The development of the hindbrain afferent projections in the axolotl: evidence for timing as a specific mechanism of afferent fiber sorting

An Integrated Perspective of Evolution and Development: From Genes to Function to Ear, Lateral Line and Electroreception

Four sensory systems (vestibular, lateral line, electroreception, auditory) are unique and project exclusively to the brainstem of vertebrates. All sensory neurons depend on a common set of genes (Eya1, Sox2, Neurog1, Neurod1) that project to a dorsal nucleus and an intermediate nucleus, which differentiate into the vestibular ear, lateral line and electroreception in vertebrates. In tetrapods, a loss of two sensory systems (lateral line, electroreception) leads to the development of a unique ear and auditory system in amniotes. Lmx1a/b, Gdf7, Wnt1/3a, BMP4/7 and Atoh1 define the lateral line, electroreception and auditory nuclei. In contrast, vestibular nuclei depend on Neurog1/2, Ascl1, Ptf1a and Olig3, among others, to develop an independent origin of the vestibular nuclei. A common origin of hair cells depends on Eya1, Sox2 and Atoh1, which generate the mechanosensory cells. Several proteins define the polarity of hair cells in the ear and lateral line. A unique connection of stereocilia requires CDH23 and PCDH15 for connections and TMC1/2 proteins to perceive mechanosensory input. Electroreception has no polarity, and a different system is used to drive electroreceptors. All hair cells function by excitation via ribbons to activate neurons that innervate the distinct target areas. An integrated perspective is presented to understand the gain and loss of different sensory systems.

Topologically correct central projections of tetrapod inner ear afferents require Fzd3

Inner ear sensory afferent connections establish sensory maps between the inner ear hair cells and the vestibular and auditory nuclei to allow vestibular and sound information processing. While molecular guidance of sensory afferents to the periphery has been well studied, molecular guidance of central projections from the ear is only beginning to emerge. Disorganized central projections of spiral ganglion neurons in a Wnt/PCP pathway mutant, Prickle1, suggest the Wnt/PCP pathway plays a role in guiding cochlear afferents to the cochlear nuclei in the hindbrain, consistent with known expression of the Wnt receptor, Frizzled3 (Fzd3) in inner ear neurons. We therefore investigated the role of Wnt signaling in central pathfinding in Fzd3 mutant mice and Fzd3 morpholino treated frogs and found aberrant central projections of vestibular afferents in both cases. Ear transplantations from knockdown to control Xenopus showed that it is the Fzd3 expressed within the ear that mediates this guidance. Also, cochlear afferents of Fzd3 mutant mice lack the orderly topological organization observed in controls. Quantification of Fzd3 expression in spiral ganglion neurons show a gradient of expression with Fzd3 being higher in the apex than in the base. Together, these results suggest that a gradient of Fzd3 in inner ear afferents directs projections to the correct dorsoventral column within the hindbrain.

Primary sensory map formations reflect unique needs and molecular cues specific to each sensory system

Interaction with the world around us requires extracting meaningful signals to guide behavior. Each of the six mammalian senses (olfaction, vision, somatosensation, hearing, balance, and taste) has a unique primary map that extracts sense-specific information. Sensory systems in the periphery and their target neurons in the central nervous system develop independently and must develop specific connections for proper sensory processing. In addition, the regulation of sensory map formation is independent of and prior to central target neuronal development in several maps. This review provides an overview of the current level of understanding of primary map formation of the six mammalian senses. Cell cycle exit, combined with incompletely understood molecules and their regulation, provides chemoaffinity-mediated primary maps that are further refined by activity. The interplay between cell cycle exit, molecular guidance, and activity-mediated refinement is the basis of dominance stripes after redundant organ transplantations in the visual and balance system. A more advanced level of understanding of primary map formation could benefit ongoing restoration attempts of impaired senses by guiding proper functional connection formations of restored sensory organs with their central nervous system targets.

Transplantation of Ears Provides Insights into Inner Ear Afferent Pathfinding Properties

Numerous tissue transplantations have demonstrated that otocysts can develop into normal ears in any location in all vertebrates tested thus far, though the pattern of innervation of these transplanted ears has largely been understudied. Here, expanding on previous findings that transplanted ears demonstrate capability of local brainstem innervation and can also be innervated themselves by efferents, we show that inner ear afferents grow toward the spinal cord mostly along existing afferent and efferent fibers and preferentially enter the dorsal spinal cord. Once in the dorsal funiculus of the spinal cord, they can grow toward the hindbrain and can diverge into vestibular nuclei. Inner ear afferents can also project along lateral line afferents. Likewise, lateral line afferents can navigate along inner ear afferents to reach hair cells in the ear. In addition, transplanted ears near the heart show growth of inner ear afferents along epibranchial placode-derived vagus afferents. Our data indicate that inner ear afferents can navigate in foreign locations, likely devoid of any local ear-specific guidance cues, along existing nerves, possibly using the nerve-associated Schwann cells as substrate to grow along. However, within the spinal cord and hindbrain, inner ear afferents can navigate to vestibular targets, likely using gradients of diffusible factors that define the dorso-ventral axis to guide them. Finally, afferents of transplanted ears functionally connect to native hindbrain vestibular circuitry, indicated by eliciting a startle behavior response, and providing excitatory input to specific sets of extraocular motoneurons.

Ear transplantations reveal conservation of inner ear afferent pathfinding cues

Vertebrate inner ear neurons project into the correct brainstem nuclei region before target neurons become postmitotic, or even in their absence. Moreover, afferents from transplanted ears in frogs have been shown to navigate to vestibular nuclei, suggesting that ear afferents use molecular cues to find their target. We performed heterochronic, xenoplastic, and heterotopic transplantations in chickens to investigate whether inner ear afferents are guided by conserved guidance molecules. We show that inner ear afferents can navigate to the vestibular nuclei following a delay in afferent entry and when the ear was from a different species, the mouse. These data suggest that guidance molecules are expressed for some time and are conserved across amniotes. In addition, we show that chicken ears transplanted adjacent to the spinal cord project dorsally like in the hindbrain. These results suggest that inner ear afferents navigate to the correct dorsoventral brainstem column using conserved cues.

Wilhelm His' lasting insights into hindbrain and cranial ganglia development and evolution

Wilhelm His (1831-1904) provided lasting insights into the development of the central and peripheral nervous system using innovative technologies such as the microtome, which he invented. 150 years after his resurrection of the classical germ layer theory of Wolff, von Baer and Remak, his description of the developmental origin of cranial and spinal ganglia from a distinct cell population, now known as the neural crest, has stood the test of time and more recently sparked tremendous advances regarding the molecular development of these important cells. In addition to his 1868 treatise on 'Zwischenstrang' (now neural crest), his work on the development of the human hindbrain published in 1890 provided novel ideas that more than 100 years later form the basis for penetrating molecular investigations of the regionalization of the hindbrain neural tube and of the migration and differentiation of its constituent neuron populations. In the first part of this review we briefly summarize the major discoveries of Wilhelm His and his impact on the field of embryology. In the second part we relate His' observations to current knowledge about the molecular underpinnings of hindbrain development and evolution. We conclude with the proposition, present already in rudimentary form in the writings of His, that a primordial spinal cord-like organization has been molecularly supplemented to generate hindbrain 'neomorphs' such as the cerebellum and the auditory and vestibular nuclei and their associated afferents and sensory organs.

Sensing External and Self-Motion with Hair Cells: A Comparison of the Lateral Line and Vestibular Systems from a Developmental and Evolutionary Perspective

Detection of motion is a feature essential to any living animal. In vertebrates, mechanosensory hair cells organized into the lateral line and vestibular systems are used to detect external water or head/body motion, respectively. While the neuronal components to detect these physical attributes are similar between the two sensory systems, the organizational pattern of the receptors in the periphery and the distribution of hindbrain afferent and efferent projections are adapted to the specific functions of the respective system. Here we provide a concise review comparing the functional organization of the vestibular and lateral line systems from the development of the organs to the wiring from the periphery and the first processing stages. The goal of this review is to highlight the similarities and differences to demonstrate how evolution caused a common neuronal substrate to adapt to different functions, one for the detection of external water stimuli and the generation of sensory maps and the other for the detection of self-motion and the generation of motor commands for immediate behavioral reactions.

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.

Spiral Ganglion Neuron Projection Development to the Hindbrain in Mice Lacking Peripheral and/or Central Target Differentiation

We investigate the importance of the degree of peripheral or central target differentiation for mouse auditory afferent navigation to the organ of Corti and auditory nuclei in three different mouse models: first, a mouse in which the differentiation of hair cells, but not central auditory nuclei neurons is compromised (Atoh1-cre; Atoh1f/f ); second, a mouse in which hair cell defects are combined with a delayed defect in central auditory nuclei neurons (Pax2-cre; Atoh1f/f ), and third, a mouse in which both hair cells and central auditory nuclei are absent (Atoh1-/-). Our results show that neither differentiated peripheral nor the central target cells of inner ear afferents are needed (hair cells, cochlear nucleus neurons) for segregation of vestibular and cochlear afferents within the hindbrain and some degree of base to apex segregation of cochlear afferents. These data suggest that inner ear spiral ganglion neuron processes may predominantly rely on temporally and spatially distinct molecular cues in the region of the targets rather than interaction with differentiated target cells for a crude topological organization. These developmental data imply that auditory neuron navigation properties may have evolved before auditory nuclei.

Ontogenetic Development of Vestibulo-Ocular Reflexes in Amphibians

Vestibulo-ocular reflexes (VOR) ensure gaze stability during locomotion and passively induced head/body movements. In precocial vertebrates such as amphibians, vestibular reflexes are required very early at the onset of locomotor activity. While the formation of inner ears and the assembly of sensory-motor pathways is largely completed soon after hatching, angular and translational/tilt VOR display differential functional onsets and mature with different time courses. Otolith-derived eye movements appear immediately after hatching, whereas the appearance and progressive amelioration of semicircular canal-evoked eye movements is delayed and dependent on the acquisition of sufficiently large semicircular canal diameters. Moreover, semicircular canal functionality is also required to tune the initially omnidirectional otolith-derived VOR. The tuning is due to a reinforcement of those vestibulo-ocular connections that are co-activated by semicircular canal and otolith inputs during natural head/body motion. This suggests that molecular mechanisms initially guide the basic ontogenetic wiring, whereas semicircular canal-dependent activity is required to establish the spatio-temporal specificity of the reflex. While a robust VOR is activated during passive head/body movements, locomotor efference copies provide the major source for compensatory eye movements during tail- and limb-based swimming of larval and adult frogs. The integration of active/passive motion-related signals for gaze stabilization occurs in central vestibular neurons that are arranged as segmentally iterated functional groups along rhombomere 1–8. However, at variance with the topographic maps of most other sensory systems, the sensory-motor transformation of motion-related signals occurs in segmentally specific neuronal groups defined by the extraocular motor output targets.

Effects of brain-derived neurotrophic factor (BDNF) on the cochlear nucleus in cats deafened as neonates

Many previous studies have shown significant neurotrophic effects of intracochlear delivery of BDNF in preventing degeneration of cochlear spiral ganglion (SG) neurons after deafness in rodents and our laboratory has shown similar results in developing cats deafened prior to hearing onset. This study examined the morphology of the cochlear nucleus (CN) in a group of neonatally deafened cats from a previous study in which infusion of BDNF elicited a significant improvement in survival of the SG neurons. Five cats were deafened by systemic injections of neomycin sulfate (60 mg/kg, SQ, SID) starting one day after birth, and continuing for 16-18 days until auditory brainstem response (ABR) testing demonstrated profound bilateral hearing loss. The animals were implanted unilaterally at about 1 month of age using custom-designed electrodes with a drug-delivery cannula connected to an osmotic pump. BDNF (94 μg/ml; 0.25 μl/hr) was delivered for 10 weeks. The animals were euthanized and studied at 14-23 weeks of age. Consistent with the neurotrophic effects of BDNF on SG survival, the total CN volume in these animals was significantly larger on the BDNF-treated side than on the contralateral side. However, total CN volume, both ipsi- and contralateral to the implants in these deafened juvenile animals, was markedly smaller than the CN in normal adult animals, reflecting the severe effects of deafness on the central auditory system during development. Data from the individual major CN subdivisions (DCN, Dorsal Cochlear Nucleus; PVCN, Posteroventral Cochlear Nucleus; AVCN, Anteroventral Cochlear Nucleus) also were analyzed. A significant difference was observed between the BDNF-treated and control sides only in the AVCN. Measurements of the cross-sectional areas of spherical cells showed that cells were significantly larger in the AVCN ipsilateral to the implant than on the contralateral side. Further, the numerical density of spherical cells was significantly lower in the AVCN ipsilateral to the implant than on the contralateral side, consistent with the larger AVCN volume observed with BDNF treatment. Together, findings indicate significant neurotrophic effects of intracochlear BDNF infusion on the developing CN.

Neuroanatomical Tracing Techniques in the Ear: History, State of the Art, and Future Developments

The inner ear has long been at the cutting edge of tract tracing techniques that have shaped and reshaped our understanding of the ear's innervation patterns. This review provides a historical framework to understand the importance of these techniques for ear innervation and for development of tracing techniques in general; it is hoped that lessons learned will help to quickly adopt transformative novel techniques and their information and correct past beliefs based on technical limitations. The technical part of the review presents details of our protocol as developed over the last 30 years. We also include arguments as to why these recommendations work best to generate the desired outcome of distinct fiber and cell labeling, and generate reliable data for any investigation. We specifically focus on two tracing techniques, in part developed and/or championed for ear innervation analysis: the low molecular multicolor dextran amine tract tracing technique and the multicolor tract tracing technique with lipophilic dyes.

The Order and Place of Neuronal Differentiation Establish the Topography of Sensory Projections and the Entry Points within the Hindbrain

Establishing topographical maps of the external world is an important but still poorly understood feature of the vertebrate sensory system. To study the selective innervation of hindbrain regions by sensory afferents in the zebrafish embryo, we mapped the fine-grained topographical representation of sensory projections at the central level by specific photoconversion of sensory neurons. Sensory ganglia located anteriorly project more medially than do ganglia located posteriorly, and this relates to the order of sensory ganglion differentiation. By single-plane illumination microscopy (SPIM) in vivo imaging, we show that (1) the sequence of arrival of cranial ganglion inputs predicts the topography of central projections, and (2) delaminated neuroblasts differentiate in close contact with the neural tube, and they never loose contact with the neural ectoderm. Afferent entrance points are established by plasma membrane interactions between primary differentiated peripheral sensory neurons and neural tube border cells with the cooperation of neural crest cells. These first contacts remain during ensuing morphological growth to establish pioneer axons. Neural crest cells and repulsive slit1/robo2 signals then guide axons from later-differentiating neurons toward the neural tube. Thus, this study proposes a new model by which the topographical representation of cranial sensory ganglia is established by entrance order, with the entry points determined by cell contact between the sensory ganglion cell bodies and the hindbrain.

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.

Ear manipulations reveal a critical period for survival and dendritic development at the single-cell level in Mauthner neurons

Second-order sensory neurons are dependent on afferents from the sense organs during a critical period in development for their survival and differentiation. Past research has mostly focused on whole populations of neurons, hampering progress in understanding the mechanisms underlying these critical phases. To move toward a better understanding of the molecular and cellular basis of afferent-dependent neuronal development, we developed a new model to study the effects of ear removal on a single identifiable cell in the hindbrain of a frog, the Mauthner cell. Ear extirpation at various stages of Xenopus laevis development defines a critical period of progressively-reduced dependency of Mauthner cell survival/differentiation on the ear afferents. Furthermore, ear removal results in a progressively decreased reduction in the number of dendritic branches. Conversely, addition of an ear results in an increase in the number of dendritic branches. These results suggest that the duration of innervation and the number of inner ear afferents play a quantitative role in Mauthner cell survival/differentiation, including dendritic development.

Sensory afferent segregation in three-eared frogs resemble the dominance columns observed in three-eyed frogs

The formation of proper sensory afferent connections during development is essential for brain function. Activity-based competition is believed to drive ocular dominance columns (ODC) in mammals and in experimentally-generated three-eyed frogs. ODC formation is thus a compromise of activity differences between two eyes and similar molecular cues. To gauge the generality of graphical map formation in the brain, we investigated the inner ear projection, known for its well-defined and early segregation of afferents from vestibular and auditory endorgans. In analogy to three eyed-frogs, we generated three-eared frogs to assess to what extent vestibular afferents from two adjacent ears could segregate. Donor ears were transplanted either in the native orientation or rotated by 90 degrees. These manipulations should result in either similar or different induced activity between both ears, respectively. Three-eared frogs with normal orientation showed normal swimming whereas those with a rotated third ear showed aberrant behaviors. Projection studies revealed that only afferents from the rotated ears segregated from those from the native ear within the vestibular nucleus, resembling the ocular dominance columns formed in three-eyed frogs. Vestibular segregation suggests that mechanisms comparable to those operating in the ODC formation of the visual system may act on vestibular projection refinements.

Connecting ears to eye muscles: evolution of a 'simple' reflex arc

Developmental and evolutionary data from vertebrates are beginning to elucidate the origin of the sensorimotor pathway that links gravity and motion detection to image-stabilizing eye movements--the vestibulo-ocular reflex (VOR). Conserved transcription factors coordinate the development of the vertebrate ear into three functional sensory compartments (graviception/translational linear acceleration, angular acceleration and sound perception). These sensory components connect to specific populations of vestibular and auditory projection neurons in the dorsal hindbrain through undetermined molecular mechanisms. In contrast, a molecular basis for the patterning of the vestibular projection neurons is beginning to emerge. These are organized through the actions of rostrocaudally and dorsoventrally restricted transcription factors into a 'hodological mosaic' within which coherent and largely segregated subgroups are specified to project to different targets in the spinal cord and brain stem. A specific set of these regionally diverse vestibular projection neurons functions as the central element that transforms vestibular sensory signals generated by active and passive head and body movements into motor output through the extraocular muscles. The large dynamic range of motion-related sensory signals requires an organization of VOR pathways as parallel, frequency-tuned, hierarchical connections from the sensory periphery to the motor output. We suggest that eyes, ears and functional connections subserving the VOR are vertebrate novelties that evolved into a functionally coherent motor control system in an almost stereotypic organization across vertebrate taxa.

Neuronal birth order identifies a dimorphic sensorineural map

Spatially distributed sensory information is topographically mapped in the brain by point-to-point correspondence of connections between peripheral receptors and central target neurons. In fishes, for example, the axonal projections from the mechanosensory lateral line organize a somatotopic neural map. The lateral line provides hydrodynamic information for intricate behaviors such as navigation and prey detection. It also mediates fast startle reactions triggered by the Mauthner cell. However, it is not known how the lateralis neural map is built to subserve these contrasting behaviors. Here we reveal that birth order diversifies lateralis afferent neurons in the zebrafish. We demonstrate that early- and late-born lateralis afferents diverge along the main axes of the hindbrain to synapse with hundreds of second-order targets. However, early-born afferents projecting from primary neuromasts also assemble a separate map by converging on the lateral dendrite of the Mauthner cell, whereas projections from secondary neuromasts never make physical contact with the Mauthner cell. We also show that neuronal diversity and map topology occur normally in animals permanently deprived of mechanosensory activity. We conclude that neuronal birth order correlates with the assembly of neural submaps, whose combination is likely to govern appropriate behavioral reactions to the sensory context.

Activity-independent specification of synaptic targets in the posterior lateral line of the larval zebrafish

The development of functional neural circuits requires that connections between neurons be established in a precise manner. The mechanisms by which complex nervous systems perform this daunting task remain largely unknown. In the posterior lateral line of larval zebrafish, each afferent neuron forms synaptic contacts with hair cells of a common hair-bundle polarity. We investigated whether afferent neurons distinguish hair-cell polarities by analyzing differences in the synaptic signaling between oppositely polarized hair cells. By examining two mutant zebrafish lines with defects in mechanoelectrical transduction, and by blocking transduction during the development of wild-type fish, we found that afferent neurons could form specific synapses in the absence of stimulus-evoked patterns of synaptic release. Asking next whether this specificity arises through intrinsically generated patterns of synaptic release, we found that the polarity preference persisted in two mutant lines lacking essential synaptic proteins. These results indicate that lateral-line afferent neurons do not require synaptic activity to distinguish hair-cell polarities and suggest that molecular labels of hair-cell polarity guide prepatterned afferents to form the appropriate synapses.

Development of multisensory convergence in the Xenopus optic tectum

The adult Xenopus optic tectum receives and integrates visual and nonvisual sensory information. Nonvisual inputs include mechanosensory inputs from the lateral line, auditory, somatosensory, and vestibular systems. While much is known about the development of visual inputs in this species, almost nothing is known about the development of mechanosensory inputs to the tectum. In this study, we investigated mechanosensory inputs to the tectum during critical developmental stages (stages 42-49) in which the retinotectal map is being established. Tract-tracing studies using lipophilic dyes revealed a large projection between the hindbrain and the tectum as early as stage 42; this projection carries information from the Vth, VIIth, and VIIIth nerves. By directly stimulating hindbrain and visual inputs using an isolated whole-brain preparation, we found that all tectal cells studied received both visual and hindbrain input during these early developmental stages. Pharmacological data indicated that the hindbrain-tectal projection is glutamatergic and that there are no direct inhibitory hindbrain-tectal ascending projections. We found that unlike visual inputs, hindbrain inputs do not show a decrease in paired-pulse facilitation over this developmental period. Interestingly, over this developmental period, hindbrain inputs show a transient increase followed by a significant decrease in the alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA)/N-methyl-D-aspartate (NMDA) ratio and show no change in quantal size, both in contrast to visual inputs. Our data support a model by which fibers are added to the hindbrain-tectal projection across development. Nascent fibers form new synapses with tectal neurons and primarily activate NMDA receptors. At a time when retinal ganglion cells and their tectal synapses mature, hindbrain-tectal synapses are still undergoing a period of rapid synaptogenesis. This study supports the idea that immature tectal cells receive converging visual and mechanosensory information and indicates that the Xenopus tectum might be an ideal preparation to study the early development of potential multisensory interactions at the cellular level.

Morphological characteristics of the lateral line receptors of Hynobius leechii (Urodela: Hynobiidae)

Morphological characteristics of the lateral line receptors, specifically the superficial neuromast, the pit organ, and the ampullary organ, are described and compared among larval, metamorphosed, and both female and male adult Hynobius leechii. The distribution and morphologies of the lateral line receptors are similar to those of other urodeles. During the transition from a larval to an adult salamander, the number of receptors is generally reduced, but their sizes increase. Other morphological changes also occur, such as the development of a more distinct mantle cell boundary at the epidermal surface of the adult ampullary and pit organs. In particular, we found kinocilia on the sensory hair cells of the ampullary organs of both larval and adult salamanders, and, for the first time, we describe two different morphologies of the pit organ in urodeles. The detailed description of the pit organ represents only the second study of its kind in urodeles. However, our comparison of lateral line receptors between larval and adult salamanders is the first of its kind for salamanders that utilize external fertilization.

Embryonic origin of gustatory cranial sensory neurons

Cranial nerves VII, IX and X provide both gustatory (taste) and non-gustatory (touch, pain, temperature) innervation to the oral cavity of vertebrates. Gustatory neurons innervate taste buds and project centrally to the rostral nucleus of the solitary tract (NTS), whereas neurons providing general epithelial innervation to the oropharynx project to non-gustatory hindbrain regions, i.e., spinal trigeminal nucleus. In addition to this dichotomy in function, cranial ganglia VII, IX and X have dual embryonic origins, comprising sensory neurons derived from both cranial neural crest and epibranchial placodes. We used a fate mapping approach to test the hypothesis that epibranchial placodes give rise to gustatory neurons, whereas the neural crest generates non-gustatory cells. Placodal ectoderm or neural crest was grafted from Green Fluorescent Protein (GFP) expressing salamander embryos into unlabeled hosts, allowing us to discern the postembryonic central and peripheral projections of each embryonic neuronal population. Neurites that innervate taste buds are exclusively placodal in origin, and their central processes project to the NTS, consistent with a gustatory fate. In contrast, neural crest-derived neurons do not innervate taste buds; instead, neurites of these sensory neurons terminate as free nerve endings within the oral epithelium. Further, the majority of centrally directed fibers of neural crest neurons terminate outside the NTS, in regions that receive general epithelial afferents. Our data provide empirical evidence that embryonic origin dictates mature neuron function within cranial sensory ganglia: specifically, gustatory neurons derive from epibranchial placodes, whereas neural crest-derived neurons provide general epithelial innervation to the oral cavity.