Comparative anatomy of melanin pigment in the stria vascularis. Evidence for a distinction between melanocytes and intermediate cells in the cat

The Stria Vascularis: Renewed Attention on a Key Player in Age-Related Hearing Loss

Age-related hearing loss (HL), or presbycusis, is a complex and heterogeneous condition, affecting a significant portion of older adults and involving various interacting mechanisms. Metabolic presbycusis, a type of age-related HL, is characterized by the dysfunction of the stria vascularis, which is crucial for maintaining the endocochlear potential necessary for hearing. Although attention on metabolic presbycusis has waned in recent years, research continues to identify strial pathology as a key factor in age-related HL. This narrative review integrates past and recent research, bridging findings from animal models and human studies, to examine the contributions of the stria vascularis to age-related HL. It provides a brief overview of the structure and function of the stria vascularis and then examines mechanisms contributing to age-related strial dysfunction, including altered ion transport, changes in pigmentation, inflammatory responses, and vascular atrophy. Importantly, this review outlines the contribution of metabolic mechanisms to age-related HL, highlighting areas for future research. It emphasizes the complex interdependence of metabolic and sensorineural mechanisms in the pathology of age-related HL and highlights the importance of animal models in understanding the underlying mechanisms. The comprehensive and mechanistic investigation of all factors contributing to age-related HL, including cochlear metabolic dysfunction, remains crucial to identifying the underlying mechanisms and developing personalized, protective, and restorative treatments.

A critical evaluation of "leakage" at the cochlear blood-stria-barrier and its functional significance

The blood-labyrinth-barrier (BLB) is a semipermeable boundary between the vasculature and three separate fluid spaces of the inner ear, the perilymph, the endolymph and the intrastrial space. An important component of the BLB is the blood-stria-barrier, which shepherds the passage of ions and metabolites from strial capillaries into the intrastrial space. Some investigators have reported increased "leakage" from these capillaries following certain experimental interventions, or in the presence of inflammation or genetic variants. This leakage is generally thought to be harmful to cochlear function, principally by lowering the endocochlear potential (EP). Here, we examine evidence for this dogma. We find that strial capillaries are not exclusive, and that the asserted detrimental influence of strial capillary leakage is often confounded by hair cell damage or intrinsic dysfunction of the stria. The vast majority of previous reports speculate about the influence of strial vascular barrier function on the EP without directly measuring the EP. We argue that strial capillary leakage is common across conditions and species, and does not significantly impact the EP or hearing thresholds, either on evidentiary or theoretical grounds. Instead, strial capillary endothelial cells and pericytes are dynamic and allow permeability of varying degrees in response to specific conditions. We present observations from mice and demonstrate that the mechanisms of strial capillary transport are heterogeneous and inconsistent among inbred strains.

Migration and fate of vestibular melanocytes during the development of the human inner ear

Melanocytes are present in various parts of the inner ear, including the stria vascularis in the cochlea and the dark cell areas in the vestibular organs, where they contribute to endolymph homeostasis. Developmental studies describing the distribution of vestibular melanocytes are scarce, especially in humans. In this study, we investigated the distribution and maturation of the vestibular melanocytes in relation to the developing dark cell epithelium in inner ear specimens from week 5 to week 14 of development and in surgical specimens of the adult ampulla. Vestibular melanocytes were located around the utricle and the ampullae of the semicircular canals before week 7 and were first seen underneath the transitional zones and dark cell areas between week 8 and week 10. At week 10, melanocytes made intimate contact with epithelial cells, interrupting the local basement membrane with their dendritic processes. At week 11, most melanocytes were positioned under the dark cell epithelia. No melanocytes were seen around or in the saccule during all investigated developmental stages. The dark cell areas gradually matured and showed an adult immunohistochemical profile of the characteristic ion transporter protein Na⁺/K⁺‐ATPase α1 by week 14. Furthermore, we investigated the expression of the migration‐related proteins ECAD, PCAD, KIT, and KITLG in melanocytes and dark cell epithelium. This is the first study to describe the spatiotemporal distribution of vestibular melanocytes during the human development and thereby contributes to understanding normal vestibular function and pathophysiological mechanisms underlying vestibular disorders.

Endothelial cell, pericyte, and perivascular resident macrophage-type melanocyte interactions regulate cochlear intrastrial fluid-blood barrier permeability

The integrity of the fluid-blood barrier in the stria vascularis is critical for maintaining inner ear homeostasis, especially for sustaining the endocochlear potential, an essential driving force for hearing function. However, the mechanisms that control intrastrial fluid-blood barrier permeability remain largely unknown. At the cellular level, the intrastrial fluid-blood barrier comprises cochlear microvascular endothelial cells connected to each other by tight junctions (TJs), an underlying basement membrane, and a second line of support consisting of cochlear pericytes and perivascular resident macrophage-type melanocytes. In this study, we use a newly established primary cell culture-based in vitro model to show that endothelial cells, pericytes, and perivascular resident macrophage-type melanocytes interact to control intrastrial fluid-blood barrier permeability. When the endothelial cell monolayer was treated with pericyte--or perivascular resident macrophage-type melanocyte--conditioned media, the permeability of the endothelial cell monolayer was significantly reduced relative to an untreated endothelial cell monolayer. Further study has shown the pericytes and perivascular resident macrophage-type melanocytes to regulate TJ expression in the endothelial cell monolayer. The new cell culture-based in vitro model offers a unique opportunity to obtain information on the organ-specific characteristics of the cochlear blood/tissue barrier. Our finding demonstrates the importance of signaling among pericytes, endothelial cells, and perivascular resident macrophage-type melanocytes to the integrity of the intrastrial fluid-blood barrier.

Perivascular-resident macrophage-like melanocytes in the inner ear are essential for the integrity of the intrastrial fluid-blood barrier

The microenvironment of the cochlea is maintained by the barrier between the systemic circulation and the fluids inside the stria vascularis. However, the mechanisms that control the permeability of the intrastrial fluid-blood barrier remain largely unknown. The barrier comprises endothelial cells connected to each other by tight junctions and an underlying basement membrane. In a recent study, we found that the intrastrial fluid-blood barrier also includes a large number of perivascular cells with both macrophage and melanocyte characteristics. The perivascular-resident macrophage-like melanocytes (PVM/Ms) are in close contact with vessels through cytoplasmic processes. Here we demonstrate that PVM/Ms have an important role in maintaining the integrity of the intrastrial fluid-blood barrier and hearing function. Using a cell culture-based in vitro model and a genetically induced PVM/M-depleted animal model, we show that absence of PVM/Ms increases the permeability of the intrastrial fluid-blood barrier to both low- and high-molecular-weight tracers. The increased permeability is caused by decreased expression of pigment epithelial-derived factor, which regulates expression of several tight junction-associated proteins instrumental to barrier integrity. When tested for endocochlear potential and auditory brainstem response, PVM/M-depleted animals show substantial drop in endocochlear potential with accompanying hearing loss. Our results demonstrate a critical role for PVM/Ms in regulating the permeability of the intrastrial fluid-blood barrier for establishing a normal endocochlear potential hearing threshold.

Novel structures in marginal and intermediate cells presumably relate to functions of apical versus basal strial strata

Prior ultrastructural studies showed that K+ supplied to the stria vascularis came from recycling ions from the organ of Corti or perilymph to strial basal cells. A newly distinguished basal subtype of intermediate cell (BIC) completely covered the basal cells with a leaf-like horizontal process and appeared situated to absorb from them all of the recycled K+. The basal region of marginal cells (MCs) projected foot-like and enlarged processes to border BICs opposite an unique ca. 150 angstroms space. These basal MC processes appeared positioned to resorb part of the K+ recycled to BICs. A second, upper subtype of IC (UIC), occupying middle to upper strial strata, contacted BIC's extensively. UICs were thus located to resorb from BICs the portion of the recycled K+ not forwarded to basal MC processes. The apical segment of MCs projected mitochondria-filled primary processes and numerous associated secondary processes. The Na,K-ATPase-rich secondary processes populated mid to upper stria where they could siphon K+ from UICs and resorb and secrete the ions thus generating the 150 mM [KCl] of endolymph. The morphologic relationship of basal marginal cell processes to BICs differed so strikingly from the relation of upper MC processes to UICs as to suggest a different function for basal stria, one possibly concerned with generating the endocochlear potential.

Postnatal vascular development in the lateral wall of the cochlear duct of gerbils: quantitative analysis by electron microscopy and confocal laser microscopy

The development of the capillary network in the stria vascularis and in the underlying spiral ligament of gerbils was systematically and quantitatively investigated by conventional electron microscopy and confocal laser microscopy in association with vascular labeling with fluorescent gelatin. The developmental changes of capillaries in the lateral wall were observed as the following series of events. (i) At 0 days after birth (DAB) capillaries already existed in the spiral ligament as a network. (ii) At 3-9 DAB the capillary network developed into two layers starting from the scala vestibuli side to the scala tympani side; one layer was located in the stria and the other in the spiral ligament. (iii) At 9 DAB capillaries in the stria became separated from the spiral ligament, and the capillary network consisting of a two-layered structure was complete. (iv) Total capillary length and capillary density in the lateral wall increased until 9 DAB and leveled off thereafter, but changes in the relative position of capillaries in the stria toward the luminal surface of marginal cells continued until 31 DAB. On the basis of the above observations, we propose two possible mechanisms underlying the vascular development in the lateral wall: (i) the formation of new vasculature (angiogenesis), and (ii) changes in the position of cellular components relative to capillaries in association with the differentiation and maturation of marginal cells and intermediate cells.

Quantitative differences in endolymphatic calcium and endocochlear potential between pigmented and albino guinea pigs

A number of previous studies have suggested that melanin may play a role in Ca2+ homeostasis of endolymph. In the present study, endolymph Ca2+ levels and endocochlear potential (EP) were measured in all four cochlear turns of pigmented or albino guinea pigs. Auditory sensitivity was also evaluated using cochlear action potential (AP) thresholds. In pigmented animals we found that endolymph Ca2+ tended to increase from base to apex of the cochlea, while EP systematically decreased towards the apex. In contrast, no significant Ca2+ gradient was found in albinos and the EP decline was far less. As a result, the apical turn of albino animals had significantly lower Ca2+ and significantly higher EP than in pigmented animals. AP thresholds pooled across all test frequencies were significantly lower in albino animals although no differences at individual frequencies reached significance. Even after correction for EP differences, the endolymph Ca2+ levels in albino animals were significantly lower than in pigmented ones. These results confirm that there are significant physiologic differences between pigmented and albino animals, which are a likely consequence of the absence of melanin in the albino cochlea. They are consistent with the involvement of melanin in the active transport of Ca2+ into endolymph.

Morphological and functional characteristics of the different cell types in the stria vascularis: a comparison between cells obtained from fresh tissue preparations and cells cultured in vitro

Endolymph is the only extracellular fluid in the body which is characterized by an intracellular-like ion composition. The molecular mechanisms responsible for the production of endolymph and the regulation of endolymph composition are still unknown to a large extent, although the stria vascularis (SV) is believed to play an important role for these functions. A basic requirement for investigating the function of different cell types in the SV is the establishment of a method, which increases the accessibility of the tissue with maintained cell viability and function. In this study, fresh tissue preparations and cultured cells from SV, harvested from pigmented guinea pigs, were established. Marginal cells, intermediate/melanocyte-like cells and fibroblasts could be discerned in the cell cultures with bright-field microscopy, transmission and scanning electron microscopy as well as immunohistochemistry, using polyclonal antibodies against cytokeratin and vimentin. In order to study functional characteristics of the fresh tissue preparations and the cell cultures, changes in the cytoplasmic free Ca2+ concentration were determined with the fura-2 method. The cultured cells, of different types in the SV, are a suitable model for future studies of the molecular mechanisms behind the production of endolymph and the regulation of endolymph composition.

Differential effects of gentamicin on the distribution of cochlear function in albino and pigmented guinea pigs

It has been suggested that the high affinity of melanin pigment for aminoglycoside antibiotics may cause these drugs to bind preferentially to the pigmented inner ear, producing greater ototoxicity than in the amelanotic albino cochlea. However, evidence of greater ototoxicity in albinos has led to the hypothesis that melanin inhibits the toxicity of these drugs in the pigmented inner ear. On the other hand, ototoxicity in the pigmented animals may simply be delayed relative to the albinos, only to become equal or even more severe with time. The present study was conducted to determine whether a relatively low dose of gentamicin (68.5 mg/kg) would produce differential ototoxicity between albino and pigmented guinea pigs which would persist long after drug exposure had stopped. Nine pigmented and eight albino guinea pigs were given gentamicin sulfate for 14 consecutive days, and were then allowed a two-month recovery period before cochlear analysis; 11 pairs of saline-injected or untreated albino and pigmented guinea pigs served as controls. The results showed that the gentamicin-treated albinos had significantly elevated thresholds for the compound action potential from the auditory nerve (CAP), and significantly lower endocochlear potentials (EP) and cochlear microphonic (CM) input-output voltage functions when compared to their respective controls, or to either group of pigmented guinea pigs. The CAP in drug-treated pigmented animals did not differ significantly from controls, and the differences in EP and CM were marginally significant. The results indicate that the pigmented cochlea is less susceptible to gentamicin than the albino cochlea, and support the hypothesis that melanin may inhibit aminoglycoside ototoxicity in the pigmented inner ear.

Distribution and features of melanocytes during inner ear development in pigmented and albino rats

In this developmental study, the distribution and features of melanocytes in the inner ear of pigmented and albino rats was investigated with the use of an antibody, which specifically reacts with a melanocyte differentiation antigen present in the membranes of (pre)melanosomes. Melanocyte precursors could be traced from 13 days post conception onwards and the course was followed to their targets in the inner ear. Melanocytes which settle in the modiolus appeared to reach their target along another pathway than strial and vestibular melanocytes. No difference was observed in the melanocyte distribution between pigmented and albino rats. The integration of melanocytes into the stria vascularis was associated with an increased rate of melanosome production in both strains, but in the albinos far fewer melanosomes were produced. After the stria had reached maturity, melanosome production was arrested and melanosomes were subject to lysosomal digestion. In the stria of the pigmented rats, cells with aggregations of disintegrating melanosomes appeared and persisted into adulthood. In the adult, the majority of the intermediate cells contained only a few scattered melanosomes, while melanosomes could only rarely be detected in the albinos. These observations indicate that there is a close relationship between melanosome production and the process of interdigitation of melanocytes with the marginal cells. It seems unlikely that melanosomes or melanin make any important contribution to the function of the adult stria vascularis. Outside the stria, the features of melanocytes in both strains were similar to skin melanocytes.

Dysgenesis of melanocytes and cochlear dysfunction in mutant microphthalmia (mi) mice

In order to evaluate the cytological homology of intermediate cells and melanocytes, and to investigate the function of melanocytes in the inner ear, hearing acuity and cochlear pathology were studied in three strains of mice, namely, wild type mice (+/+), albino mice without melanin (c2J/c2J), and microphthalmia mice with no melanocytes (mibw/mibw). Our histochemical data indicated that intermediate cells showed cytological characteristics almost identical to those of melanocytes and that disorders of melanin and/or melanocytes were reflected in the stria vascularis of each mouse. While c2J/c2J presented the same normal hearing acuity and normal structure of the stria vascularis as +/+, the hearing acuity of mibw/mibw mutants was severely impaired. Their stria vascularis was abnormally thin, lacking intermediate cells. According to these results, lack of melanin has little influence on hearing acuity; however, the absence of intermediate cells or melanocytes causes severe hearing loss, presumably due to a strial dysfunction.

Ongoing proliferation of melanocytes in the stria vascularis of adult guinea pigs

The intermediate cells of the stria vascularis are melanocytes derived from the neural crest. These internalized pigment cells have been thought to be a static population, distinct from those found in the skin. However, this investigation demonstrates that the melanocytes of the adult stria vascularis undergo continuous replication. Cell proliferation was studied using [3H]-thymidine autoradiography and bromodeoxyuridine (BrdU) immunohistochemistry. Single or multiple injections of [3H]-thymidine within a six hour period labeled a mean of 9 intermediate cells. In pigmented guinea pigs, single daily injections of [3H]-thymidine for 2 or 4 days labeled a mean of 24 and 69 intermediate cells, respectively; Pigmented guinea pigs given BrdU once daily for 2 or 4 days labeled a mean of 38 and 75 intermediate cells. By contrast, albino littermates also given BrdU averaged only 23 and 42 labeled intermediate cells in the same 2 and 4 day experiments. The mean number of proliferating cells/mm of stria per 24 h was 1.54 in the pigmented animals and 0.88 in the albinos. Both the total number and density of labeled intermediate cells were significantly smaller in the albino than the pigmented guinea pigs. These results demonstrate that the melanocytes in the stria vascularis undergo continuous baseline mitosis, and at a rate comparable to the melanocytes of the skin. This surprising similarity promotes the speculation that the proliferative rate of the strial melanocytes may be influenced by some of the same factors known to affect replication and pigment production in the skin.

Auditory brainstem responses in cats with Chediak-Higashi syndrome

Auditory brainstem responses ABRs were recorded in cats with Chediak-Higashi syndrome using monaural stimulation. The components appearing between 1 and 3 ms after stimulus onset were greatly attenuated in the ABRs recorded using a reference contralateral to the stimulated ear. These data suggest that abnormalities exist in the brainstem auditory pathway in the region of the superior olivary complex in cats with Chediak-Higashi syndrome.

Strial dysfunction in a melanocyte deficient mutant rat (Ws/Ws rat)

A homozygous mutant rat at the white spotting (Ws) locus showing a deficiency of melanocytes has recently been found (4, 5). The function and morphology of the inner ear of the Ws/Ws rat were examined by auditory brainstem response (ABR), endocochlear DC potential (EP), and electron microscopy. The mean ABR threshold of the Ws/Ws rat was significantly higher than that of the control +/+ rat. Most Ws/Ws rats showed no or very little EP. In electron microscopy, the stria vascularis of the Ws/Ws rat proved to be very thin and flat with poor interdigitation of marginal cells, and absence of intermediate cells. The organ of Corti appeared to be intact in both the +/+ rat and the Ws/Ws rat. These electrophysiological and morphological findings suggest that the Ws/Ws rat suffered from severe hearing loss caused by strial dysfunction.

Albino and pigmented gerbil auditory function: influence of genotype and gentamicin

Auditory function of albino and pigmented gerbils was examined before and after treatment with the ototoxic aminoglycoside antibiotic gentamicin. Auditory brainstem responses (ABRs) and cochlear nerve compound action potentials (CAPs) were measured in response to pure tones having frequencies between 2 and 32 kHz. Age-matched albinos had significantly lower CAP, but not ABR, thresholds than pigmented gerbils. Gentamicin treatment elevated CAP and ABR thresholds in both genotypes, but pigmented gerbils were less severely affected. Compared to the ABR, the CAP is a more sensitive measure of ototoxicity and pigmentation differences. CAP tuning curves (TCs) were another sensitive measure of genotypic differences in susceptibility to ototoxicity. TC tip thresholds from pigmented animals given gentamicin were not as elevated as the TC tip thresholds of albinos.

Turn-specific differences in the endocochlear potential between albino and pigmented guinea pigs

Recent findings indicate that structural differences exist in the stria vascularis (SV) between albino and pigmented guinea pigs. In the higher cochlear turns, volume density for marginal cells in the albino SV is abnormally large, while that for intermediate cells (melanocytes) is abnormally small. These anatomical variations suggest that functional differences between albino and pigmented inner ears also may be found. To examine this possibility, four strains of guinea pigs were studied, consisting of Hartley albino (N = 9) and NIH outbred pigmented (N = 15) guinea pigs, as well as albino (N = 11) and pigmented (N = 15) guinea pig siblings born to mixed litters. Tracheotomy and carotid artery cannulation were performed. Animals were mechanically ventilated, with periodic samples drawn for arterial blood gas analysis. Blood pressure, heart rate and rectal temperature were monitored. Compound action potentials were measured first to assess cochlear viability. Positive endocochlear potentials (+EP) then were recorded, beginning with the fourth turn, followed by the first, second and third turns. Results showed that the +EP in albinos remained relatively constant across cochlear turns, but decreased significantly from base to apex in the pigmented inner ears. Across all animals, mean +EPs (mV +/- S.E.M.) for turns 1-4 in albinos were: 72.5 (2.5), 68.7 (2.3), 59.2 (2.7), 68.1 (3.3); pigmented values were: 72.9 (2.9), 66.9 (2.6), 53.8 (3.0), 57.0 (2.7). One-way ANOVAs did not show a significant difference in albino +EPs between any of the cochlear turns, but did indicate a highly significant difference between turns in the pigmented inner ears (P < 0.000004). Post hoc comparisons demonstrated +EPs in turns 3 and 4 were smaller than in turn 1. Since turn 3 was recorded last in these experiments, and was reduced in value relative to turn 4 in both groups, it is likely that cochlear deterioration contributed to this result more than any other factor. These results, combined with previous anatomical data, indicate that a diminution of melanocyte cell volume in the albino SV is accompanied by an increase in marginal cell volume density and larger +EPs in the higher cochlear turns, at least at resting levels.

Electron microscopical observations of melanin in the endolymphatic sac

Endolymphatic sacs of pigmented guinea pigs were examined by light and electron microscopy. Melanocytes were located not only in the subepithelial connective tissue but also in the intercellular spaces between the epithelial cells. They projected their dendrites into the epithelium extensively. Melanosomes in several stages of maturation and well-developed Golgi apparatus were seen in the cytoplasm of the melanocytes. Melanosomes were also observed in the membrane-bound vacuoles of the epithelial cells. These findings suggest that melanocytes in the endolymphatic sac are highly capable of producing tyrosinase, synthesizing melanin and transferring melanosomes to the epithelial cells, while melanocytes in the other parts of the inner ear do not show signs of melanogenesis under normal conditions.

Cochlear degeneration in aged rats of four strains

Animals with various degrees of inbreeding, some of which are albino, are frequently used in biological research. Albinos do not produce melanin and it is therefore absent from the cochlea. While the function of melanin is unknown, it has been hypothesized that it is involved in cochlear homeostasis. It is possible then, that age-related degeneration may be affected by the presence or absence of melanin. We therefore evaluated young (2-6 months old) and aged (24-36 months old) cochleas in 4 different rat strains: albino Fischer 344 and Lewis rats and pigmented Lewis-Brown Norway F1 rats and Brown Norway rats. Cochlear morphology was the same across all strains of young adult animals with the exception that the pigmented animals had small, darkly stained granules in the stria vascularis. The aged pigmented animals all had large granules as well as small ones. Degeneration of spiral ganglion cells in the apical region of the ganglion had occurred in the old animals of all strains. Strial degeneration at the apex was also present in aged animals. There was no correlation between the presence or absence of melanin and the magnitude of cochlear degenerative changes in the aged animals. The presence or absence of melanin therefore, appears to have no effect on cochlear degeneration in the aged rat cochlea.

Hyperpigmentation of chinchilla stria vascularis following acoustic trauma

This report describes morphological alterations of the chinchilla stria vascularis seen 30 days after exposure to impulse noise. The observed changes included a dramatic increase in strial melanin content which occurred in 7 of 36 animals exposed to electronically synthesized impluses presented in various temporal patterns at either 135 or 150 dB peak SPL. In these animals, densely pigmented areas of stria 1.5 to 3 mm in length were found in the basal cochlear turn. Light and electron microscopic study revealed that these areas contained large numbers of melanin granules situated primarily in pale-staining cells of the middle layer of the stria. Unlike the pigment granules present in normal chinchilla stria, the melanosomes found in the noise-exposed material clearly showed ultrastructural features characteristic of eumelanin. Melanin granules were also observed in marginal and basal cells of the noise-exposed stria. In some cases, pigment granules which had apparently been expelled from the marginal cells were present in the endolymphatic space beneath Reissner's membrane and on the strial surface. These findings support the view that the melanin-bearing cells of the inner ear are capable of markedly increased activity in response to stressful conditions.

The development of melanin in the stria vascularis of the gerbil

One factor that influences noise susceptibility is pigmentation. The aim of this study was to investigate the development of melanocytes, other melanin-containing cells and the amount of melanin in stria vascularis from birth to adult age in the gerbil which has a uniform pigmentation of the fur and eyes, is born without hearing but establishes hearing function at 14-18 days after birth. Changes in the melanin morphology, concentration and distribution have been correlated to the development of the inner ear and to the time period during which hearing function is established, which indicates that the melanocytes in stria vascularis are of importance for the hearing function.

Turn-specific and pigment-dependent differences in the stria vascularis of normal and gentamicin-treated albino and pigmented guinea pigs

The aims of the present study were to determine which structures in the stria vascularis (SV) may depend upon the presence of pigmented melanocytes both for normal morphology and for the expression of gentamicin ototoxicity in the inner ear. These pigment-dependent influences were inferred through comparisons of the SV in pigmented guinea pigs and in albinos containing nonpigmented melanocytes. Results were obtained from 6 albino and 8 pigmented guinea pigs given gentamicin, and from 3 albino and 3 pigmented control animals not receiving the drug. One-month old animals received gentamicin daily (100 mg/kg) for 14 days and recovered for an additional 14 days before being prepared for electron microscopy. The SV from each of the 4 cochlear turns was analyzed using stereological point counting procedures. In control animals, differences were found in the higher cochlear turns, where volume density for the marginal cells in albinos was abnormally large (turns 3 and 4), while the volume density for intermediate cells (melanocytes) was abnormally small (turn 3). Cell volume estimates for the intermediate cells were significantly smaller in the albino than pigmented control animals in the higher cochlear turns, indicating that functional abnormalities may be found in the albino cochlea. In animals exposed to gentamicin, marginal cell volume density was reduced significantly in turn 4 of albinos, but not in any region of the pigmented inner ears. Radial area of SV and estimates of the absolute volumes for marginal cells in albinos given gentamicin also were significantly reduced in turn 1 compared to their controls; such differences were not observed in the pigmented animals. The results indicate that marginal cell size is significantly reduced in albino but not pigmented animals 14 days after gentamicin exposure, and further suggest a role of pigmented melanocytes in ameliorating gentamicin-induced cochlear damage.

Identification of two types of melanocyte within the stria vascularis of the mouse inner ear

We have distinguished two types of melanocyte within the intermediate layer of the stria vascularis in the cochlea of normally pigmented mice: light and dark intermediate cells. The light intermediate cells are present in the stria from birth and have the typical appearance of a melanocyte. They are large and dendritic with electron-lucent cytoplasm containing numerous vesicles that show tyrosinase activity, and pigment granules in various stages of development. These granules have the ultrastructural and histochemical characteristics of premelanosomes and melanosomes. The light intermediate cells persist throughout life, but less frequently contain pigment in older animals. The dark intermediate cells, present only in adult mice, vary considerably in number and distribution between animals. Pigment granules, bound within an electron-dense acid phosphatase-rich matrix, form the main component of the dark intermediate cells. The intermediate cells may comprise either two distinct cell populations or different developmental stages of the same cell type; ultrastructural observations suggest the latter. In young mice, light intermediate cells contain the electron-dense matrices, which at later stages of development are found almost exclusively in dark cells. The dark intermediate cells contain few cell organelles other than pigment granules accumulated within lysosomal bodies and they often have pycnotic nuclei. These observations suggest that the dark intermediate cells are a degenerate form of the light intermediate cells. Clusters of melanosomes also occur in the basal cells, and to a much lesser extent in the marginal cells. These cells do not stain after incubation in DOPA, suggesting that they are not capable of melanin synthesis, and therefore probably acquire melanin by donation from adjacent melanocytes. Pigment clusters are also found within the spiral ligament at all stages of development.

Normal somatotopy in SI of a tyrosinase-negative albino cat

Because of known abnormalities in both the visual and auditory pathways of tyrosinase-negative albino cats, we mapped the primary somatosensory cortex (SI) in one such cat electrophysiologically. We detected absolutely no sign of abnormality in terms of somatotopy, and conclude that if anomalies do exist in the albino somatosensory system, they are either very subtle or lie outside SI.

Endocochlear potential generation is associated with intercellular communication in the stria vascularis: structural analysis in the viable dominant spotting mouse mutant

Deafness in the viable dominant spotting mouse mutant is due to a primary defect of the stria vascularis which results in absence of the positive endocochlear potential in scala media. Endocochlear potentials were measured and the structure of stria vascularis of mutants with potentials close to zero was compared with that in normal littermate controls by use of morphometric methods. The stria vascularis was significantly thinner in mutants. Marginal cells were not significantly different from controls in terms of volume density or intramembrane particle density but the network density of tight junctions was significantly reduced in the mutants. A virtual absence of gap junctions between basal cells and marginal or intermediate cells was observed, but intramembrane particle density and junctional complexes between adjacent basal cells were not different from controls. The volume density of basal cells was significantly greater in mutants. Intermediate cells accounted for a significantly smaller volume density of the stria vascularis in mutants and had a lower density of intramembrane particles than controls. Melanocytes were not identified in the stria vascularis of mutants. These results suggest that communication between marginal, intermediate and basal cells might be important to the normal function of the stria vascularis.

Normal and abnormal visual field maps in albinos. Central effects of non-matching maps

The abnormal chiasmatic crossing characteristic of all albino mammals brings two discordant representations of the visual field to the central visual relays. The representation from the nasal retina is normal whereas the one from the temporal retina is disrupted, a part representing the contralateral visual field as is normal and a part coming from the ipsilateral visual field as a mirror image of a part of the normal representation. Experiments that were designed to define the rules on the basis of which the abnormal representations can be established in the lateral geniculate nucleus and visual cortex are described. These experiments are related to more recent studies of albino cats, and the observations of the visual pathways are related speculatively to abnormalities seen in the auditory pathways of albinos. The possibility is raised that the auditory abnormalities are secondary to the visual abnormalities, produced by a failure of the two systems to establish normally matching maps of sensory space.

Differential susceptibility to gentamicin ototoxicity between albino and pigmented guinea pigs

The known chemical affinity of melanin pigment for aminoglycoside antibiotics has led to the suggestion that higher concentrations of these drugs will bind to the pigmented inner ear and produce greater ototoxicity compared to the nonpigmented albino cochlea. Although this has provided a compelling hypothesis, results from the few investigations to address this question have been equivocal. In the present study, cochlear microphonic (CM) thresholds were recorded from albino and pigmented guinea pigs both before and two weeks after exposure for 14 consecutive days to 100 mg/Kg gentamicin. Cochleae were dissected and half-turn segments prepared for surface examination of the organ of Corti. After gentamicin exposure, threshold shifts averaged a statistically reliable 33 dB in albinos and 19 dB for the pigmented animals. Anatomical studies revealed a significant 44% mean outer hair cell loss in albinos compared to a 21% loss in the pigmented inner ears. The results showed that albinos display greater ototoxicity from gentamicin than do pigmented guinea pigs. Aminoglycosides are known to exert toxicity through interaction with polyphosphoinositides found in high concentrations in the inner ear. Cochleae in both albino and pigmented animals appear to possess significant phospholipid concentrations and bind toxic levels of these drugs independent of inner ear pigment content. However, evidence showing that melanin can inhibit aminoglycoside activity in vitro suggests that, once these drugs bind to pigmented tissue, they may undergo inactivation in a manner unavailable to the nonpigmented albino cochlea. The present results are consistent with the possibility that cochlear melanin may inhibit gentamicin activity in vivo and decrease the severity of aminoglycoside ototoxicity in the pigmented inner ear.