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Stenopelmatidae (or "Jerusalem crickets") belong to the atympanate Ensifera, lacking hearing organs in the foreleg tibiae. Their phylogenetic position is controversial, either as a taxon in Tettigonioidea or within the clade of Gryllacridoidea. Similarly, the origin of tibial auditory systems in Ensifera is controversial. Therefore, we investigated the neuronal structures of the proximal tibiae of Stenopelmatus spec. with the hypothesis that internal sensory structures are similar to those in tympanate Ensifera. In Stenopelmatus the complex tibial organ consists of three neuronal parts: the subgenual organ, the intermediate organ, and a third part with linearly arranged neurons. This tripartite organization is also found in tympanate Ensifera, verifying our hypothesis. The third part of the sense organ found in Stenopelmatus can be regarded by the criterion of position as homologous to auditory receptors of hearing Tettigonioidea. This crista acustica homolog is found serially in all thoracic leg pairs and contains 20 +/- 2 chordotonal neurons in the foreleg. The tibial organ was shown to be responsive to vibration, with a broad threshold of about 0.06 ms(-2) in a frequency range from 100-600 Hz. The central projection of tibial sensory neurons terminates into two equally sized lobes in the primary sensory neuropil, the medial ventral association center. The data are discussed comparatively to those of other Ensifera and mapped phylogenetically onto recently proposed phylogenies for Ensifera. The crista acustica homolog could represent a neuronal rudiment of a secondarily reduced ear, but neuronal features are also consistent with an evolutionary preadaptation.

Although γ-aminobutyric acid (GABA) and glutamate are known to be present in the vestibular sensory epithelia of a variety of species, the functional relationship between these two transmitters is not clear. The present study addresses the three-dimensional spatial distribution of GABA and glutamate immunoreactivity in the vestibular labyrinth of the oyster toadfish by using whole end organs labeled by immunofluorescence with monoclonal anti-GABA and/or antiglutamate antibodies and visualized as whole mounts by multiphoton confocal microscopy. We find glutamate-immunoreactive hair cells present throughout the sensory epithelium. In contrast, prominent GABA immunoreactivity is restricted to a small population of hair cells located in the central region of the crista. Double immunofluorescence reveals two distinct staining patterns in GABA-labeled hair cells. Most (∼80%) GABA-labeled cells show trace levels of glutamate, appropriate for the metabolic/synthetic role of cytoplasmic glutamate. The remainder of the GABA-stained cells contain substantial levels of both GABA and glutamate, suggesting transmitter colocalization. In the toadfish utricle, glutamatergic hair cells are present throughout the macula. GABA-immunoreactive hair cells follow the arc of the striola, and most GABA-labeled receptor cells coexpress glutamate. The localization of GABA was explored in other species as well. In the pigeon, GABAergic hair cells are present throughout the crista ampullaris. Our findings demonstrate that multiple, neurochemically distinct types of hair cells are present in vestibular sensory epithelia. These observations, together with the excitatory activity generally associated with 8th nerve afferent fibers, strongly suggest that GABA serves an important, specific, and complex role in determining primary afferent response dynamics. J. Comp. Neurol. 471:1–10, 2004. © 2004 Wiley-Liss, Inc.

The simple primordium of the inner ear (otocyst) differentiates into many cell types, including sensory neurons and hair cells. We examined expression of the divergent homeobox transcription factor, cProx1, during otocyst development in chickens. Nuclear cProx1 protein is not evident in the otic placode but emerges in the otic cup by stage 12. At stage 16, cProx1-positive nuclei are scattered continuously throughout the neuroepithelium, from anteroventral to posteromedial. These labeled cells are neural precursors; they express betaIII-tubulin and migrate to the cochleovestibular ganglion between stages 13 and 21. By stage 18, two areas develop a dense pattern of cProx1 expression in which every nucleus is labeled. These areas emerge at the anterior and posterior extremes of the band of scattered cProx1 expression and express the sensory markers cSerrate1 and Cath1 by stage 23. Four discrete patches of dense cProx1 expression appear by stage 23 that correspond to the future superior crista, lateral crista, saccular macula, and posterior crista, as confirmed by immunolabeling for hair cell antigen (HCA) by stage 29. The remaining sensory epithelia display a dense pattern of cProx1 expression and label for HCA by stage 29. In the basilar papilla, nuclear cProx1 expression is down-regulated in most hair cells by stage 37 and in many supporting cells by stage 40. Our findings show that regions of the otocyst that give rise to neurons or hair cells are distinguished by their relative density of cProx1-positive nuclei, and suggest a role for cProx1 in the genesis of these cell types.

We imaged the horizontal semicircular canal (HSCC) crista and cupula of toadfish, Opsanus tau, by using a) confocal light microscopy of isolated vital HSCC; b) serial sections of fixed, trichrome-stained HSCC; and c) scanning electron microscopy of fixed HSCCs. HSCC were dissections which included an ampulla and an attached canal tube (long and slender canal portion), and, in some cases, a small portion of the utricular wall. Cupulae were seen as multipartite mucus connective tissue shells rising from the crista and extending toward the ampullary roof. They were composed of several refractile bands traversing the cupulae perpendicular to longitudinal fibers extending from the cupular base to its apex. Alcian green-stained cupulae showed an asymmetric alcianphilic, dark, X-shaped structure, indicating that the pillar is rich in mucin and carbohydrate, an interpretation supported by images of trichrome-stained sections. The cupular antrum is devoid of prominent refractile fibers. No tubes or channels were observed in the cupula or antrum of vital preparations. Cupular shell fibers cover the surface of the crista, are roughly parallel, and are associated with a translucent material having a refractive index greater than the surrounding endolymph. Stereocilia were thin, 100-microm-long structures, with little longitudinal curvature, which end with no end bulb. No strands extend from stereocilia to the roof or other portions of the cupular antrum. Gross movements of stereocilia were not seen in mechanically quiescent preparations. Within the cupular antrum, stereocilia were parallel to connective tissue fibers, all embedded in an isotropic gel. This fiber-reinforced gel and cupular matrix are sensitive to N-acetlyneuraminidase and beta-N-acetyl glucosaminidase, and minimally sensitive to beta-N-acetyl hexosaminidase. Connective tissue fibers may serve to stiffen the gel, whose matrix would restrict lateral motion of embedded fibers and stereocilia thereby providing mechanical support for stereocilia.

Localization of expression of the adenosine 5'-triphosphate (ATP)-gated ion channel P2X2 receptor subunit (P2X2R) in the rat inner ear at different stages of development was achieved by using in situ mRNA hybridization. In the adult, P2X2R mRNA was strongly expressed in many of the cells bordering the cochlear endolymphatic compartment. This included the interdental cells of the spiral limbus, all cells of the inner sulcus and organ of Corti, and cells of the spiral prominence. In the vestibular labyrinth, strong expression was noted in the transitional cells at the base of the crista ampullaris and in the sensory epithelium of the crista and maculae. During development, P2X2R mRNA expression was evident in the precursors of these structures at the earliest period studied, embryonic day 12 (E12). Expression increased during the ontogeny in both the cochlear and the vestibular end organs. In addition, both the spiral and vestibular ganglia showed developmental expression. In contrast to the supporting cells of the organ of Corti, both inner and outer hair cells exhibited P2X2R mRNA only after postnatal day 10 (P10) through P12, concomitant with the onset of hearing. P2X2R expression levels in all cells fell from a maximum at P12-P18 to lower levels in the adult. In the adult, P2X2R mRNA levels were modest in outer hair cells in the basal (high-frequency) encoding region of the cochlea, and inner hair cell labeling was low throughout the cochlea. Reissner's membrane, which maintains an electrochemical barrier between scala vestibuli and scala media, showed considerable expression of P2X2R mRNA in early postnatal development, and expression was maintained at moderate levels in the adult cochlea. These data are consistent with a role for the P2X2R subunit in the processes of labyrinthine development and the regulation of the electrochemical gradients supporting auditory and vestibular sensory transduction.

The chinchilla crista ampullaris was studied in 10 samples, each containing 32 consecutive ultrathin sections of the entire neuroepithelium. Dissector methods were used to estimate the incidence of various synaptic features, and results were confirmed in completely reconstructed hair cells. There are large regional variations in cellular and synaptic architecture. Type I and type II hair cells are shorter, broader, and less densely packed in the central zone than in the intermediate and peripheral zones. Complex calyx endings are most common centrally. On average, there are 15-20 ribbon synapses and 25-30 calyceal invaginations in each type I hair cell. Synapses and invaginations are most numerous centrally. Central type II hair cells receive considerably fewer afferent boutons than do peripheral type II hair cells, but have similar numbers of ribbon synapses. The numbers are similar because central type II hair cells make more synapses with the outer faces of calyx endings and with individual afferent boutons. Most afferent boutons get one ribbon synapse. Boutons without ribbon synapses were only found peripherally, and boutons getting multiple synapses were most frequent centrally. Throughout the neuroepithelium, there is an average of three to four efferent boutons on each type II hair cell and calyx ending. Reciprocal synapses are rare. Most synaptic ribbons in type I hair cells are spherules; those in type II hair cells can be spherical or elongated and are particularly heterogeneous centrally. Consistent with the proposal that the crista is concentrically organized, the intermediate and peripheral zones are each similar in their cellular and synaptic architecture near the base and near the planum. An especially differentiated subzone may exist in the middle of the central zone.

Using transmission electron microscopy, we have identified a new type of afferent terminal structure in the crista ampullaris of the goldfish Carassius auratus. In addition to the bouton-type afferent terminals previously described in the ear of this species, the crista also contained enlarged afferent terminals that enveloped a portion of the basolateral hair cell membrane. The hair cell membrane was evaginated and protruded into the afferent terminal in a glove-and-finger configuration. The membranes of the two cells were regularly aligned in the protruded region of the contact and had a distinct symmetrical electron density. The electron-dense profiles of these contacts were easily identified and were present in every crista sampled. In some cases, efferent terminals synapsed onto the afferents at a point where the hair cell protruded into the terminal. The ultrastructural similarities of the goldfish crista afferents to calyx afferents found in amniotes (birds, reptiles, and mammals) are discussed. The results of the study support the hypothesis that structural variation in the vertebrate inner ear may have evolved much earlier in evolution than previously supposed.

The zebrafish provides an important model for vertebrate inner ear development. The otic placode becomes visible at approximately 16 hours (at 28.5 degrees C) and forms a vesicle with a lumen by cavitation at approximately 18 hours. Two otoliths appear in the lumen by 19.5 hours, and at about 24 hours the first sensory hair cells are seen, grouped in two small patches, one beneath each otolith, corresponding to future maculae. Staining with fluorescent phalloidin reveals 10-20 hair cells in each macula by 42 hours; between 3 days and 7 days the numbers grow to approximately 80 per macula. Neurons of the statoacoustic ganglion are first visible by staining with HNK-1 antibody at about 24 hours. Serial sections and time-lapse films show that the neuronal precursors originate by delamination from the ventral face of the otocyst; the peak period of delamination is from 22 hours to 30 hours. The system of semicircular canals forms between 42 hours and 72 hours by outgrowth of protrusions from the walls of the otocyst to form pillars of tissue spanning the lumen. Three further clusters of hair cells also become visible in this period, forming the three cristae. Thus, by the end of the first week, all key components of the ear are present. Subsequent growth produces thousands more hair cells; additional neurons probably derive from proliferation of neuronal precursors within the ganglion. Although the timetable is species-specific, the principles of inner ear development in the zebrafish seem to be the same as in other vertebrates.

Physiological studies in many vertebrates indicate that vestibular primary afferents are not a homogeneous population. Such data raise the question of what structural mechanisms underlie these physiological differences and what functional role is played by afferents of each type. We have begun to answer these questions by characterizing the architecture of 110 afferents innervating the posterior canal of Pseudemys scripta. We emphasize their spatial organization because experimental evidence suggests that afferent physiological properties exhibit significant spatial heterogeneity. The sensory surface of the posterior canal comprises paired, triangular hemicristae, which are innervated by two afferent types. Bouton afferents (66% of total afferents) are found over the entire sensory surface. They differ significantly in the shape and size of their collecting areas, number of boutons, soma size, and axon diameter; this morphological variation is systematically related to the afferent's spatial position. In addition, multivariate analyses suggest that bouton afferents may comprise two subtypes: alpha afferents have delicate processes and are found throughout the crista; beta afferents are more robust and are concentrated preferentially toward the canal center. Calyx-bearing afferents comprise two morphological subtypes: dimorphs (13% of total afferents) bear calyceal and bouton endings; calyceal afferents (21%) bear calyceal endings only. Both types occur exclusively in an elliptical region near the center of each hemicrista; their morphology varies with radial distance from the center of this elliptical region. Our data provide evidence that in Pseudemys: (1) the classical vestibular afferent types (bouton, calyx, dimorph) are structurally heterogeneous, and (2) their spatial sampling characteristics are highly structured and distinctive for each type. These spatial patterns may shed light on regional differences in physiological profiles of vestibular afferents, and they raise questions about the role of this spatial heterogeneity in signaling head movement.

The subgenual chordotonal organ complex in insects is modified in ensiferan taxa like Gryllidae and Tettigoniidae into hearing organs with specific sets of auditory receptors. Here, this sensory organ complex is documented in the nonhearing splay-footed cricket Comicus calcaris. The tibial chordotonal organ consists of three parts: the subgenual organ, the intermediate organ, and the crista acustica homolog. The latter is an array of linearly organized neurons homologous to auditory receptors in the tibial hearing organs of Tettigoniidae. The tibial organ is structurally similar in all three leg pairs, with similar neuron numbers in the fore- and midleg, but lower numbers in the hindleg. The foreleg crista acustica homolog consists of 34±4 neurons, the highest number in an atympanate Ensiferan. Additionally, an accessory chordotonal organ with 15±5 neurons innervated by nerve 5B1 is present in the foreleg. The central projection of the tibial organreveals ipsilateral sensory terminals in the primary sensory neuropil, the medial ventral association center with terminations close to the midline. As determined from extracellular recordings, the entire tibial organ is vibrosensitive. The organization of the tibial organ is compared to other ensiferan auditory and nonauditory tibial organs. Spatial orientation of neurons in the crista acustica homolog is not reminiscent of auditory structures, and the neuroanatomy is discussed with respect to stridulation behavior and the evolutionary origin of hearing in Ensifera.

Most ensiferan insects possess sets of highly specialized chordotonal organs in the proximal tibiae to detect conspecific auditory/vibratory signals or approach of predators. To date, most auditory/vibratory afferents have been classified according to their physiological properties and axonal projection morphology, but not to somatotopic origins. Hence, the functional specialization of identified receptor cells in the tibial organs remains uncertain. To address this question from an anatomical aspect, we investigated the structure of the weta, Hemideina femorata, tibial organs (the most elaborated tibial chordotonal organs among ensiferans) and their central projections by staining small numbers of receptor afferents from identified tibial organs. These organs comprise the "complex tibial organ," including the subgenual organ (primary vibration detector) and its posterior complement, the accessory organ, and the crista acustica (primary auditory organ) and its proximal complement, the intermediate organ. Unlike reports of a membranous organ structure for homologs in other ensiferans, weta tibial organs contain receptor cells embedded in thick solid masses. Primary afferents project ipsilaterally to the medial ventral association center of thoracic ganglia, where axon terminals are arrayed topographically in different areas specific to each organ, except for almost complete overlap of afferents originating from the distal part of the crista acustica and from the intermediate organ. In contrast to somatotopic reflection of sensilla position on limbs, as known for mechanoreceptor hairs, the somatotopic projection map of the insect ear reveals topographic association with acoustic tracheae or tibial cuticular attachment sites, which in turn must reflect determinants of response sensitivity (e.g., frequency or threshold).

The distribution of the cell adhesion molecule BEN in the developing chick inner ear is described. BEN is first detected in the otic placode at stage 11. As the placode begins to invaginate, BEN becomes concentrated in a ventromedial region extending from the anterior to the posterior end of the otic pit. BEN expression levels increase in this region as the pit closes to form the otocyst, and distinct boundaries become defined along the dorsal and ventral edges of the ventromedial band of BEN expression. BEN expression also becomes concentrated dorsally within the otic epithelium as the pit closes and is observed in the condensing otic ganglion. By stage 22, the ventromedial band of BEN expression splits into two distinct regions, a small caudal patch within which the posterior crista will develop, and a larger anterior patch. By stage 26, this larger anterior patch of cells expressing BEN becomes subdivided into five separate areas corresponding to the regions within which the anterior crista, the lateral crista, the utricle, the saccule, and both the basilar papilla and lagenar macula form. Hair cells only develop within these regions defined by BEN distribution. The data suggest that the ventromedial patch of BEN expression observed from stage 11 onwards defines a single sensory competent zone from which all sensory organs of the inner ear develop. BEN immunoreactivity in the inner ear declines after stage 38. In response to noise exposure, upregulation of BEN expression is mainly detected in regions of the posthatch papilla where the damage is severe and regenerating hair cells are not observed. The regenerating hair and supporting cells do not express BEN, highlighting a molecular difference between the processes of development and regeneration. J. Comp. Neurol. 434:275?288, 2001. © 2001 Wiley-Liss, Inc.

Individual nerve fibers were traced on consecutive serial 1-micron-thick cross-sections to determine their position of origin within the horizontal ampullary nerve trunk (HANT) and the site at which they penetrated the basilar lamina of the horizontal crista in the guitarfish, Rhinobatos productus. Of the 132 fibers successfully traced, 87.1% followed an anatomical projection pattern consistent with the dynamic response characteristics of the nerve fibers. That projection pattern was: from the peripheral HANT region to the ipsilateral slope; from the central HANT region to the crest; and from the intermediate HANT region both to the ipsilateral slope and the crest of the horizontal crista. The 12.9% of "error" fibers not following this projection pattern were also consistent with the functional neurophysiological data. The terms crista bulbus and crista angustarum were introduced to designate specific anatomical regions of the dumb-bell shaped crista ampullaris.

The location in time and space of the terminal mitoses of type I and type II sensory hair cells (HCI and HCII) of the developing crista ampullaris in rat lateral semicircular canal and macula utriculi was determined by radioautographs of specimens exposed to tritiated thymidine from the 13th to the 20th day of gestation. Qualitative analysis and statistical treatment of the percentages of labeled HCI and HCII show that the terminal mitoses occur first in the macula utriculi with a maximum percentage of the 14th day of gestation, for the HCI, and on the 15th day of gestation, for the HCII. In the lateral crista, the maximum percentage of labeled HCI occurs on the 17th and 18th day of gestation and on the 19th day of gestation for the HCII. A spatial distribution of this labeling activity is also described: the older cells are located at the top of the crista and at the level of the striola of the macula utriculi while the younger cells are found at the bottom of the crista and on the sides of the utricle. A study of the vestibular receptors in the fetuses shows that synaptic contacts already exist on the 18th day of gestation in the macula utriculi at the level of the striola and on the 19th day at the top of the crista; the cells situated on the periphery are still immature. The first hair cells to undergo their terminal mitoses are, therefore, connected first. These results also suggest that the two types of cells are genetically programmed and that the HCI start functioning first during the development of the labyrinth.

Both scanning electron and light microscopy have been used to observe the tiny sensura neglecta which is located in the inferior utricular sinus of the vestibular labyrinth in the pigeon. The following new observations were made. The end organ, which occupies one-third of the floor of the inferior utricular sinus, is ovoid, measuring 200 μm in length × 80 μm in width and 30 μm in height. The neuroepithelium of the neglecta is uniformly innervated by the neglectar nerve which contains about 60 to 90 myelinated nerve fibers that fasciculate from the posterior ampullary nerve. The fiber spectrum is unimodal with a mode diameter of 1–2 μm. From 150 to 200 Type I and Type II hair cells are found on the surface of the neglecta. The surface features of the hair cells, i.e., the pipe organ patterns of the stereocilia are identical to that of the other vestibular sensory regions. No hair cells are found on the sides of the neglecta and all hair cells project their tufts upward from the top surface. The sensory region of the neglecta is covered by a gelatinous membrane. In no instance are statoconia observed on this covering. The non-sensory epithelium contiguous to the sensura neglecta is comprised only of “light cells” (Dohlman, '67). The above observations indicate that the sensura neglecta in the pigeon has both crista- and macula-like features. It is hypothesized that this is reasonable from an evolutionary and teleological point of view. It is further suggested that regardless of its similarities to the statolithic and canalicular organs, the mode of stimulation of the neglecta can best be described by analogy to the culmen type of system associated with the auditory papilla of the lizard (Wever, '71).

The spinal cord and the acoustic primordium of the chick embryo (3 to 7-day-old) were investigated by electron microscopy. Special attention was devoted to the structure of neurofilaments and tubules in the neuroblasts and cells of the crista acustica. The neurofilaments were observed to be related at early stages of development to the RNA particles of the cytoplasm (Palade's particles). This relationship is observed in particular fields which are clearly distinguished because they show fewer particles than the surrounding cytoplasm. The first stage is characterized by the presence in these fields of short branched filaments, which frequently are in contact with the ribosomes. The filaments grow in length and become ordered during a later period. At the same time, the fields become free of the basophilic particles. Tubules about 300 A thick were found in neuroblasts and their prolongations (central nervous system); in the acoustic nerve, fibers of the “tubule type” or of the “neurofilament type” are described. Filaments and tubules showing similar morphology were also observed in the acoustic epithelium (crista acustica). The filaments seem to originate in the same way as in the neuroblasts. A close relationship between tubules and centrioles was found. The origin of the neurofilaments is discussed in relation to the filaments of proteinic nature described in other cell types.

The scanning electron microscope (SEM) was used to investigate the morphology of the neuroepithelial regions of the vestibular ampullary structures in 47 White King pigeons. The specific neural surfaces studied were (1) the cristae ampullares of the vertical and lateral membranous ampullae, (2) the hair cells lining the cristae, (3) the ampullary nerve fibers, and (4) the bipolar cells of the vestibular (Scarpa's) ganglion. Additionally, some observations of the gross anatomical structures of the bony labyrinth are given. Arguments are advanced which show that if the surface area of a given semicircular canal can be projected onto one of the three normal head planes, then that canal can be made to respond to motion in the appropriate plane, provided that the projected area is sufficiently large to achieve a threshold pressure as determined by a generalized form of Groen's equation ('57). With regard to the cristae ampullares, it is hypothesized that their surface areas can be described by means of a revolved catenary, i.e., a catenoid of revolution. (The catenary is found in nature as the approximate shape taken by a flexible cable when it is suspended at two points). The surface area of a catenoid provides a minimum surface of revolution. In the context of a crista, this implies that the given number of hair cells could not be fitted onto a smaller surface area. One advantage of this is that nature is able to utilize a thinner cupula than would be possible with other configurations and therefore an increased sensitivity to cupular motion can be realized. A second important factor is that all hair cells must revolve (by way of cupular motion) about the same centre of rotation in response to angular acceleration. Thus, all of the orthogonally-positioned hair cell tufts on the cristae surface may be stimulated simultaneously by way of a tangential shear. Other arguments show that the classical “swinging door” type of cupular motion is not consistent with SEM and other recent observations. Two alternate modes of cupular motion are presented, each of which requires far less energy expenditure than does the “swinging door” cupula. The suggestion is then made that, during normal head movements, the cupula behaves as a drum much like the tympanic membrane and that only for large, non-physiological motions does the “swinging door” mode of cupular motion take place. It must be remembered, however, that cupular motions during normal physiological head movements are infinitesimally small (Oman and Young, '72).

Efferent neurons innervating the vestibular labyrinth and cochlea of the pigeon have been identified by means of a variety of retrograde tracers: [3H]-adenosine (Ad), horseradish peroxidase (HRP), Evan's Blue (EB) and Bisbenzimide (Bb). Discrete injections into individual cristae ampullares of the semicircular canals, into the macula utriculi, or into several of these end organs resulted in similar patterns of neuronal labelling. Efferent vestibular neurons were always found within a small portion of the nucleus reticularis pontis caudalis (RP), ventrolateral to the abducens nucleus on both sides. No systemic difference in the locations of labelled cells was found following injection into different sensory epithelia. Cell counts following injections into individual cristae did not differ significantly from those following injections into all three cristae. The injections into all cristae in both labyrinths yielded cell counts that were much lower than twice the number of cells labelled by injections into the three cristae on one side only. When HRP was injected into the right lateral canal crista and Ad into the right posterior canal crista, a high proportion of neurons was labelled with both compounds (61% of the HRP-labelled cells and 67% of the Ad-labelled cells). Injection of EB into all three cristae on the right side and Bb into all three cristae on the left side produced a smaller percentage of doubly labelled cells (10% of the EB-labelled cells and 6% of the Bb-labelled cells). It is concluded, therefore, that there is a considerable degree of collateralization within one labyrinth. Fewer collaterals of efferent neurons are directed to both labyrinths. Since each semicircular canal represents head rotation in one direction and one plane, it is unlikely that efferents which contact several different movement sensors can provide sensory motor control that is specific for directions and planes of head movements. Control injections of these tracers into the cochlea yielded labelled cells in a different reticular structure, the nucleus reticularis paragigantocellularis lateralis (Pgc), on both sides, as well as in the RP. It is proposed that the Pgc cells represent cochlear efferents, while the RP neurons are related to the macula lagenae, an otolithic organ of balance in the pigeon.

By means of scanning electron microscopy, the differentiation and maturation of sensory hair bundles have been studied in the ampullar cristae of the mouse during development from gestational day 13 (GD13) to postnatal day 10 (PD10). Two gradients of ciliary differentiation were demonstrated, one from the apex to the base and the other from the center to the periphery. The different hair bundles that appeared on the crista during the fetal period originated in an initial ciliary stage found in regions undergoing differentiation, differentiation, which was visible at the apex of the crista starting on GD14. This stage of ciliary development is gradually followed by a juvenile one at GD15. Starting on GD18, the maturation of hair bundles in the central apical area differed from that in the peripheral areas and prefigured the regional specialization of the crista found in the adult stage. In the discussion, we suggest that when the vestibular epithelial cells begin to exhibit ciliary differentiation, they are simultaneously contacted by nerve fibers.