LX Secretion and Absorption of Endolymph

The ultrastructure of a stria vascularis in the auditory organ of the cuban crocodile (Crocodylus rhombifer)

Background: An endocochlear potential (EP) exists in the mammalian cochlea generated by the stria vascularis and an associated fibrocyte network. It plays an essential role for sensory cell function and hearing sensitivity. In non-mammalian ectothermic animals the endocochlear potential is low and its origin somewhat unclear. In this study, we explored the crocodilian auditory organ and describe the fine structure of a stria vascularis epithelium that has not been verified in birds. Material and Methods: Three Cuban crocodiles (Crocodylus rhombifer) were analyzed with light and transmission electron microscopy. The ears were fixed in glutaraldehyde The temporal bones were drilled out and decalcified. The ears were dehydrated, and embedded and was followed by semi-thin and thin sectioning. Results: The fine structure of the crocodile auditory organ including the papilla basilaris and endolymph system was outlined. The upper roof of the endolymph compartment was specialized into a Reissner membrane and tegmentum vasculosum. At the lateral limbus an organized, multilayered, vascularized epithelium or stria vascularis was identified. Discussion: Electron microscopy demonstrates that the auditory organ in Crocodylus rhombifer, unlike in birds, contains a stria vascularis epithelium separate from the tegmentum vasculosum. It is believed to secrete endolymph and to generate a low grade endocochlear potential. It may regulate endolymph composition and optimize hearing sensitivity alongside the tegmentum vasculosum. It could represent a parallel evolution essential for the adaptation of crocodiles to their diverse habitats.

The expression domain of two related homeobox genes defines a compartment in the chicken inner ear that may be involved in semicircular canal formation

Homeobox-containing genes encode a class of proteins that control patterning in developing systems, in some cases by acting as selector genes that define compartment identity. In an effort to demonstrate a similar role for such genes during ear development in the chicken, we present a detailed expression study of two related homeobox-containing genes, SOHo-1 and GH6, using in situ hybridization. At otocyst stages the two genes define a broad lateral domain of expression, which may represent a developmental compartment. Three-dimensional computer reconstructions of SOHo-1 expression at these and later stages revealed that the lateral domain becomes progressively restricted to the three semicircular canals. Thus, SOHo-1 and GH6 are among a small group of markers for a specific structural component of the inner ear. The gene expression domain initially includes the sensory regions of the semicircular canals, known as the cristae ampullaris, but none of the other four sensory organs which were recognizable by BMP4 expression during early morphogenesis (stages 19-24). Significantly, two of the sensory organs (the superior and posterior cristae) were found at the limits, or boundaries, of the SOHo-1/GH6 expression domain, suggesting that compartment boundaries may be involved in specifying sensory organ location as well as identity. Maintained expression at the boundaries may aid in specifying the location of canal outgrowth. These concepts are presented as a formal model which emphasizes that patterning information could be provided at the boundaries of gene expression domains in the inner ear.

Differential expression of bone morphogenetic proteins in the developing vestibular and auditory sensory organs

The genes responsible for the formation of various sensory organs in the inner ear are not known. There are eight sensory organs in the chick inner ear, and our previous study showed that all presumptive sensory organs initially express bone morphogenetic protein 4 (BMP4), a member of the transforming growth factor (TGF)-beta gene family. To address the potential role of BMPs in the patterning of different sensory organ structures, we investigated the expression of BMP4, BMP5, and BMP7 during sensory organ differentiation in the chick inner ear. The gene expression pattern of BMP5, although similar to that of BMP4, was transient and disappeared by embryonic day 3.5 (E3.5). In contrast, BMP7 gene expression was quite extensive, starting in the otic placode. By E5, gene expression patterns of BMP4 and BMP7 differed among vestibular and auditory sensory organs. In the vestibular sensory organs, BMP7 gene expression segregated from the main sensory tissue areas at the onset of differentiation, whereas BMP4 expression concentrated in supporting cells. In the cochlea, however, BMP7 gene expression became restricted to sensory tissue over time and eventually concentrated in supporting cells, whereas BMP4 gene expression was localized to hair cells. The different BMP expression patterns in developing auditory and vestibular sensory organs may help to shape each respective sensory structure. Furthermore, the expression of BMP4 in the cochlea also revealed an interesting pattern of sensory cell differentiation: the distal portion of the cochlea differentiates first, and the tall hair cells develop before the short hair cells.

Planum semilunatum of the rat: new light and electron microscopy observations

Planum semilunatum (PSL) cells of the rat ampullae were studied by light, transmission, and scanning electron microscopy. The observed ultrastructure of rat PSL cells was similar to that described in other species, but is in disagreement with earlier reports of rat PSL cells, most probably because of previous divergent definitions of the PSL. Regions of PSL cells were easily distinguished from other nonsensory epithelia at the lateral ends of the crista ampullaris. The PSL region consisted of irregular-shaped columnar to cuboidal pentagonal or hexagonal cells that interdigitated with one another by lateral membrane infoldings. In the PSL region the subepithelial reticular layer appeared thickened and formed wartlike impressions in the basal surface of the PSL cells. These morphological characteristics of the subepithelial reticular layer were unique to the PSL region in the ampulla and may reflect special adaptations of the PSL region to mechanical stress. Furthermore, the thick subepithelial reticular layer may have implications for transport across the PSL region.

Cholinesterase activity in vestibular organs of young and old mice

The activities of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) were studied by histochemical methods in the semicircular canal end organs, the utricle and the saccule of young and old mice. AChE was located on the plasma membrane of efferent nerve terminals beneath vestibular hair cells, and along the basement membrane. In the ampulla, stained efferent terminals were more prevalent on the slopes of the crista than in the central region. In all organs examined, there were no discernible differences in AChE activity between young and old mice. BChE activity was observed in the epithelial light cells and supporting cells of the saccule, utricle, and ampulla. Its distribution was similar in both young and old mice in the ampulla, but decreased significantly with age in the utricle. Preliminary data suggest that BChE activity is also weak in old saccular supporting cells. Unlike the utricle, old saccular light cells retained intense BChE activity.

Morphometry of ampullar dark cells

The morphologic characteristics of rat ampullar dark cells were analyzed quantitatively. Four parameters were measured: cell area per micron baseline length, membrane surface density, volume density of intercellular space, and volume density of vacuoles. No statistically significant difference was observed for any parameter, either between dark cell regions of the superior, lateral, and posterior ampullae or between different regions of dark cells within a given ampulla. The membrane surface density of the ampullar dark cells was similar to that reported for principal cells of the cortical collecting duct in the kidney. The volume density of the intercellular spaces of the dark cells was very constant, unlike that reported for epithelia involved in isotonic fluid transport. Vacuoles were found to comprise only a small portion of protoplasm in the dark cells. This new morphometry provides a basis on which to examine the response of ampullar dark cells to differing experimental conditions.

Melanocytes in vestibular dark cell areas in human fetuses

Melanocytes in human vestibular dark cell areas were observed to have round or spindle-shaped cell bodies with several processes. They were located below the basement membrane but the melanocyte processes were found in the dark cell epithelial layer closely adjacent to the dark cell processes or bodies without any separation from the basement membrane. Submicroscopic observations revealed melanin pigment in various stages of accumulation on the tyrosinase filaments in melanosomes. Three-dimensional stereoscopic observations clearly revealed the morphological characteristics of tyrosinase filaments, their helicoidal structure and some variations. Mature melanin granules developed from premelanosomes as the melanin pigments were gradually deposited onto the filaments. Through this progress of pigment accumulation, the melanosomes became darker. In the four mid-term fetuses used in the present investigation, few differences were seen in the mode of melanin deposition on the tyrosinase filaments. In view of the close spatial arrangement and contact of the melanocytes with the dark cells, it seems plausible that the melanocytes play some role in vestibular metabolism via the dark cells.

Ultrastructural localization of carbonic anhydrase in the vestibular end organs of the guinea pig

Carbonic anhydrase activity was demonstrated cytochemically on an ultrastructural level in the vestibular end organs of the guinea pig. Reaction product was found in the dark cells, transitional cells, cells of the planum semilunatum and supporting cells. In the dark cells, reaction product was observed in the cytoplasm as well as in the basal infoldings. Reaction product was also observed in the basal infoldings of the transitional cells and the cells of the planum semilunatum. The globular structures inside the supporting cells, transitional cells and the cells of the planum semilunatum were also surrounded by the reaction product. These findings suggest that carbonic anhydrase may have different functions, such as water and ion transport, respiration, nutrition and calcium carbonate deposition in the vestibular end organs.

Submicroscopic study of the vestibular dark cell area in human fetuses

The purpose of this study was to determine the characteristic ultra-microstructure of the vestibular dark cell area related to inner ear metabolism at mid-term human embryonic development. This is when the general inner ear structures apparently attain their final development. Two types of epithelial cells, dark cells and light cells, are discernible in the vestibular dark cell areas. The morphology of the dark cells is described and their role in the metabolism of endolymph and otoconia is indicated. Little is known about the nature and presence of the light cells in mammals. The present study has revealed the ultra-microstructure of the light cells and indicates their secretory function in otoconia metabolism. The dark and light cells seem to be closely related to each other in the metabolic function of the dark cell area.

Ultrastructural study of the semicircular canal cells of the frog Rana esculenta

The ultrastructure of the nonsensory cells (dark cells, transitional cells, and undifferentiated cells) of the frog semicircular canal was studied by using transmission electron microscopy in an attempt to correlate the structure with the functions of these epithelial cells. All the nonsensory cells were linked by tight junctions and desmosomes; this suggested that there is little paracellular ionic transport from perilymph to endolymph. In the dark cell epithelium, the apical intercellular spaces were dilated; in the basal part, numerous basolateral plasma membrane infoldings, containing mitochondria, delimited electron-lucent spaces. The undifferentiated cells and the transitional cells were devoid of any basal membrane infolding. Surrounding the semicircular canal, very flattened and interdigitated mesothelial cells constituted a thin multilayer tissue which limited the perilymphatic space. The morphological aspect of the dark cells suggests that they may play a role in the secretion and/or in the reabsorption of endolymph, which bathes the apical pole of these cells. The undifferentiated and transitional cells can play a role in the maintenance of the endolymphatic ionic composition because of their apical tight junctions and desmosomes.

Cytochemical study of K+-p-nitrophenyl phosphatase (K+-NPPase) in planum semilunatum and dark cells of the squirrel monkey

We examined the ultrastructure of the planum semilunatum, and also cytochemically demonstrated K+-dependent p-nitrophenyl phosphatase (K+-NPPase) activity in planum semilunatum and ampullary dark cells of the squirrel monkey. K+-NPPase activity was found in the basal part of the planum semilunatum. The reaction products were limited to the cytoplasmic side of the plasma membranes. A more intense reaction was found in the basal infoldings and the lateral interdigitations of the dark cells. On the other hand, the apical cell surface of both cell types showed no K+-NPPase activity. The present results show that both the planum semilunatum and the dark cells may both be involved in maintaining endolymph homeostasis.

Electron-microscopic study of the vestibular dark cells in the crista ampullaris of the guinea pig

The ultrastructure of the vestibular dark cells in the crista ampullaris of the guinea pig was observed using both transmission and scanning electron microscopy. The dark cells had numerous vacuoles of varying size and electron density, and also characteristic well-developed basal infoldings. These findings strongly suggest that the dark cells play an important role in fluid transport. Unique meshwork structures were observed on the luminal surface of the dark cells. Otoconia showing varying degree of degeneration were occasionally recognized on and near these structures. Electron microscopy revealed that the meshwork was comprised of cytoplasmic processes in a reticular arrangement. They seem to be engaged in the metabolism of otoconia, and perhaps also in fluid transport.

Further observations of vestibular ototoxicity in the chick: effects of streptomycin on the ampullary sensory epithelium

Starting eight days after hatching, chicks received daily subcutaneous injections of streptomycin sulfate, either 400 mg/kg for 30 days or 1,200 mg/kg for 15 days. Randomly selected chicks from each group were killed at intervals during the injection period, and the ampullae were examined for signs of vestibulotoxicity. Ampullary cell types differed in sensitivity to streptomycin. First, dark cell processes withered. Second, vacuoles formed in the apices of the light cells of the planum semilunatum. Third, nerve terminals swelled, and their organelles and ground substance clumped together, creating cleared areas. Last, hair cells and supporting cells became slightly vacuolated. However, these cells showed less overt damage than the other cell types. The onset of damage was earlier and the damage more severe with the 1,200-mg than with the 400-mg dosage. There was no evidence of hair cell loss during the experiment.

Carbonic anhydrase activity in stria vascularis and dark cells in vestibular labyrinth

Carbonic anhydrase activity was demonstrated ultracytochemically in the dark cells of the vestibular labyrinth and in the stria vascularis of the guinea pig. Reaction products were found both in the dark cells and in the stria vascularis. Localization of carbonic anhydrase was very similar between the dark cells and the intermediate cells, but the marginal cells had a different localization pattern. In both the dark cells and the intermediate cells, the reaction products were deposited in the cytoplasm, especially in the infoldings. In the marginal cells, carbonic anhydrase activity was observed on the lateral plasma membrane of the apical area and in a few apical vesicles, but was not detected in the infoldings of the deep portion. Reaction products were also observed in the intercellular spaces between the marginal cells. No reaction products were observed in the basal cells and transition cells. Carbonic anhydrase activity was observed in the cytoplasm of capillary endothelial cells and the fibrocytes of the spiral ligament. It is considered that carbonic anhydrase may play a major role in water and electrolyte transport in both the dark cells and stria vascularis.

Specific pathology of the stria vascularis in postnatal progressive genetic inner ear disorder

After reaching normal morphological maturation the stria vascularis of the Shaker-2 (Sh-2) mouse postnatally underwent a progressive degeneration, especially of the intermediate cell layer. Considerable changes were present on the 3rd week after birth. The marginal cells became atrophic and showed a reduction in the number and extent of the cellular extensions towards the intermediate cell layer. The intermediate cells disintegrated but were present 9 weeks after birth. The basal cells showed a normal morphology. Specific inclusion bodies were identified in the marginal cells from the 3rd week on. Occasionally such inclusions occurred in marginal cells. In the Shaker-1 (Sh-1) a flattening of intermediate cells occurred but otherwise no pathological changes were found 6--9 weeks after birth.

Vestibular ototoxicity in the chick: effects of streptomycin on equilibrium and on ampullary dark cells

Starting a week after they were hatched, chicks received daily subcutaneous injections of streptomycin sulfate for 15 or 30 days at one of three dosages: 400, 800, or 1,200 mg/kg body weight. During the period of administration, the chicks were weighed, examined for signs of systemic intoxication, and tested for impairment of equilibrium. At intervals some birds from each group were sacrificed and the end-organs of the semicircular canals were examined for damage. After the fifteenth injection, the weights of the control and experimental chicks were similar. By comparison, the chicks that received streptomycin injections showed varying degrees of impairment of equilibrium. First, some birds in the three experimental groups began to tremble at least slightly by the third injections, but others, particularly at the highest dosage (1,200 mg/kg body weight), trembled severely by the fifth injections. However, trembling began to subside in the lowest-dosage (400 mg/kg body weight) group by the fourteenth injection. Second, the chicks' ability to perch on dowels, either hooded or unhooded, and their ability to perch on the investigator's fingers in the dark deteriorated. Perching performances on the dowel deteriorated conspicuously only at higher dosages, while changes in perching on the finger were detected earlier and at lower dosages. Streptomycin damaged dark cells before other cell types. The cuboidal dark cells were most sensitive, followed by the pyriform cells. The eminential cells were least sensitive. Although the hair cells were functionally damaged by either the primary or the secondary actions of streptomycin, as evidenced by the chicks' early impairment of equilibrium, they showed distinct cytologic lesions later than did the dark cells.

Hereditary deafness with hydrops and anomalous calcium phosphate deposits

The temporal bones from a 58-year-old white woman who had had hereditary congenital deafness were examined with the techniques of microdissection and surface preparations followed by sectioning of the modiolus. There was bilateral, almost total sensorineural degeneration, which also involved the saccule. The degeneration of the distal processes of the cochlear neurons in the osseous spiral lamina was almost complete, whereas numerous ganglion cells and proximal processes remained in the modiolus and the internal auditory canal. Severe cochleo-saccular hydrops was present in the left ear with Reissner's membrane bulging into the horizontal canal. X-ray diffraction and electron probe analysis were used to study the abnormal crystalline deposits in both ears. On the left side the saccular otoconia were composed of calcite, but the utricular macula was covered by a crust of apatite spherulites. More apatite occurred around the maculae and in the scala media. The cupulae were composed of apatite and octacalcium phosphate. On the right side the utricular otoconia were of normal calcite, but there was a deposit of apatite on the macula sacculi. The upper part of the scala media was completely filled by a deposit of apatite and octacalcium phosphate.

Adenylate cyclase activity in the fetal and the early postnatal inner ear of the mouse

The adenylate cyclase activity was analyzed in fetal, early postnatal and adult inner ears of the CBA/CBA mouse and also in approximately one month old inner ears from Shaker -1 and Shaker -2 mice. A comparison was made with the maturation of potassium levels in endolymph as investigated with the X-ray energy dispersive technique. Adenylate cyclase activity in the developing normal inner ear shows two significant periods of increases: from the 16th to the 19th gestational day in both the cochlear and vestibular parts of the labyrinth, and from birth to day 6 after birth in the lateral wall tissues of the scala media. During the first period the anatomical boundaries of the secretory epithelia are developing. The postnatal rise in adenylate cyclase activity correlates with the morphological maturation of stria vascularis at the cellular and subcellular levels and the rise in potassium content of endolymph. The rise of enzyme activity in the cochlear during the maturation of endolymph supports a link between adenylate cyclase and the control of inner ear fluids. Adenylate cyclase activity in stria vascularis/spiral ligament of Shaker -1 and Shaker -2 mice were at normal levels and correlated better with the rather normal morphology of the tissues than the abnormal composition of endolymph in these mutants.

Obliteration of the ductus reuniens

The ductus reuniens was successfully obliterated in 52 guinea pig ears. Histopathological study showed that a majority of these specimens demonstrated cochlear hydrops, saccular collapse and normal utricle. These results support the theory of longitudinal flow of endolymph from the cochlea toward the endolymphatic sac via the ductus reuniens and saccule. A major source of endolymph in the saccule appears to be the scala media. In another set of 11 animals in which the ductus reuniens was first obstructed and two months later the endolymphatic duct was blocked, endolymphatic hydrops was shown in the cochleae, saccules, and utricles of all but one. The evidence suggests that cochlear hydrops was caused by obliteration of the ductus reuniens, and the saccular and utricular hydrops occurred subsequently as the result of blockage of the endolymphatic duct. Remnants of otolithic membrane which were attached to the distended saccular wall indicate that the membrane which had collapsed onto the macula after obliteration of the ductus reuniens is capable of subsequent distension. This experiment supports the concept of endolymph flow from the utricle and canals toward the endolymphatic sac. A blocked cutus reuniens might also explain the pathophysiological basis for the auditory form of Meniere's disease.

Sensorineural hearing loss in a Scandinavian old English sheepdog. A case report

Inner ear disease is extremely rare in the old English sheepdog. The present case would constitute the second known case in Scandinavia and the first one observed in Norway. The morphological investigation revealed complete degeneration of the organ of Corti. A single layer of cells was covering the anatomical location of the stria vascularis. The dog did not show clinical signs of vestibular dysfunction. Hair cells were present in the cristae ampullares and macula utriculi. The histopathological features support a genetic basis for the sensorineural hearing loss, though other (rare) causes of exogenically induced inner ear degeneration cannot be completely excluded on the morphological findings alone. The brain revealed signs of chronic meningitis.

Experimentally (atoxyl) induced ampullar degeneration and damage to the maculae utriculi

Atoxyl administration to guinea pigs may cause vesicular degeneration of both the secretory and the sensory regions of the cristae ampullares and macula utriculi. Some of the severely damaged secretory cells were even expelled from the surface into the endolymphatic space. The nerve chalices of type I hair cells disintegrated. The degeneration of the secretory region will thus block the endolymph circulation and the electrolyte balance is likely to collapse. Whether hair cell degeneration can best be explained on this basis (indirect atoxyl effect) or by a direct action of atoxyl on the hair cells and the nerve chalices of type I hair cells is discussed.

A scanning electron microscopic study of the morphology and geometry of neural surfaces and structures associated with the vestibular apparatus of the pigeon

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 "swing 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 "swing 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).

Sodium and potassium concentrations in the endolymph and perilymph of the cat

Inner ear fluid from the scala vestibuli, scala tympani, scala media, utricle, and cerebrospinal fluid were collected from 47 normal cats, and analyzed for sodium and potassium concentrations with a flame photometer. Each compartment was found to have its own different values for sodium and potassium concentration. Perilymph in the scala vestibuli possesses a lower sodium concentration than perilymph in the scala tympani; in potassium concentration the perilymph in the scala vestibuli shows a higher value than that of the scala tympani. Compared with cochlear endolymph, utricular endolymph contains a higher concentration of sodium and a lower concentration of potassium. These concentration differences, which are related to the characteristics of membrane transport in each compartment of inner ear fluid, are considered to be adequate to sustain normal biological conditions of the inner ear.