Adenine nucleotides of the stria vascularis

Summary The levels of the adenine nucleotides ATP, ADP, and AMP in the stria vascularis were measured under normal conditions, and following various durations of ischemia. The concentrations of these compounds were used for the calculation of the adenylate energy charge, the energy status and the phosphorylation state of the stria. Following 10 min of ischemia the adenylate energy charge had decreased three fold, the energy status seven fold and the phosphorylation state 14 fold. To study the potential for recovery of strial function following various brief and prolonged ischemic intervals, a method for the perfusion of the ear via the anterior inferior cerebellar artery was developed. For various reasons it was found advantageous to use artificial blood as perfusate, relying upon fluorocarbons as oxygen carriers. The endolymphatic potential was used as electrical indicator of strial function. Recovery of the endolymphatic potential following brief periods of ischemia was paralleled by a corresponding increase of the ATP levels and a drastic decrease of the AMP levels of the stria vascularis. Preliminary results on the effects of substrate-free perfusion are presented.Supported by NIH grant NS 06575, and NSF grant BNS 77-16842     

Scanning ultrastructure of the stria vascularis

In the eleven years since the introduction of the first commercially available scanning electron microscope, a number of otologic studies have made use of this new technique. However, the literature contains only one report of scanning ultrastructure of the endolymphatic surface of the stria vascularis. We have studied the stria vascularis with a technique allowing examination of the mid-modiolar cut surface as well as the endolymphatic surface.     

Scanning ultrastructure of the stria vascularis

Summary In the eleven years since the introduction of the first commercially available scanning electron microscope, a number of otologic studies have made use of this new technique. However, the literature contains only one report of scanning ultrastructure of the endolymphatic surface of the stria vascularis. We have studied the stria vascularis with a technique allowing examination of the mid-modiolar cut surface as well as the endolymphatic surface.Abbreviations used Figure legends — 1SP lower spiral prominence - uSP upper spiral prominence - E endolymphatic - CS cut surface - c capillaries - S sinus - SL spiral ligament Text abbreviations SEM scanning electron microscope (microscopy) - TEM transmission electron microscope - Å angstrom     

Passive transport of the stria vascularis

Summary The transport and diffusion of the stria vascularis were studied by injection of ferritin and horseradish peroxidase into the endolymphatic space of the guinea pig. The observation was made by electron microscopy under the conditions with or without the apposition of NaCl crystals on the round window membrane.In the cochlea with no NaCl crystals on the round window membrane, ferritin or HRP was found only in the apical cytoplasm of the marginal cells. No trace of particles was seen in the intermediate, basal, endothelial cells and intercellular spaces in the stria vascularis.The ferritin passed through the marginal cells, intercellular spaces, basement membrane and capillary walls by vesicular transport under the condition of change of osmotic pressure by apposition of NaCl crystals on the round window membrane.HRP labeled the marginal cells but was not seen in the intermediate, basal, endothelial cells and intercellular spaces.The functional significance of these findings were discussed.

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Zusammenfassung Transport und Diffusion der Stria vascularis wurden durch Injektion von Ferritin und Meerrettich-peroxidase (MRP) in den Endolymphraum des Meerschweinchens untersucht und die Veränderungen elektronenmikroskopisch dargestellt. Einem Teil der Tiere wurden Kochsalzkristalle auf die Membran des runden Fensters appliziert. Bei den Schnecken ohne Kochsalzkristalle am runden Fenster wurde Ferritin oder MRP nur im apikalen Teil des Cytoplasmas der Marginalzellen gefunden. Die intermediären, die basalen und die Endothelzellen sowie die intercellulären Räume der Stria vascularis waren frei.Bei Änderung des osmotischen Druckes nach Einlage von Kochsalzkristallen auf die runde Fenstermembran passiert Ferritin die Marginalzellen, die intercellulären Räume, die Basalmembran und die Capillarwände mittels vesiculären Transports. MRP fand sich in den Marginalzellen, nicht aber in den intermediären, basalen und endothelialen Zellen und den intercellulären Räumen. Die funktionelle Bedeutung dieser Befunde wird diskutiert.

     

Development of the human stria vascularis.

Fifteen human fetal cochleas were investigated by light microscopy and transmission electron microscopy in order to observe the development of the stria vascularis. The earliest signs of strial cell differentiation take place during the 11th week of gestation. Subsequently, the first stages of the stria vascularis development occur quickly. At week 14 the three types of cells, namely, marginal, intermediate and basal cells are discernable. Moreover at this stage, signs of specific activity are already present. The adult-like appearance of the stria vascularis is reached by week 21 but its maturation is completed only during the last trimester of pregnancy. This is in good agreement both with the development of the organ of Corti structures and with the maturation of the human auditory function.     

Development of the stria vascularis in the rat

The rat is an altricial animal and is thus a useful model for the study of auditory development. The endocochlear potential (EP) undergoes a rapid increase in magnitude from the end of the 1st week to the beginning of the 3rd postnatal week. The purpose of this study was to examine the ultrastructure of the developing stria vascularis in the rat pup in order to correlate functional changes with structural alterations. Rat pups of various ages underwent EP measurement under Rompun anesthesia. The cochleas were rapidly removed under deep pentobarbital anesthesia. The tissues were fixed in 2.5% glutaraldehyde and postfixed in 1.5% osmium tetroxide. Thin sections were viewed and photographed using a Hitachi H7000 transmission electron microscope. A series of distinct developmental changes were observed. Intermediate and basal cells became more distinct from one another, and basal cells became more elongated. Marginal cells underwent progressive development of basolateral infoldings. These cytologic changes may signal the development of ion transport mechanisms necessary for EP development.     

Quantitative assessment of the rat stria vascularis

Stria vascularis tissues from standardized regions in the basal, middle and apical turns of the rat cochlear duct were assessed quantitatively. Strial width, number of marginal cells across the strial width, radial area, as well as the volume density of the different components of the stria vascularis were determined for each standardized region. Strial width, number of marginal cells across the strial width and the radial area were greatest in the basal region and least in the apical region of the cochlea. The volume density of intermediate cells and capillary space was statistically unchanged in the three examined regions of the stria vascularis. However, the volume density of marginal cells and that of basal cells were different between regions. The volume density of marginal cells was highest in the basal turn while the volume density of basal cells was greatest in the apical turn. An objective assessment of the response of the stria vascularis to environmental conditions can be made by kant of its cellular architecture, providing a means to compare the effects of various agents between animal models used to study human inner ear dysfunction.     

Response of the stria vascularis to corticosteroids

Structural changes in the cellular architecture of the stria vascularis in adrenalectomized rats were quantitated by stereological methods after the administration of either aldosterone or dexamethasone. The volume densities of the differing strial components from steroid-administered animals were determined to approximate those of sham-adrenalectomized animals in general. The increased volume density of intercellular space as was observed following adrenalectomy, however, was only restored after the administration of aldosterone. These data correlate a recovery of the cellular architecture of the stria vascularis in adrenalectomized animals with the restoration of endogenous levels of adrenal steroids. These findings provide further information with regard to the effects of varying serum levels of corticosteroids on inner ear morphology.     

Adenine nucleotides of the stria vascularis.

The levels of the adenine nucleotides ATP, ADP, and AMP in the stria vascularis were measured under normal conditions, and following various durations of ischemia. The concentrations of these compounds were used for the calculation of the adenylate energy charge, the energy status and the phosphorylation state of the stria. Following 10 min of ischemia the adenylate energy charge had decreased three fold, the energy status seven fold and the phosphorylation state 14 fold. To study the potential for recovery of strial function following various brief and prolonged ischemic intervals, a method for the perfusion of the ear via the anterior inferior cerebellar artery was developed. For various reasons it was found advantageous to use "artifical blood" as perfusate, relying upon fluorocarbons as oxygen carriers. The endolymphatic potential was used as electrical indicator of strial function. Recovery of the endolymphatic potential following brief periods of ischemia was paralleled by a corresponding increase of the ATP levels and a drastic decrease of the AMP levels of the stria vascularis. Preliminary results on the effects of substrate-free perfusion are presented.     

Freeze fracturing of the human stria vascularis

The stria vascularis is an important functional element in the mammalian cochlea. This special tissue is considered to be the source of the endocochlear potential and thus the driving force for the production of a receptor response to the auditory stimulus. In order to maintain its function, the stria vascularis needs to be separated from the endolymphatic space by a tight seal. This seal is comprised of tight junctions in the marginal cell layer. The junctional arrangement in the stria vascularis is described, utilizing the freeze-fracturing technique which allows the visualization of large expansions of plasma membrane. The marginal cells are generally separated by tight junctions of the moderately tight to tight type. In places, however, even so-called leaky junctions with only a few sealing strands are present. Whereas the intermediate cell layer seems to lack tight junctions, the basal cells are connected by extensive tight junctions more or less covering the entire cell. These junctions seem to form an extremely tight barrier against the spiral ligament. Gap junctions are also present in the tissue. Intermediate cells as well as the basal cells are coupled by gap junctions. In the basal cell layer, gap junctional elements may also be found inside the large tight junctions comprising so-called mixed junctions.     

Marginal cells of the stria vascularis in vitro]

Explants of stria vascularis and spiral ligament of the guinea pig cochlea were cultivated and after 2 days fibroblast-like cells were found growing around the explant. Marginal cells advanced at 15 microns/day to the border of the explant, and after 2 weeks they proliferated on top of a thin layer of fibroblast-like cells outside the explant, replacing several layers of fibroblast-like cells. Tight junctions and interdigitations of the lateral membranes were found between all neighbouring marginal cells. Their apical surface was covered by microvillus-like membrane extensions. The basal membrane of the new marginal cells did not interdigitate with the underlying membranes of fibroblast-like cells; there was always a gap between the two cell types. The results demonstrate that marginal cells of the stria vascularis are capable of repairing damage to the epithelium, such as may be caused by endolymphatic hydrops, even if the luminal side contains perilymph-like fluid. Furthermore, the cell culture allows living, clearly identified marginal cells to be studied in vivo.     

Electrophysiological measurements of the stria vascularis potentials in vivo

Glass microelectrodes were introduced into the stria vascularis (SV) to measure the DC potential profile of the SV in vivo from its apical as well as basal side. The K+ concentration gradient in the lateral cochlear wall was measured by means of double barrel K+-selective microelectrodes. A positive potential 10.4 +/- 6.3 mV higher than the endocochlear potential (EP) was found in the SV. In noise-exposed animals the positive potential found in the SV was 14.6 +/- 7.2 mV higher than the EP. K+ concentration observed during penetration into the SV was in the range of 50-100 mmol/l. Simultaneous measurement of the DC positive potential in the SV and EP showed a nearly parallel time course during anoxia with a tendency to increase the difference between the SV potential and the EP. The difference reached approximately 30 mV 20 min after the beginning of anoxia. It may be assumed that the electrogenic pump localized at the basolateral membrane of the marginal cell (MC) is a source of the positive potential inside the MC and is hence a source of the potential drop across the luminal side of the MC membrane.     

Age-dependent degeneration of the stria vascularis in human cochleae

OBJECTIVE: Aging is a common cause of acquired hearing impairments. This study investigated age-related morphologic changes in human cochleae, with a particular focus on degeneration of the stria vascularis (SV) and the spiral ganglion (SG). STUDY DESIGN: Retrospective case review. METHODS: The study group comprised 91 temporal bones from individuals aged 10 to 85 years who had no history or audiometric findings suggestive of specific causes of cochlear degeneration. We quantified the SV and SG atrophy at each cochlear turn using morphometric measurements. Correlations of the SV and SG atrophy with age, audiometric patterns of hearing loss, and auditory thresholds were statistically investigated. RESULT: The SV and the SG both showed a tendency for progressive atrophy to develop with age. However, statistically significant correlations were observed between aging and SV atrophy only in the apical and basal cochlear turns. These findings were consistent with those reported previously in gerbils. No significant correlations were detected between SV or SG atrophy and audiometric findings. CONCLUSION: SV atrophy appears to be the most prominent anatomic characteristic of aged human cochleae.     

Differentiation of cyst-forming stria vascularis tissues in vitro

The marginal cells of the stria vascularis possess distinctive morphological characteristics associated with their role in endolymph production. Interestingly, when stria-derived epithelial cells are grown in association with the underlying mesenchyme, the final differentiation of these cell types does not occur. Beyond the rudimentary polarity that is established, similar to that shown in epithelial monolayers, cells in culture bear only a slight resemblance to their marginal cell counterparts in vivo. The ultrastructural features that typify these epithelia, extensive cytoplasmic invaginations, with an abundance of mitochondria, and darkly stained cytoplasm, are not evident under standard culture conditions. In order to determine whether fluid transport, a key function of the stria vascularis, has an effect on the ultrastructural morphology, we examined de novo stria vascularis tissues that formed a fluid-filled cyst in vitro. We found that only cells associated with the luminal structure demonstrated dark cytoplasmic staining and amplification of the basolateral membrane of the marginal cells. Additionally, other epithelial features, such as mitochondria-rich and microvilli-rich cells, were observed in cyst-forming tissues. The enhancement of the marginal cell specializations was not as robust as that observed in vivo; however, they were clearly more extensive when compared to cells in the same culture that were not associated with a fluid-filled lumen. Thus it appears that fluid transport may be necessary to maximize differentiation of stria vascularis tissues in vitro.