. Nonmammalian vertebrates possess a third otolith endorgan, the macula lagena. Different functions have been ascribed to the lagena in arboreal birds, including hearing, equilibrium, homing behavior, and magnetoreception. However, no conclusive evidence on the function of the lagena in birds is currently available. The present study is aimed at providing a neuroanatomical substrate for the function of the lagena in the chicken as an example of terrestrial birds. The afferents from the lagena of chick embryos (E19) to the brainstem and cerebellum were investigated by the sensitive lipophilic tracer Neuro Vue Red in postfixed ears. The results revealed that all the main vestibular nuclei, including the tangential nucleus, received lagenar projections. No lagenar terminals were found in auditory centers, including the cochlear nuclei. In the cerebellum, the labeled terminals were found variably in all of the cerebellar nuclei. In the cerebellar cortex, the labeled fibers were found mostly in the uvula, with fewer afferents in the flocculus and paraflocculus. None was seen in the nodulus. The absence of lagenar afferent projections in auditory nuclei and the presence of a projection pattern in the vestibular nuclei and cerebellum similar to that of the utricle and saccule suggest that the primary role of the lagena in the chick lies in the processing of vestibular information related to linear acceleration and static head position.

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)].

Mosaic evolution of the mammalian auditory periphery

The classical mammalian auditory periphery, i.e., the type of middle ear and coiled cochlea seen in modern therian mammals, did not arise as one unit and did not arise in all mammals. It is also not the only kind of auditory periphery seen in modern mammals. This short review discusses the fact that the constituents of modern mammalian auditory peripheries arose at different times over an extremely long period of evolution (230 million years; Ma). It also attempts to answer questions as to the selective pressures that led to three-ossicle middle ears and the coiled cochlea. Mammalian middle ears arose de novo, without an intermediate, single-ossicle stage. This event was the result of changes in eating habits of ancestral animals, habits that were unrelated to hearing. The coiled cochlea arose only after 60 Ma of mammalian evolution, driven at least partly by a change in cochlear bone structure that improved impedance matching with the middle ear of that time. This change only occurred in the ancestors of therian mammals and not in other mammalian lineages. There is no single constellation of structural features of the auditory periphery that characterizes all mammals and not even all modern mammals.

Evolutionary paths to mammalian cochleae

Evolution of the cochlea and high-frequency hearing (>20 kHz; ultrasonic to humans) in mammals has been a subject of research for many years. Recent advances in paleontological techniques, especially the use of micro-CT scans, now provide important new insights that are here reviewed. True mammals arose more than 200 million years (Ma) ago. Of these, three lineages survived into recent geological times. These animals uniquely developed three middle ear ossicles, but these ossicles were not initially freely suspended as in modern mammals. The earliest mammalian cochleae were only about 2 mm long and contained a lagena macula. In the multituberculate and monotreme mammalian lineages, the cochlea remained relatively short and did not coil, even in modern representatives. In the lineage leading to modern therians (placental and marsupial mammals), cochlear coiling did develop, but only after a period of at least 60 Ma. Even Late Jurassic mammals show only a 270 ° cochlear coil and a cochlear canal length of merely 3 mm. Comparisons of modern organisms, mammalian ancestors, and the state of the middle ear strongly suggest that high-frequency hearing (>20 kHz) was not realized until the early Cretaceous (~125 Ma). At that time, therian mammals arose and possessed a fully coiled cochlea. The evolution of modern features of the middle ear and cochlea in the many later lineages of therians was, however, a mosaic and different features arose at different times. In parallel with cochlear structural evolution, prestins in therian mammals evolved into effective components of a new motor system. Ultrasonic hearing developed quite late-the earliest bat cochleae (~60 Ma) did not show features characteristic of those of modern bats that are sensitive to high ultrasonic frequencies.

Morphology and innervation of the vestibular lagena in pigeons

The morphological characteristics of the pigeon lagena were examined using histology, scanning electron microscopy, and biotinylated dextran amine (BDA) neural tracers. The lagena epithelium was observed to lie partially in a parasagittal plane, but was also U-shaped with orthogonal (lateral) directed tips. Hair cell planar polarities were oriented away from a central reversal line that ran nearly the length of the epithelium. Similar to the vertebrate utricle and saccule, three afferent classes were observed based upon their terminal innervation pattern, which include calyx, dimorph, and bouton fibers. Calyx and dimorph afferents innervated the striola region of the lagena, whereas bouton afferents innervated the extrastriola and a small region of the central striola known as the type II band. Calyx units had large calyceal terminal structures that innervated only type I hair cells. Dimorph afferents innervated both type I and II hair cells, with calyx and bouton terminals. Bouton afferents had the largest most complex innervation patterns and the greatest terminal areas contacting many hair cells.

The spiral ganglion: connecting the peripheral and central auditory systems

In mammals, the initial bridge between the physical world of sound and perception of that sound is established by neurons of the spiral ganglion. The cell bodies of these neurons give rise to peripheral processes that contact acoustic receptors in the organ of Corti, and the central processes collect together to form the auditory nerve that projects into the brain. In order to better understand hearing at this initial stage, we need to know the following about spiral ganglion neurons: (1) their cell biology including cytoplasmic, cytoskeletal, and membrane properties, (2) their peripheral and central connections including synaptic structure; (3) the nature of their neural signaling; and (4) their capacity for plasticity and rehabilitation. In this report, we will update the progress on these topics and indicate important issues still awaiting resolution.

Dissecting the molecular basis of organ of Corti development: Where are we now?

This review summarizes recent progress in our understanding of the molecular basis of cochlear duct growth, specification of the organ of Corti, and differentiation of the different types of hair cells. Studies of multiple mutations suggest that developing hair cells are involved in stretching the organ of Corti through convergent extension movements. However, Atoh1 null mutants have only undifferentiated and dying organ of Corti precursors but show a near normal extension of the cochlear duct, implying that organ of Corti precursor cells can equally drive this process. Some factors influence cochlear duct growth by regulating the cell cycle and proliferation. Shortened cell cycle and premature cell cycle exit can lead to a shorter organ of Corti with multiple rows of hair cells (e.g., Foxg1 null mice). Other genes affect the initial formation of a cochlear duct with or without affecting the organ of Corti. Such observations are consistent with evolutionary data that suggest some developmental uncoupling of cochlear duct from organ of Corti formation. Positioning the organ of Corti requires multiple genes expressed in the organ of Corti and the flanking region. Several candidate factors have emerged but how they cooperate to specify the organ of Corti and the topology of hair cells remains unclear. Atoh1 is required for differentiation of all hair cells, but regulation of inner versus outer hair cell differentiation is still unidentified. In summary, the emerging molecular complexity of organ of Corti development demands further study before a rational approach towards regeneration of unique types of hair cells in specific positions is possible.

Fossil evidence on evolution of inner ear cochlea in Jurassic mammals

The coiled cochlea is a key evolutionary innovation of modern therian mammals. We report that the Late Jurassic mammal Dryolestes, a relative to modern therians, has derived bony characteristics of therian-like innervation, but its uncoiled cochlear canal is less derived than the coiled cochlea of modern therians. This suggests a therian-like innervation evolved before the fully coiled cochlea in phylogeny. The embryogenesis of the cochlear nerve and ganglion in the inner ear of mice is now known to be patterned by neurogenic genes, which we hypothesize to have influenced the formation of the auditory nerve and its ganglion in Jurassic therian evolution, as shown by their osteological correlates in Dryolestes, and by the similar base-to-apex progression in morphogenesis of the ganglion in mice, and in transformation of its canal in phylogeny. The cochlear innervation in Dryolestes is the precursory condition in the curve-to-coil transformation of the cochlea in mammalian phylogeny. This provides the timing of the evolution, and where along the phylogeny the morphogenetic genes were co-opted into patterning the cochlear innervation, and the full coiling of the cochlea in modern therians.

Comparative aspects of cochlear functional organization in mammals

This review addresses the functional organization of the mammalian cochlea under a comparative and evolutionary perspective. A comparison of the monotreme cochlea with that of marsupial and placental mammals highlights important evolutionary steps towards a hearing organ dedicated to process higher frequencies and a larger frequency range than found in non-mammalian vertebrates. Among placental mammals, there are numerous cochlear specializations which relate to hearing range in adaptation to specific habitats that are superimposed on a common basic design. These are illustrated by examples of specialist ears which evolved excellent high frequency hearing and echolocation (bats and dolphins) and by the example of subterranean rodents with ears devoted to processing low frequencies. Furthermore, structural functional correlations important for tonotopic cochlear organization and predictions of hearing capabilities are discussed.

Analysis of magnetic elements in otoliths of the macula lagena in homing pigeons with inductively coupled plasma mass spectrometry

OBJECTIVE

The macula lagena in birds is located at the apical end of the cochlea and contains many tiny otoliths. The macula lagena is innervated and has neural projections to the brainstem, but its physiological function is still unclear. It remains disputable that it is because otoliths in the lagena are rich in elements Fe and Zn that birds can obtain geomagnetic information for homing. To clarify this issue, we carried out a study to determine whether or not otoliths in the lagena of homing pigeons are richer in magnetic elements than those in the saccule and the utricle.

METHODS

The contents of ferromagnetic elements (Fe, Co, Ni) and other metal elements in lagenal otoliths of adult homing pigeons were precisely analyzed with inductively coupled plasma mass spectrometry (ICP-MS) of high sensitivity, and then they were compared with those in saccular and utricular otoliths (all the contents were normalized to Ca).

RESULTS

In adult homing pigeons, the contents of ferromagnetic elements (Fe, Co, Ni) in lagenal otoliths were less than 0.7% (normalized to Ca element) and were the same order in magnitude as those in saccular and utricular otoliths. The content of Fe in lagenal otoliths was not significantly different from that in utricular otoliths and was even lower than that in saccular otoliths. The content of Co in lagenal otoliths was lower than that in saccular otoliths and higher than that in utricular otoliths. The content of Ni in lagenal otoliths was not significantly different from that in saccular otoliths and was higher than that in utricular otoliths. The contents of other metal elements Na, Mg, K, Al, Mn and Pb in lagenal otoliths were not significantly different from those in utricular and saccular otoliths. The contents of metal elements Zn, Ba and Cu in lagenal otoliths were lower than those in saccular otoliths.

CONCLUSION

The contents of magnetic elements in lagenal otoliths of homing pigeons are not much higher than those in utricular and saccular otoliths, which does not support the hypothesis that birds depend on high contents of Fe and Zn in lagenal otoliths for sensation of geomagnetic information. Similarities in morphology, element ingredient and element content between lagenal otoliths and utricular otoliths suggest that the two types of otolithic organs may play similar roles in sensing gravitational and acceleration signals.

Hey2 regulation by FGF provides a Notch-independent mechanism for maintaining pillar cell fate in the organ of Corti

The organ of Corti, the auditory organ of the inner ear, contains two types of sensory hair cells and at least seven types of supporting cells. Most of these supporting cell types rely on Notch-dependent expression of Hes/Hey transcription factors to maintain the supporting cell fate. Here, we show that Notch signaling is not necessary for the differentiation and maintenance of pillar cell fate, that pillar cells are distinguished by Hey2 expression, and that-unlike other Hes/Hey factors-Hey2 expression is Notch independent. Hey2 is activated by FGF and blocks hair cell differentiation, whereas mutation of Hey2 leaves pillar cells sensitive to the loss of Notch signaling and allows them to differentiate as hair cells. We speculate that co-option of FGF signaling to render Hey2 Notch independent also liberated pillar cells from the need for direct contact with surrounding hair cells, and enabled evolutionary remodeling of the complex cellular mosaic of the inner ear.

Evolutionary insights into the unique electromotility motor of mammalian outer hair cells

Prestin (SLC26A5) is the molecular motor responsible for cochlear amplification by mammalian cochlea outer hair cells and has the unique combined properties of energy-independent motility, voltage sensitivity, and speed of cellular shape change. The ion transporter capability, typical of SLC26A members, was exchanged for electromotility function and is a newly derived feature of the therian cochlea. A putative minimal essential motif for the electromotility motor (meEM) was identified through the amalgamation of comparative genomic, evolution, and structural diversification approaches. Comparisons were done among nonmammalian vertebrates, eutherian mammalian species, and the opossum and platypus. The opossum and platypus SLC26A5 proteins were comparable to the eutherian consensus sequence. Suggested from the point-accepted mutation analysis, the meEM motif spans all the transmembrane segments and represented residues 66-503. Within the eutherian clade, the meEM was highly conserved with a substitution frequency of only 39/7497 (0.5%) residues, compared with 5.7% in SLC26A4 and 12.8% in SLC26A6 genes. Clade-specific substitutions were not observed and there was no sequence correlation with low or high hearing frequency specialists. We were able to identify that within the highly conserved meEM motif two regions, which are unique to all therian species, appear to be the most derived features in the SLC26A5 peptide.

Foxg1 is required for morphogenesis and histogenesis of the mammalian inner ear

The forkhead genes are involved in patterning, morphogenesis, cell fate determination, and proliferation. Several Fox genes (Foxi1, Foxg1) are expressed in the developing otocyst of both zebrafish and mammals. We show that Foxg1 is expressed in most cell types of the inner ear of the adult mouse and that Foxg1 mutants have both morphological and histological defects in the inner ear. These mice have a shortened cochlea with multiple rows of hair cells and supporting cells. Additionally, they demonstrate striking abnormalities in cochlear and vestibular innervation, including loss of all crista neurons and numerous fibers that overshoot the organ of Corti. Closer examination shows that some anterior crista fibers exist in late embryos. Tracing these fibers shows that they do not project to the brain but, instead, to the cochlea. Finally, these mice completely lack a horizontal crista, although a horizontal canal forms but comes off the anterior ampulla. Anterior and posterior cristae, ampullae, and canals are reduced to varying degrees, particularly in combination with Fgf10 heterozygosity. Compounding Fgf10 heterozygotic effects suggest an additive effect of Fgf10 on Foxg1, possibly mediated through bone morphogenetic protein regulation. We show that sensory epithelia formation and canal development are linked in the anterior and posterior canal systems. Much of the Foxg1 phenotype can be explained by the participation of the protein binding domain in the delta/notch/hes signaling pathway. Additional Foxg1 effects may be mediated by the forkhead DNA binding domain.

Morphometry of otoliths in chicken macula lagena

The macula lagena located at the apical end of the cochlea in birds is characterized by the presence of numerous otoliths with unclear sensory functions. These otoliths are reported to be similar to those in the vestibular system but their detailed features in morphology are unknown. In the present study, we examined the number, size and shape of otoliths from the macula lagena in Chinese domestic chickens (Gallus Ling Nan) with a scanning electron microscope for morphometry. For chickens aged 10-15 post-hatch days, the otoliths in each macula lagena were counted to be 16,055 +/- 4038 (mean +/- S.D., n = 4). The average length and width were 12.98 +/- 3.70 microm and 5.10 +/- 1.48 microm (n = 526 otoliths), respectively. The ratio of length to width for the otolith was 2.58 +/- 0.39 (n = 526 otoliths) and remained relatively constant despite their variations in physical size. Almost all the otoliths were in regular shape and appeared like isolated cylinders with smooth facets at each end, but a few of them (0.025% of 64,221 otoliths screened) were found to be in odd shapes, such as T-shape and cross-shape. The results suggest that otoliths in the macula lagena and those in the vestibular system of bird's inner ear have similar physical properties and may play a similar role in sensing gravitational and acceleration signals.

Quantitative analysis of stereociliary arrays on vestibular hair cells

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

Protection from acoustic trauma is not a primary function of the medial olivocochlear efferent system

The medial olivocochlear (MOC) efferent system is an important component of an active mechanical outer hair cell system in mammals. An extensive neurophysiological literature demonstrates that the MOC system attenuates the response of the cochlea to sound by reducing the gain of the outer hair cell mechanical response to stimulation. Despite a growing understanding of MOC physiology, the biological role of the MOC system in mammalian audition remains uncertain. Some evidence suggests that the MOC system functions in a protective role by acting to reduce receptor damage during intense acoustic exposure. For the MOC system to have evolved as a protective mechanism, however, the inner ears of mammals must be exposed to potentially damaging sources of noise that can elicit MOC-mediated protective effects under natural conditions. In this review, we evaluate the possibility that the MOC system evolved to protect the inner ear from naturally occurring environmental noise. Our survey of nonanthropogenic noise levels shows that while sustained sources of broadband noise are found in nearly all natural acoustic environments, frequency-averaged ambient noise levels in these environments rarely exceed 70 dB SPL. Similarly, sources reporting ambient noise spectra in natural acoustic environments suggest that noise levels within narrow frequency bands are typically low in intensity (<40 dB SPL). Only in rare instances (e.g., during frog choruses) are ambient noise levels sustained at moderately high intensities (~70-90 dB SPL). By contrast, all experiments in which an MOC-mediated protective effect was demonstrated used much higher sound intensities to traumatize the cochlea (100-150 dB SPL). This substantial difference between natural ambient noise levels and the experimental conditions necessary to evoke MOC-mediated protection suggests that even the noisiest natural acoustic environments are not sufficiently intense to have selected for the evolution of the MOC system as a protective mechanism. Furthermore, although relatively intense noise environments do exist in nature, they are insufficiently distributed to account for the widespread distribution of the MOC system in mammals. The paucity of high-intensity noise and the near ubiquity of low-level noise in natural environments supports the hypothesis that the MOC system evolved as a mechanism for "unmasking" biologically significant acoustic stimuli by reducing the response of the cochlea to simultaneous low-level noise. This suggested role enjoys widespread experimental support.

Differential spatial organization of otolith signals in frog vestibular nuclei

Activation maps of pre- and postsynaptic field potential components evoked by separate electrical stimulation of utricular, lagenar, and saccular nerve branches in the isolated frog hindbrain were recorded within a stereotactic outline of the vestibular nuclei. Utricular and lagenar nerve-evoked activation maps overlapped strongly in the lateral and descending vestibular nuclei, whereas lagenar amplitudes were greater in the superior vestibular nucleus. In contrast, the saccular nerve-evoked activation map coincided largely with the dorsal nucleus and the adjacent dorsal part of the lateral vestibular nucleus, corroborating a major auditory and lesser vestibular function of the frog saccule. The stereotactic position of individual second-order otolith neurons matched the distribution of the corresponding otolith nerve-evoked activation maps. Furthermore, particular types of second-order utricular and lagenar neurons were clustered with particular types of second-order canal neurons in a topology that anatomically mirrored the preferred convergence pattern of afferent otolith and canal signals in second-order vestibular neurons. Similarities in the spatial organization of functionally equivalent types of second-order otolith and canal neurons between frog and other vertebrates indicated conservation of a common topographical organization principle. However, the absence of a precise afferent sensory topography combined with the presence of spatially segregated groups of particular second-order vestibular neurons suggests that the vestibular circuitry is organized as a premotor map rather than an organotypical sensory map. Moreover, the conserved segmental location of individual vestibular neuronal phenotypes shows linkage of individual components of vestibulomotor pathways with the underlying genetically specified rhombomeric framework.