An electron microscopic study of the function of the root cells in the external spiral sulcus of the cochlea

Histology of the Cochlear Outer Sulcus Cells in Normal Human Ears, Presbycusis, and Menière's Disease

HYPOTHESIS

Outer sulcus cell features and distribution are hypothesized to differ throughout regions of the human cochlea and between diseased and normal specimens.

BACKGROUND

Outer sulcus cells play a role in inner ear fluid homeostasis. However, their anatomy and distribution in the human are not well described.

METHODS

Temporal bone specimens with normal hearing (n = 10), Menière's disease (n = 10), presbycusis with flat audiograms (n = 4), and presbycusis with sloping audiograms (n = 5) were examined by light microscopy. Outer sulcus cells were assessed quantitatively and qualitatively in each cochlear turn. One specimen was stained for tubulin immunofluorescence and imaged using confocal microscopy.

RESULTS

Outer sulcus cells interface with endolymph throughout the cochlea, with greatest contact in the apical turn. Mean outer sulcus cell counts in the upper apical turn (8.82) were generally smaller (all p < 0.05) than those of the upper basal (17.71), lower middle (18.99) upper middle (18.23), and lower apical (16.42) turns. Mean outer sulcus cell counts were higher (p < 0.05) in normal controls (20.1) than in diseased specimens (15.29). There was a significant correlation between mean cell counts and tonotopically expected hearing thresholds in the upper basal (r = -0.662, p = 0.0001), lower middle (r = -0.565, p = 0.0017), and upper middle (r = -0.507, p = 0.0136) regions. Other differences in cell morphology, distribution, or relationship with Claudius cells were not appreciated between normal and diseased specimens. Menière's specimens had no apparent unique features in the cochlear apex. Immunofluorescence staining demonstrated outer sulcus cells extending into the spiral ligament in bundles forming tapering processes which differed between the cochlear turns in morphology.

CONCLUSION

Outer sulcus cells vary throughout the cochlear turns and correlate with hearing status, but not in a manner specific to the underlying diagnoses of Menière's disease or presbycusis.

Age-dependent alterations of Kir4.1 expression in neural crest-derived cells of the mouse and human cochlea

Age-related hearing loss (or presbyacusis) is a progressive pathophysiological process. This study addressed the hypothesis that degeneration/dysfunction of multiple nonsensory cell types contributes to presbyacusis by evaluating tissues obtained from young and aged CBA/CaJ mouse ears and human temporal bones. Ultrastructural examination and transcriptomic analysis of mouse cochleas revealed age-dependent pathophysiological alterations in 3 types of neural crest-derived cells, namely intermediate cells in the stria vascularis, outer sulcus cells in the cochlear lateral wall, and satellite cells in the spiral ganglion. A significant decline in immunoreactivity for Kir4.1, an inwardly rectifying potassium channel, was seen in strial intermediate cells and outer sulcus cells in the ears of older mice. Age-dependent alterations in Kir4.1 immunostaining also were observed in satellite cells ensheathing spiral ganglion neurons. Expression alterations of Kir4.1 were observed in these same cell populations in the aged human cochlea. These results suggest that degeneration/dysfunction of neural crest-derived cells maybe an important contributing factor to both metabolic and neural forms of presbyacusis.

The Membrane Properties of Cochlear Root Cells are Consistent with Roles in Potassium Recirculation and Spatial Buffering

Auditory transduction, amplification, and hair cell survival depend on the regulation of extracellular [K(+)] in the cochlea. K(+) is removed from the vicinity of sensory hair cells by epithelial cells, and may be distributed through the epithelial cell syncytium, reminiscent of "spatial buffering" in glia. Hypothetically, K(+) is then transferred from the epithelial syncytium into the connective tissue syncytium within the cochlear lateral wall, enabling recirculation of K(+) back into endolymph. This may involve secretion of K(+) from epithelial root cells, and its re-uptake via transporters into spiral ligament fibrocytes. The molecular basis of this secretion is not known. Using a combination of approaches we demonstrated that the resting conductance in guinea pig root cells was dominated by K(+) channels, most likely composed of the Kir4.1 subunit. Dye injections revealed extensive intercellular gap junctional coupling, and delineated the root cell processes that penetrated the spiral ligament. Following uncoupling using 1-octanol, individual cells had Ba(2+)-sensitive weakly rectifying currents. In the basal (high-frequency encoding) cochlear region K(+) loads are predicted to be the highest, and root cells in this region had the largest surface area and the highest current density, consistent with their role in K(+) secretion. Kir4.1 was localized within root cells by immunofluorescence, and specifically to root cell process membranes by immunogold labeling. These results support a role for root cells in cochlear K(+) regulation, and suggest that channels composed of Kir4.1 subunits may mediate K(+) secretion from the epithelial gap junction network.

Localization and functional studies of pendrin in the mouse inner ear provide insight about the etiology of deafness in pendred syndrome

Immunolocalization studies of mouse cochlea and vestibular end-organ were performed to study the expression pattern of pendrin, the protein encoded by the Pendred syndrome gene (PDS), in the inner ear. The protein was restricted to the areas composed of specialized epithelial cells thought to play a key role in regulating the composition and resorption of endolymph. In the cochlea, pendrin was abundant in the apical membrane of cells in the spiral prominence and outer sulcus cells (along with their root processes). In the vestibular end-organ, pendrin was found in the transitional cells of the cristae ampullaris, utriculi, and sacculi as well as in the apical membrane of cells in the endolymphatic sac. Pds-knockout (Pds-/-) mice were found to lack pendrin immunoreactivity in all of these locations. Histological studies revealed that the stria vascularis in Pds-/- mice was only two-thirds the thickness seen in wild-type mice, with the strial marginal cells showing irregular shapes and sizes. Functional studies were also performed to examine the role of pendrin in endolymph homeostasis. Using double-barreled electrodes placed in both the cochlea and the utricle, the endocochlear potential and endolymph potassium concentration were measured in wild-type and Pds-/- mice. Consistent with the altered strial morphology, the endocochlear potential in Pds-/- mice was near zero and did not change during anoxia. On the other hand, the endolymphatic potassium concentration in Pds-/- mice was near normal in the cochlea and utricle. Together, these results suggest that pendrin serves a key role in the functioning of the basal and/or intermediate cells of the stria vascularis to maintain the endocochlear potential, but not in the potassium secretory function of the marginal cells.

Constitutive expression of Hsp27 in the rat cochlea

Heat shock protein-27 (Hsp27) is known to function as both a stress-inducible molecular chaperone and regulator of actin polymerization. For many cells in the cochlea, actin is part of the cytoskeleton and plays an important role in the maintenance of cochlear function. To understand the molecular processes by which the cochlear actin cytoskeleton is maintained and regulated during normal auditory function, we examined the expression and localization of Hsp27 in the normal rat cochlea. Reverse transcription-polymerase chain reaction and Western blot showed constitutive expression of Hsp27 in the normal rat cochlea. Immunofluorescence microscopy showed Hsp27-like staining is localized to the cuticular plate and lateral wall of outer hair cells. Hsp27-like immunostaining is also found in tension fibroblasts, in the root cells of the spiral limbus and in Reissner's membrane. The presence of Hsp27 in the actin-rich tension fibroblasts and outer hair cells suggests a potential role in the regulation and maintenance of the actin cytoskeleton in these cells. The presence of high levels of constitutive Hsp27 may also provide a mechanism for pre-protecting these cells against environmental stressors.

Proteoglycan arrays in the cochlear basement membrane

Indirect immunofluorescence and transmission electron microscopy were used to investigate the composition and assembly of proteoglycans in the basement membranes of the spiral limbus, basilar membrane, spiral ligament, Reissner's membrane, myelinated nerve fibers, and blood capillaries of the spiral ligament and stria vascularis in the chinchilla cochlea. Four types of basement membrane components: laminin, entactin/nidogen, type IV collagen and heparan sulfate proteoglycans were immunolocalized in all basement membranes in association with heparan sulfate proteoglycans. beta 1 and alpha 1 integrin subunits were also detected along these basement membranes. The concentration of the basement membrane-associated proteins and integrin subunits differed according to the adjacent cell type. Electron microscopy showed that all basement membranes, with exception of those of stria vascularis, consist of two layers: lamina lucida and lamina densa. In the stria vascularis only a homogeneous lamina densa was observed. Cuprolinic blue treatment revealed heterogeneity in the ultrastructure and arrangement of proteoglycans in the cochlear basement membranes. Proteoglycans of the subepithelial basement membrane in the spiral limbus and spiral ligament formed quasi-regular, linear arrays within the lamina lucida, or were located at both sides of the lamina densa in the basilar membrane and Reissner's membrane. In the basement membranes of nerve fibers, and capillaries in the spiral ligament and stria vascularis, proteoglycans were scattered throughout these basement membranes, but showed different concentration and ultrastructural appearance, which may be related to different filtration and mechanical properties. In the basilar membrane, PGs were located above and below the lamina densa. An additional layer of PGs below the lamina densa may function as increased mechanical support of organ of Corti by its interaction with underlying fibrillar collagen layer. In the stria vascularis capillaries, PGs were stained considerably less with Cuprolinic blue and were scattered through the lamina densa of the basement membrane compared to capillaries of spiral ligament. This observation is compatible with a higher permeability of the strial capillaries.

Expression of the major basement membrane-associated proteins during postnatal development in the murine cochlea

The major basement membrane-associated proteins, including laminin-1, fibronectin, heparan sulfate proteoglycan (HSP), and entactin, were examined by immunofluorescence microscopy during postnatal development of the mouse cochlea. Samples were collected every 2 days through 8 days, and again at 14 days after birth. In the neonate, staining for HSP entactin and laminin-1 was barely visible; however, antibodies against fibronectin displayed intense immunoreactivity in nearly every cochlear tissue. Fibronectin is progressively inactivated in all tissues except the basilar membrane where it persists at high levels to adulthood. Laminin-1, entactin, and HSP illustrate remarkable temporal and spatial coordinate regulation. Elevated expression of these proteins is observed at 2 postnatal days (PND), and persists in the membranes surrounding the spiral ganglion cell bodies. Transient expression of laminin-1 and entactin, an to a lesser extent HSP, is observed from PND4 to PND8 in a track of membrane running from the interdental cells of the spiral limbus down the inner sulcus, across the basilar membrane, up the external sulcus to the spiral prominence, and branching into the spiral ligament ensheathing the root cell processes. By PND14 the abundance of these proteins is greatly reduced along this track. The abundance and dynamic regulation of these major basement membrane-associated proteins suggests that they play an important role in postnatal cochlear development.

Expression of basement membrane type IV collagen chains during postnatal development in the murine cochlea

An immunofluorescence study was performed to examine the temporal and spatial patterns of expression for the different type IV collagen chains during postnatal cochlear development. At birth, the classical chains (4A1 and 4A2) were widely expressed, while the novel chains (4A3, 4A4, and 4A5) were completely absent. Activation of the novel chains was observed at 4 days of age, with intense, widely distributed immunostaining suggesting that most of the cells in the cochlea express the novel chains at this developmental stage. From day 8 through day 14, developmental inactivation of the novel chains results in a reduction of generalized immunoreactivity with a concomitant elevation of specific staining in the membranous structures bounding the interdental cells of the spiral limbus, the inner sulcus, the basilar membrane, and in a fibrous bed of staining radiating from the spiral prominence into the region of the spiral ligament which corresponds to the location of the root cell processes. This pattern of intense immunostaining for the novel chains persists through adulthood. The classical chains are expressed in these same anatomical regions only transiently (from day 6 to day 10), after which a gradual developmental inactivation leads to the adult expression pattern where classical collagen chains are found primarily in the perineurium, in the membranes surrounding the spiral ganglion cell bodies, and in the vascular basement membranes of the spiral ligament and the stria vascularis. The complex developmental pattern of expression for the type IV collagen chains in the murine cochlea is similar to that observed in the murine kidney, which is the other major site for basement membrane pathology in Alport syndrome.

Atrial natriuretic peptide-like immunoreactive cells in the guinea pig inner ear

Using specific antibodies against cardiodilatin/atrial natriuretic peptide (CDD/ANP) in a conventional immuno-histochemical method (PAP) we located ANP/CDD-like immuno-reactive cells related to the secretory area, to the sensory and to the neuronal area in the compartments of the inner ear (cochlea, utricle/ampulla, and endolymphatic sac). Immunoreactive cells were unevenly distributed in the different compartments as well as within the cochlear space. Our findings suggest that ANP/CDD may play a role in the local control of fluid and electrolyte homeostasis of the inner ear. ANP/CDD-binding sites and ANP/CDD-like immunoreactivity in the inner ear may also indicate that the peptide has an additional paracrine and/or autocrine function in the organ.