Glycoconjugates in the human fetal endolymphatic sac as detected by lectins

Innate immune defense in the inner ear - mucines are expressed by the human endolymphatic sac

The human endolymphatic sac has been shown recently to have immunological capacities and has thus been proposed as the main entity protecting the inner ear from pathogen invasion, equivalent to mucosa-associated lymphoid tissue (MALT). Although the sac expresses molecules of the innate immune system, the potential expression of members of the important mucin family has not been detailed. Thus, this paper explores endolymphatic sac expression of a number of mucins and mucin precursors. Twelve fresh tissue samples from the human endolymphatic sac were obtained during translabyrinthine surgery. The expression of Mucin 1, 2, 5B/AC and 16, as well as the core structure elements (mucin precursors) T-antigen, Tn-antigen and Sialyl-Tn-antigen was investigated by immunohistochemistry. The endolymphatic sac epithelium expressed MUC1 (both apically towards the endolymphatic sac (ES) lumen and basally towards the capillary network), MUC 16 and Tn-antigen. There was no labeling after incubation with antibodies against T-antigen, sialyl-Tn-antigen, MUC2 and MUC5B/AC. We conclude that the human endolymphatic sac epithelium expresses a number of mucin molecules, which supports the hypothesis of the sac as the primary immunological tissue structure of the inner ear, equivalent to MALT in other organs. The mucins may also play a role in the formation and continuous homeostasis of the inner ear fluids, as well as the pathogenesis of Meniere's disease.

Localization of megalin in rat vestibular dark cells and endolymphatic sac epithelial cells

CONCLUSION

Megalin immunoreactivity was observed in kidney proximal tubule cells, vestibular dark cells, and epithelial cells of the endolymphatic sac. Endocytic mechanisms appear to differ between the endolymphatic sac and proximal tubule cells. We speculate that megalin is secreted by a certain type of cell into the endolymphatic space, and is then absorbed from the endolymphatic space by another type of cell to maintain endolymphatic sac homeostasis.

OBJECTIVES

We previously detected megalin immunoreactivity in the rat cochlear duct. Megalin may be involved in endocytosis in the vestibular organ and endolymphatic sac. To examine this possibility, we extended our immunocytochemical investigation to the rat inner ear cells with special attention to vestibular dark cells and endolymphatic sac.

MATERIALS AND METHODS

We observed immunoreactivity of megalin under light and electron microscopy. The primary antibody was rabbit polyclonal antibody that had been raised against rat immunoaffinity-purified megalin.

RESULTS

The luminal membrane and subapical area of dark cells in the semicircular canal were immunolabeled. The stainable substance in the endolymphatic space was strongly stained. The cytoplasm of epithelial cells was also stained in various patterns.

Time-dependent alterations of 3-fucosyl-N-acetyl-lactosamine (CD15) expression in the endolymphatic sac of adult guinea pigs after glycerol administration

The present study examined the effect of glycerol administration on 3-fucosyl-N-acetyl-lactosamine (CD15) epitope expression in the endolymphatic sac (ES) of the guinea pig's inner ear. Adult guinea pigs were injected intravenously with glycerol (2 g/kg body wt.). CD15 expression was studied at 80 min up to 5 h after treatment. In untreated animals single cells and cell groups in the ES expressing CD15 epitope intra- and intercellularly were identified by immunohistochemistry to be mainly in the epithelial layer of the rugosal and distal part of the sac. Glycerol administration modulated the expression of CD15 epitope. In the epithelial layer, expression decreased and was nearly depleted after 3 h. After 4 h of glycerol administration, CD15 expression reappeared and reached the comparable level of controls. The numbers of CD15-positive cells in the lumen of the ES increased steadily and arrived at their the highest levels in 2-h specimens. The localization of CD15-epitope expression and its modulation after glycerol administration within the ES implies that this molecule may play a role in re-establishing the sac's normal function. In addition, we speculate that CD15 may be associated with processes of an immune response in the inner ear.

Constituents of the endolymphatic tubules as demonstrated by three-dimensional morphometry

The various constituents of the tubules of the human adult intraosseous endolymphatic sac were stained with lectins and Millon reaction to reveal some of the carbohydrate and protein-containing substances in the endolymph. The amount of each substance was determined by digitizing sequential temporal bone sections. Differing amounts of the various identified substances were found in sacs from patients with Meniere's disease, patients with labyrinthine fibrosis, and controls.

Identification of substances in the endolymphatic sac

The composition of the contents of the endolymphatic sac (ES) has yet to be fully defined. Carbohydrates have been found in the ES of human fetuses and animals but not identified in the adult human ES. In the present study, celloidin was removed from previously prepared human temporal bone sections and a histochemical method was used to detect carbohydrates and protein in the ES. Six biotinylated lectins were used to identify specific carbohydrates in 15 ears: beta-D-N-acetylglucosamine, beta-D-galactose, D-galactose, alpha-D-mannose, D-N-acetylgalactosamine and alpha-L-fucose. The intensity of staining was graded qualitatively. A substance in the ES tubules that did not stain with any lectin was identified by the Millon reaction as containing protein. The carbohydrates and protein may exist in the different tubules or in the same tubule without mixing. This finding seems to support the idea that at least some of these substances are produced locally in the ES. Our observations support the hypothesis of the existence of a secretion and degradation system in the endolymphatic sac.