Morphology of the macula neglecta in sharks of the genus Carcharhinus

Ears from several species of carcharhinid sharks were studied by gross dissection, light microscopy, transmission electron microscopy, and scanning electron microscopy. Structures along a possible sound transmission path to the ear are described, but main consideration is given to the structure of the macula neglecta. The macula neglecta is composed of two patches of sensory epithelium which line part of the posterior canal duct. In an adult shark the larger of these contains 224,000 sensory hair cells oriented so as to detect forces directed posteroventrolaterally in the duct. The smaller patch contains 43,000 hair cells oriented so as to detect oppositely directed forces. These receptor cells project through numerous small terminals to a total for both patches of 4,700 myelinated nerve fibers. Cytostructural variations throughout the hair cell population are also reported. Estimated acoustic properties of the tissues in this complex and the processing potential of the neural elements are interpreted as suggestive of auditory function. A mechanism based on the geometry of the receptor arrays is proposed to explain behaviorally observed instantaneous sound localization from the farfield. Evolution of the macula neglecta is reviewed, and evidence for homology of the macula neglecta and amphibian papilla is presented.     

The ultrastructure and innervation of the ear of the gar, Lepisosteus osseus

The endorgans of the inner ear of the gar were examined using transmission and scanning electron microscopy as well as nerve staining. The ultrastructure of the sensory hair cells and supporting cells of the gar ear are similar to cells in other bony fishes, whereas there are significant differences between the gar and other bony fishes in the orientations patterns of the sensory hair cells on the saccular and lagenar sensory epithelia. The saccular sensory epithelium has two regions, a main region and a secondary region ventral to the main region. The ciliary bundles on the main region are divided into two groups, one oriented dorsally and the other ventrally. Furthermore, as a result of curvature of the saccular sensory epithelium, the dorsal and ventral ciliary bundles on the rostral portion of the epithelium are rotated ninety degrees and are thus oriented on the animal's rostro-caudal axis. Hair cells on the secondary region are generally oriented ventrally. The lagenar epithelium has three groups of sensory hair cells. The groups on the rostral and caudal ends of the macula are oriented dorsally, whereas the middle group is oriented ventrally. Hair cell orientations on the utricular epithelium and macula neglecta are similar to those in other bony fishes. Nerve fiber diameters can be divided into three size classes, 1-8 microns, 9-13 microns, and 14 microns or more, with the smallest size class containing the majority of fibers. The distribution of the various classes of fiber diameters is not the same in nerve branches to each of the end organs. Similarly, the ratio of hair cells to axons differs in each end organ. The highest hair cell to axon ratio is in the utricle (23:1) and the smallest is in the macula neglecta (7:1). The number of sensory hair cells far exceed the number of eighth nerve axons in all sensory epithelia.     

Quantitative analyses of sex and size differences in the macula neglecta and ramus neglectus in the inner ear of the skate,Raja ocellata

Summary In Raja ocellata the macula neglecta is located in the posterior canal duct of the inner ear at the junction with the sacculus. The maximum length and width of a freeze-dried macula from a male skate of 61 cm width is 1035 m and 315 m respectively. Ultrastructural studies show that the hair cells of the macula are of two types. Orientation of hair cells is towards the periphery with a reverse direction of polarization in 5.0 to 6.5% of the cells. The axons of the associated nerve, the ramus neglectus, are myelinated, and include both efferent and afferent fibres.Electron-microscopic studies and quantitative analyses reveal significant sex differences in the macula neglecta and ramus neglectus. Hair cell and axon numbers, and total axon areas increase linearly with skate size, and are significantly different in males and females for any given representative size of skate, the females having the larger counts. Since the macula neglecta functions as a vibration detector of far-field localizations, the gender difference may be involved in the location of prey, or in mate detection. It is unknown whether such differences occur in any other vertebrate species.     

Structure and Innervation of the Inner Ear Sensory Organs in an Otophysine Fish,the Upside–down Catfish (Synodontis nigriventrisDavid)

All the sensory epithelia of the inner ear in the upside–down catfish (Synodontis nigriventrisDavid) were examined by light microscopy. The morphology of the membranous labyrinth and the orientation of the hair cells is similar to what has been found in other otophysine fishes. The sensory cells are of variable size both inter– and intraepithelially; particularly the macula sacculi is equipped with heterogeneous receptors. Regional differences in the hair cell density are presented for all the otolith organs plus the papilla neglecta. Nerve stainings reveal regional differentiation. The central areas are innervated by stout and stubbly nerve endings intermingled with a few thin nerve fibres while the peripheral parts are reached exclusively by thin axons. In the anterior region of the macula sacculi are found unique cup–shaped axon terminations which surround the basal parts of a single or a few sensory cells. The number and diameter range of the myelinated nerve fibres as well as the hair cell/axon ratio are presented. Electron microscopy demonstrates the presence of unmyelinated axons in all inner ear nerve ramuli.     

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

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

The statocyst of Vampyroteuthis infernalis (Mollusca: Cephalopoda)

The statocyst shows a remarkable combination of features of decapods and octopods confirming that Vampyroteuthis is a relic somewhere near the ancestor of both groups. The lining of the statocyst separates from the outer wall, forming an inner sac, filled with endolymph, surrounded by perilymph. This is the condition found in octopods, never in decapods. The macula is partly divided into a macula princeps and macula neglecta, as in decapods but never in octopods. There are numerous statoconia, but no large statolith has been seen. The crista has four parts as in decapods, but they are not sharply separated. There are numerous small anticristae, with the general form found in decapods, differentiated into pegs and hooks.
The wall of the inner sac contains numerous hair cells. These hairs protrude between the epithelial cells. The bases of the cells are drawn out into fine processes, presumably some dendritic and some axonal. There is thus a plexus of nerve fibres all over the wall, communicating with the crista nerve.
There is a very large posterior sac of unknown function, lying behind the crista. It contains only one large anticrista and the opening of Kölliker's canal, which is very large.     

The Number and Distribution of Calyceal Hair Cells in the Inner Ear Utricular Macula of Some Reptiles

The utricular macula in the inner ear was examined in two turtle, two lacertilian, two snake and one crocodilian species. The orientation of hair cells was found to be similar to the general pattern for this macula described previously from fish to mammals. The calyceal hair cells, characterized by their embracing afferent nerve endings, are distributed in a single belt running parallel with the anterior and lateral borders of the macula in the examined reptiles.
The number of hair cells in a large number of calyces was counted for some of the examined reptile species, and the total number of hair cells calculated in two specimens of a turtle, crocodile and snake utricular macula. In the turtle and snake maculae, the calyceal hair cells form about 10% of the total hair cell number, while the crocodiles, which were very young, only had 2% calyceal hair cells. The total number of hair cells was found to be much higher in the crocodiles than in the other reptiles examined. The presented data, as well as data from the literature of the avian and mammalian inner ear, indicate that the number of this very sensitive hair cell type in the utricular macula is independent of locomotion type.     

Hair Cell Polarization in the Inner Ear of a Caecilian,Ichthyophis glutinosus (Amphibia: Gymnophiona)

The inner ear of Ichthyophis glutinosus is described with emphasis on the position of the sensory organs and the polarization of the hair cells. The hair cell polarization patterns of the maculae, cristae and papilla basilaris is similar to previous observations in other tetrapods. The papilla amphibiorum shows a simpler bidirectional polarization than described in other amphibians. The papilla neglecta, a sensory organ in the utriculus shows a unidirectional posteriorly directed polarization. A neglecta has not been found in the utriculus of anurans and urodeles previously.     

On the Structure of the Avian Maculae

The maculae of the labyrinths of several avian species were examined. The striola of the macula utriculi and lagenae is tri-zonal, consisting of two zones of hair cells type I (HC I) located on each side of a middle zone of hair cells type II (HC II). An exception is the mute swan, in which the macula utriculi has a striola consisting of one broad zone of HC I. The macula sacculi is, in its central part, mainly consisting of HC I, and the striola does not have a tri-zonal structure. The hair cells in the macula utriculi are polarized with their kinociliar end oriented towards the striola, while in the macula sacculi and lagenae they are oriented away from this dividing line. A varying number, from 1 to 12, of HC I are enclosed within the same nerve chalice. The macula sacculi seems to contain chalices with slightly more HC I than the two other maculae do.     

Number and distribution of hair cells in the utricular macula of some avian species

The utricular macula was examined in 20 bird species belonging to 13 families. Two types of hair cells, a bouton-innervated and a calyceal hair cell, were found. The calyceal hair cells are found situated in two zones that follow the anterior and lateral borders of the utricular macula in all except two species. In the rhea and mute swan, only one zone was found, corresponding to the inner zone of the other species. The majority of calyceal hair cells are present in the anterior and lateroposterior part of the inner zone. They are often gathered in a common calyx. More hair cells are found within the same calyx in birds than described in previously examined reptile and mammalian species. In nine specimens of four species belonging to four different families, the calyceal hair cells constitute between 7 and 12% of the total number of hair cells, which varies from about 20,000 to 40,000.     

Structure and Function of the Elasmobranch Auditory System

Behavioral evidence indicates that sharks detect underwatersound at frequencies up to 1000 Hz, and that certain low frequencysignals attract sharks from large distances. It appears thatthe adequate stimulus for "sound detecting" systems of the sharkis panicle motion, as opposed to fluctuations in sound pressure.The elasmobranch ear consists of the three semi-circular canalsfor detecting angular accelerations, and otolith organs fordetecting linear motion and accelerations due to gravity. Twoof these organs, the sacculus and macula neglecta, have beenshown to be responsive to vibratory motion, with the maculaneglecta having best sensitivity to vertical movements. A directvibrational pathway exists to the macula neglecta from the parietalfossa of the dorsal chondrocranium. It is not clear at present,however, whether it is the inner ear or the lateral line systemwhich is responsible for hearing. Both detection systems aretheoretically capable of providing information to the brainabout sound source location using non-parallel arrays of directionallysensitive hair-cell receptors. Recent theories of underwatersound localization by fishes and sharks suggest that the abilityto detect a vertical displacement component of an acoustic signal(e. g., via the macula neglecta) is necessary for instantaneouslocation decisions. It is not known, however, whether the sharkslocalize by processing information about various aspects ofthe sound field simultaneously (in parallel), or whether thesound field is sampled successively at different points in space.Clearly, more experimental work on the physiology of elasmobranchacoustic behavior is called for.     

Development and organization of polarity-specific segregation of primary vestibular afferent fibers in mice

A striking feature of vestibular hair cells is the polarized arrangement of their stereocilia as the basis for their directional sensitivity. In mammals, each of the vestibular end organs is characterized by a distinct distribution of these polarized cells. We utilized the technique of post-fixation transganglionic neuronal tracing with fluorescent lipid soluble dyes in embryonic and postnatal mice to investigate whether these polarity characteristics correlate with the pattern of connections between the endorgans and their central targets; the vestibular nuclei and cerebellum. We found that the cerebellar and brainstem projections develop independently from each other and have a non-overlapping distribution of neurons and afferents from E11.5 on. In addition, we show that the vestibular fibers projecting to the cerebellum originate preferentially from the lateral half of the utricular macula and the medial half of the saccular macula. In contrast, the brainstem vestibular afferents originate primarily from the medial half of the utricular macula and the lateral half of the saccular macula. This indicates that the line of hair cell polarity reversal within the striola region segregates almost mutually exclusive central projections. A possible interpretation of this feature is that this macular organization provides an inhibitory side-loop through the cerebellum to produce synergistic tuning effects in the vestibular nuclei. The canal cristae project to the brainstem vestibular nuclei and cerebellum, but the projection to the vestibulocerebellum originates preferentially from the superior half of each of the cristae. The reason for this pattern is not clear, but it may compensate for unequal activation of crista hair cells or may be an evolutionary atavism reflecting a different polarity organization in ancestral vertebrate ears.     

Scanning electron microscope observations of saccular ultrastructure in the mudpuppy (Necturus maculosus)

Summary Scanning electron microscopy revealed two types of hair cells in the sacculus of an amphibian, the mudpuppy (Necturus maculosus). Both types were surrounded by microvilli-covered sustentacular cells. The peripheral hair cells have shorter, thinner stereocilia and longer kinocilia than the hair cells in the central macula. The hair cells generally were found to be oriented with their stereocilia gradient directed toward the periphery of the macula. A nearly semicircular stria separated those directed forward and outward from those directed rearward and outward. Two basic types of otoconia were found in the otolith, and X-ray analysis revealed the entire otolith to be composed of aragonite.We would like to thank Dean E. Hillman for suggestions regarding fixation and interpretation, R. Eric Lombard for assistance with amphibian morphology and for helpful discussions during the course of this work, H. R. Wenk for performing X-ray analysis of otolith and Sister Loretta Shimondle for technical assistance. Research sponsored by the National Science Foundation Grant GK-3845 and the United States Public Health Service Grant GM-17523.     

On a Possible Hair Cell Turn-Over in the Inner Ear of the Caecilian Ichthyophis glutinosus (Amphibia: Gymnophiona)

The inner ear of the caecilian Ichthyophis glutinosus is briefly described. An analysis of the fine structure of the utricular macula show the presence of several varieties of hair cells. At least one type seems to be degenerating, another developing. A slow hair cell turnover is proposed. The number of afferent nerve endings in contact with a few hair cells were found to be 6–13 as reconstructed from electron microscopical serial sectioning.     

On the Polarization and Innervation of the Pars Inferior Sensory Epithelia of the Herring Labyrinth

The macula sacculi and the macula lagenae of the herring, Clupea harengus L., were examined by light microscopy, the macula lagenae is large compared to what is normal among non-ostariophysan fishes, the morphological polarization of the hair cells in the inferior maculae shows a pattern which is similar to that usually seen in teleost fishes. The fibres in the nerves supplying the macula sacculi and the macula lagenae were counted and their diameters measured. The ramulus saccularis is divided in two separate ramuli innervating populations of hair cells with different morphological polarization. The saccular rostral nerve trunk contains 1800–2300 fibres, with 1300–1800 fibres in the caudal nerve trunk. The lagenar nerve is composed of 2100–4000 fibres. The fibre diameters are 1–14 μm in all ramuli. Silver staining of the nerve axoplasm reveals a unique differentiation of the maculae, which can be divided into a central area surrounded by a peripheral part. The hair cells in the central area are innervated by thick nerve fibres (5–14 μm diameter) as well as a few thin nerve fibres (about 1 μm diameter), while the receptor cells in the peripheral area are exclusively innervated by thin fibres having diameters of 2 μm or less.     

Sensory surface of the saccule and lagena in the ears of ostariophysan fishes

The inner ears of a few fishes in the teleost superorder Ostariophysi are structurally unlike those of most other teleosts. Scanning electron microscopy was used to determine if other ostariophysans share these unusual features. Examined were the families Cyprinidae, Characidae, and Gymnotidae (all of the series Otophysi), and Chanidae (of the sister series Anotophysi), representing the four major ostariophysan lineages, the auditory organs of which have not yet been well described. Among the Otophysi, the saccular and lagenar otolith organs are similar to those reported for other ostariophysans. The lagena is generally the larger of the two organs. The saccular sensory epithelium (macula) contains long ciliary bundles on the sensory hair cells in the caudal region, and short bundles in the rostral region. The saccule and the lagena each have hair cells organized into two groups having opposing directional orientations. In contrast, Chanos, the anotophysan, has a saccular otolith larger than the lagenar otolith, and ciliary bundles that are more uniform in size over most of its saccular macula. Most strikingly, its saccular macula has hair cells organized into groups oriented in four directions instead of two, in a pattern very similar to that in many nonostariophysan teleosts. We suggest that the bi-directional pattern seen consistently in the Otophysi is a derived development related to particular auditory capabilities of these species.     

A study on the fine structure of the saccular macula of the gold fish

Summary The fine structure of the saccular macula of the gold fish has been studied by means of the electron microscope.The sensory epithelium of the macula consists of sensory cells and supporting cells. The surface of the sensory cell is studded with a group of sensory hairs consisting of one kino-cilium and 50–60 stereocilia. In the dorsal half of the macula, the kino-cilium is located at the dorsal end of the sensory hair group. In the ventral half of the macula, the kino-cilium is located at the ventral end of the sensory hair group. In the intermediary portion of the macula, the sensory cells with opposite polarities are situated side-by-side. The relation between the microphonic potential and the position of the kino-cilium has been discussed.Two types of nerve terminals are found situated on the basal surface of the receptor cells. The one contains no synaptic vesicle and the other contains a cluster of synaptic vesicles and a few cored vesicles. It is considered that the former corresponds to the afferent nerve terminal and the latter to the efferent one.This investigation was supported by NIH Grant NB-06052.The author is very grateful to Prof. Taro Furukawa, Osaka City University for his invaluable advice and discussion.     

Evidence concerning the morphogenesis of saccular receptors in the bullfrog (Rana catesbeiana)

A dichotomy of hair-cell types has been found in the bullfrog sacculus, and considerable evidence supports the view that one type (“peripheral”) is transformed during macular growth to the other type (“central”). Between the periphery and the center of the macula, one finds a gradation of form from “peripheral” to “central” type. Occasionally in adults and more often in stage-26 tadpoles one finds the presumably younger peripheral type of hair cell occurring well beyond the limits of the macula proper. The apparent morphogenic sequence for saccular hair cells is (1) development of a kinocillum on an endolymphatic epithelial cell, (2) gradual transformation of microvilli into stereocilia, (3) growth of the stereocilia and development of kinociliary bulb, (4) achievement of final size and form.