Pilocarpine elicits the interdental cell secretory activity of the inner ear

Cochlear spiral ganglion neuron degeneration following cyclodextrin-induced hearing loss

Because cyclodextrins are capable of removing cholesterol from cell membranes, there is growing interest in using these compounds to treat diseases linked to aberrant cholesterol metabolism. One compound, 2-hydroxypropyl-beta-cyclodextrin (HPβCD), is currently being evaluated as a treatment for Niemann-Pick Type C1 disease, a rare, fatal neurodegenerative disease caused by the buildup of lipids in endosomes and lysosomes. HPβCD can reduce some debilitating symptoms and extend life span, but the therapeutic doses used to treat the disease cause hearing loss. Initial studies in rodents suggested that HPβCD selectively damaged only cochlear outer hair cells during the first week post-treatment. However, our recent in vivo and in vitro studies suggested that the damage could become progressively worse and more extensive over time. To test this hypothesis, we treated rats subcutaneously with 1, 2, 3 or 4 g/kg of HPβCD and waited for 8-weeks to assess the long-term histological consequences. Our new results indicate that the two highest doses of HPβCD caused extensive damage not only to OHC, but also to inner hair cells, pillar cells and other support cells resulting in the collapse and flattening of the sensory epithelium. The 4 g/kg dose destroyed all the outer hair cells and three-fourths of the inner hair cells over the basal two-thirds of the cochlea and more than 85% of the nerve fibers in the habenula perforata and more than 80% of spiral ganglion neurons in the middle of basal turn of the cochlea. The mechanisms that lead to the delayed degeneration of inner hair cells, pillar cells, nerve fibers and spiral ganglion neurons remain poorly understood, but may be related to the loss of trophic support caused by the degeneration of sensory and/or support cells in the organ of Corti. Despite the massive damage to the cochlear sensory epithelium, the blood vessels in the stria vascularis and the vestibular hair cells in the utricle and saccule remained normal.

Distribution of glycoconjugates during cochlea development in mice: light microscopic lectin study

During development, different epithelial cells in the mouse cochlea express different cell surface glycoconjugates, which may reflect membrane specialization. Some of the lectins tested in this study (SBA, succ-WGA, and PSA) labeled the sensory cells of the cochlea around birth. Other lectins (WGA, Con A, RCA-II, and PHA-E) labeled surfaces of the sensory cells, particularly the stereocilia, from early stages of development (gestation day (GD) 16) through 21 days after birth. These may be adhesion molecules needed to attach the newly forming tectorial membrane (TM) to the stereocilia. Lectin staining of the developing TM revealed that the substructures of the TM are biochemically distinct. Lectin staining also showed the temporal sequence of the expression of cytoplasmic glycoconjugates of the cochlear epithelium during development. Biochemical changes during development are probably the result of different cells being involved in the production of glycoconjugates, and may have functional significance, specifically with regard to the expression of adhesion and/or signaling molecules.

Structural evidence for ion transport and tectorial membrane maintenance in the gerbil limbus

Cells medial to the tunnel of Corti were examined to assess fine structural features relevant to their proposed role in cochlear K(+) homeostasis. A dense network of canaliculi referred to as canalicular reticulum (CR) resided in the foot body of inner pillar cells, where it bordered and could resorb ions released from inner radial and spiral nerves. Lateral interdental cells (IDCs) formed columns which connected the inner sulcus epithelium with the base of the tectorial membrane's (TM) middle zone. A spout-like neck in cells at the top of lateral IDC columns housed a dense concentration of CR which resembled that characteristic of ion transporting epithelia and appeared to be located here for transporting ions and fluid toward the TM. Clustered IDCs in the center of the limbus connected underlying limbal stroma with the TM's limbal zone and appeared capable of transporting ions from stroma to TM. Abundant CR in limbal stellate fibrocytes evidenced their capacity to transport ions and fluid, presumably from inner sulcus epithelium toward central IDCs. The most medial IDCs possibly function as the terminus of an ion cycling path from scala vestibuli to endolymph. Light fibrocytes situated between supralimbal fibrocytes and medial IDCs appeared to serve as a link in this pathway. The limbal zone of the TM overlying central IDCs consisted of three distinct regions which offered a structural basis for transformation of an amorphous matrix supplied by central IDCs into the protofibrils of the membrane's middle zone.

Ablation of inner hair cells by carboplatin alters cells in the medial K(+) flow route and disrupts tectorial membrane

The thesis that K(+) effluxing from inner hair cells (IHCs) cycles medially back to endolymph through inner sulcus and interdental cells (IDCs) was tested by comparing control chinchilla cochleas with those in which IHCs were selectively destroyed by carboplatin. By light microscopy inner sulcus cells appeared tall and nearly empty in control ears, but 4 months after the carboplatin treatment many showed vacuolization and shrinkage. Inner pillar cells also consistently developed abnormal vacuoles after carboplatin treatment. Control cochleas exhibited lateral columns and central clusters of IDCs which at their apex possessed expanded presumably hydrated phalanges. Four months after carboplatin, the IDC epithelium enclosed empty looking spaces and the apical phalangeal compartment collapsed into a thin, apparently dehydrated layer. This alteration was accompanied by changes in the tectorial membrane (TM) whereby the membrane's limbal zone thickened progressively to form a tall hollow mound in advanced lesions. The clear spaces in the epithelium and collapse of the phalanges are thought to reflect diminished flow of ions and fluid through IDCs. The accumulation of limbal TM supports the premise that IDCs secrete macromolecules for TM turnover as well as ions and fluid for promoting lateral migration of its precursor constituents. Occurring after ablation of IHCs by carboplatin, the changes in inner pillar, inner sulcus and IDCs and limbal TM can be viewed as a secondary effect of the interrupted ion efflux from IHCs and as further evidence that this effluent follows a medial route.

Characterization of otoconin-95, the major protein of murine otoconia, provides insights into the formation of these inner ear biominerals

During the course of a study aimed at identifying inner ear-specific transcripts, a 1,906-bp murine cDNA predicted to encode a secreted 469-aa protein with two domains of homology with the secreted phospholipases A2 was isolated. This transcript is specifically expressed in the inner ear from embryonic day 9.5. The encoded 95-kDa glycoprotein is the major protein of the utricular and saccular otoconia and thus was named otoconin-95. By immunohistofluorescence, otoconin-95 also was detected in the cupulae of the semicircular canals and in previously undescribed transient granular structures of the cochlea. Otoconin-95 was found to be synthesized by various nonsensory cell types, but not by the supporting cells of the sensory epithelia, which produce the otoconial precursor vesicles. In addition, multiple isoforms generated by differential splicing were observed in different combinations during development. Based on the present results, we propose a model for the formation of the otoconia.

Molecular cloning and characterization of an inner ear-specific structural protein

Molecular biological studies of the mammalian inner ear have been limited by the relatively small size of the sensory endorgans contained within. The saccular otolithic organ in teleostian fish is structurally similar to its mammalian counterpart but can contain an order of magnitude more sensory cells. The prospect of the evolutionary conservation of proteins utilized in the vertebrate inner ear and the relative abundance of teleostian saccular sensory tissue made this an attractive system for molecular biological studies. A complementary DNA obtained by differential screening of a saccular complementary DNA library was identified that encodes an inner ear-specific collagen molecule.

Incorporation of D3H glucosamine to the adult and developing cochlear tectorial membrane of normal and hypothyroid rats

The uptake of D-3H-glucosamine by the developing cochlea of normal and hypothyroid rats was examined using light microscopic radioautography. During postnatal development, normal and hypothyroid rat cochleas exhibited a layer of radiolabelling in the tectorial membrane (TM). This layer first appeared in the TM region which covers the spiral limbus and the Kölliker's organ (KO), then progressively reached the apical part of the TM covering the organ of Corti. Radiolabelling was significantly greater in hypothyroid than in normal cochleas. These findings suggests that the enormous size reached by the TM in the congenital hypothyroidism could be related to an increase of epithelial secretion, at least for carbohydrates. It also suggests that TM, in normal and hypothyroid cochleas, could be formed during development by the addition of successive layers. Older layers could be displaced upwards by the new ones. Cochleas of normal young adult rats, treated with D-3H-glucosamine, showed a very scarce and diffuse radiolabelling. Cochleas of hypothyroid young adult rats exhibited a thickened and distorted TM, which incorporated a significant amount of carbohydrates. These results suggest that TM secretion is highly reduced in young adult normal animals, while in young adult hypothyroid ones it is still active. During cochlear maturation, thyroxine seems to be necessary, not only for the synthesis of normal glycoproteins (as suggested by previous reports), but also for the control of glycoprotein secretion.