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The guinea pig provides an excellent model for studying the effects of ototoxic antibiotics on the epithelium of the endolymphatic sac (ELS). Intracochlear injection of neomycin produces consistent destruction of cochlear and vestibular sensory cells, and the response to this traumatic insult can be documented by light and electron microscopy of the ELS. The histologic findings suggest that ELS epithelial cells are relatively resistant to ototoxic levels of neomycin. The study confirms the ability of the ELS to increase its activity during periods of labyrinthine stress.

The effect of amiloride on the murine endolymphatic sac was investigated. The amiloride caused collapse of the lateral intercellular spaces in the endolymphatic sac epithelium and a subsequent mild endolymphatic hydrops. These changes indicated a decreased absorption of endolymph in the endolymphatic sac. Amiloride is known to inhibit the transcellular fluid transport without inducing any changes in the paracellular fluid transport. It is therefore suggested that amiloride specially inhibits the fluid and ion exchange in the apical portion of the epithelial cells resulting a decrease in transcellular fluid transport across the endolymphatic sac epithelium. The transcellular fluid transport seems to be one of the main mechanisms in the endolymphatic sac fluid exchange system.

The endolymphatic sac (ES) is believed to absorb endolymph. Recent studies have suggested that the ES also has a secretory capacity, a function that may be related to the regulation of inner ear fluid volume and pressure. Other studies indicate that hyperosmolar substances, such as glycerol and urea, may initiate a secretion of glycoprotein into the ES. This function was suggested to be related to a regulatory function of the ES by which it can compensate for a decrease in endolymph pressure. In order to investigate this regulatory potential of the ES, the effect of glycerol on the ES was investigated with or without the presence of pharmacological inhibition of glycoprotein secretion through colchicine treatment. Animals treated in such a way showed marked signs of impaired inner ear function, including loss of postural control and loss of Preyer's reflex. Significant ultrastructural changes were noted in the endolymphatic sac suggesting a disturbed secretory activity. The results may indicate that the endolymphatic sac may actively respond to changes in endolymph homeostasis through secretion of macromolecular substances and that alterations in this secretion may lead to functional disturbances of the inner ear.

Ethacrynic acid, but not ouabain, administered into the endolymph entails a decrease in the endolymphatic potential of frogs. Injection of either ouabain or ethacrynic acid into the perilymph causes a decrease in the DC-potential. Intraperitoneal administration of ethacrynic acid (750 mg/kg) and ouabain (0.5 mg/kg) has no effect on the endolymphatic potential of the frog.

Radioactively labeled sulphur was injected into 12 mice of the NMRI strain 20 days after pretreatment with kanamycin. The animals were decapitated after intervals ranging from 2 minutes to 24 hours after injection. After a routine autoradiographic procedure, darkly stained silver grains were detected in the endolymphatic sac (ES) and its surroundings. One hour after injection, the grains were found in the surrounding blood vessels. Eleven hours later, maximal uptake was seen in the epithelial cells of the ES in the kanamycin-treated animals. Twenty-four hours after injection, a faint S35 uptake in the ES could still be detected. An increase in the ES activity, indicated by an increased number of free-floating cells and the secretion of a sulphur-containing intraluminal substance, did not occur. The spreading pattern of sulphur, as shown in this investigation, does not support the theory that waste products from the inner ear, transported to the ES by the longitudinal flow, produces an increased activity in the sac after long pretreatment with kanamycin. In 12 control animals, no difference in the distribution of labeling was observed, but there was an additional uptake in the free intraluminal cells which was not seen in the kanamycin group.

The effects of glycerol and colchicine on the sensory end organs of the inner ear were investigated in mice. Glycerol alone induced a widening of the intercellular spaces lining vestibular dark and transitional cells as well as the marginal cells of the stria vascularis. This was noted within 30 min after the injection of glycerol and was normalized again within 4 h after the injection. Colchicine induced some morphological changes in the inner ear sensory cells, such as dissociation of Golgi complexes etc. These isolated glycerol or colchicine injections did not cause any signs of inner ear functional impairment. Treatment with glycerol following pretreatment with colchicine, however, induced marked inner ear dysfunction with impaired sense of balance and audition. The inner ear morphology revealed a combination of changes as compared with what was observed after isolated treatment with glycerol or colchicine i.e. edema of the stria vascularis, and vestibular dark and transitional cells as well as dissociation of Golgi complexes in the sensory cells. The cochlea showed moderate endolymphatic hydrops. These findings indicate that colchicine affects the inner ear fluid regulating mechanisms which may lead to severe functional derangement after additional glycerol treatment. It is conceivable that the present experiment may serve as a useful model for further studies on inner ear changes related to endolymphatic hydrops and Ménière's disease.

The murine endolymphatic sac (ES) was studied 15 min to 8 hours after intravenous glycerol administration. Initially the ES showed varying degrees of obliteration and this was mostly pronounced at 15-60 min after the injection. After 2 hours the normal volume was regained and after 4 hours the lumen was dilated to 160% of its normal volume. After 6-8 hours the ES had almost regained its normal appearance. The epithelial lining showed an increase in the number of granular cells which, after two hours, reached a peak of 15.8% (p less than 0.01) compared to normal controls which showed 6.1% granular cells of total cell population in the ES. The increase of granular cells was accompanied by filling of the ES lumen with a stainable substance. The epithelial reaction may serve the purpose of counteracting decreases in endolymph pressure either in the ES or in the entire labyrinth.

Previous studies have shown that the application of topical otic drops to the external ear canals of animals with patent tympanostomy tubes may result in hearing impairment and cochlear hair cell loss. Otic drops are used in patients with tympanostomy tubes or tympanic membrane perforations and could have deleterious effects on the human membranous labyrinth. This report describes the inner ear damage that occurred after direct application of aminoglycoside-containing otic drops to the middle ears of experimental animals. The membranous labyrinths of 25 chinchillas were studied two days to five months after application of Cortisporin otic suspension (which contains neomycin, polymyxin B, hydrocortisone, and propylene glycol) to the middle ear cavity. Application of 0.5 ml of Cortisporin resulted in degeneration of all inner and outer hair cells throughout the cochlea, as well as severe damage to the stria vascularis. Moderate to severe degeneration of the vestibular receptor organs was also observed. The endolymphatic sacs showed dark-staining endolymph, cellular debris, and macrophages in the sac lumina, as well as increased activity of the epithelial lining.

The effect of acetazolamide on the ultrastructures of the murine endolymphatic sac was investigated. The animals were given a single intravenous dose of acetazolamide (100 mg/b.w.) and were sacrificed 0, 15, 30, 60 and 120 min after the injection, respectively. Prominent changes in the fine structure of the epithelial cells could be observed after 30 min. These alterations were even more pronounced after 1 h. After 2 h, the normal cell structure became to be reestablished. The most conspicuous change was a general reduction in the electron density of the dark cells. This was accompanied with a decreased number of cell organelles, especially ribosomes. Some light cells also underwent temporal modifications in their structure in the form of a reduced nuclear stainability associated with a loss of pinocytotic vesicles near the apical plasmalemma. In general, the dark cells seemed to be more influenced by acetazolamide than the light cells. The possibility that the dark cell changes are related to a modification of transepithelial ion and water flow is discussed.

Autoradiographic uptake of labelled sulphur (S35) in the endolymphatic sac was analysed after performing surgical labyrinthectomy and after systemic administration of glycerol and ethacrynic acid. Accumulation of radioactive substance was observed after surgical labyrinthectomy and associated with a deposition of stainable substance in the endolymphatic sac lumen. Increased activity was noted in the epithelial layer after administration of ethacrynic acid, whereas glycerol did not increase radioactivity in the endolymphatic sac despite the deposition of stainable substance therein. This fact was believed to depend on a too short time interval between the injection and the sacrifice of the animals, as well as a weaker concentration of the sulphated compound within the endolymphatic sac. The results suggest that the endolymphatic sac may secrete sulphur-containing substances presumably associated with a deposition of a stainable substance in the endolymphatic sac.

The effects of glycerol, urea and mannitol on the structure of the murine endolymphatic sac were investigated at various time intervals after their administration. Within 15 min after administration of glycerol and urea the lumen of the endolymphatic sac was partially collapsed and the density of its contents was increased. At the same time many light epithelial cells displayed abundant intracytoplasmic granules containing floccular and/or lamellar material. A similar type of material was observed in the lumen in close proximity to these cells. This suggested that macromolecular substances were secreted into the endolymphatic sac lumen from these cells. Mannitol induced similar changes, though less pronounced. It is proposed that the changes in the endolymphatic sac following systemic administration of hyperosmolar substances reflect an ability of the endolymphatic sac to compensate for endolymphatic volume and pressure changes caused by these agents.

During certain stages of mammalian inner ear development, small crystallized bodies which resemble otoconia may be found in the endolymphatic sac. In order to examine whether the endolymphatic sac plays any part in the process of degradation and dissolution of otoconia, we made an electron-microscopic examination on the endolymphatic sac of fetuses and adult guinea pigs injected with streptomycin sulfate (SM). In 30-day-old fetal guinea pigs we found miniature otoconium-like bodies (OLBs) in the endolymphatic sac and a giant OLB in the endolymphatic duct. In adult animals we found no otoconia in the endolymphatic sac following SM intoxication. However, the results suggested that both the dark cells of the utricle, as well as the non-sensory epithelium of the saccule, may be engaged in the absorption and dissolution of otoconia.

The combined effects of glycerol and colchicine on the endolymphatic sac were investigated in mice. Glycerol induced signs of secretion from the epithelium with formation of secretory granules in the light epithelial cells. Other characteristics of the epithelial lining were also changed resulting in an increased widening of the lateral intercellular spaces, a partial collapse of the lumen and with a deposition of a stainable substance within the lumen. This reaction lasted from 30 min to 24 h following the injection. Pretreatment with colchicine was found to decrease or inhibit the glycerol-induced secretion of macromolecules into the sac. The lumen collapsed but frequently there was no presence of stainable substance. Animals treated with both glycerol and colchicine showed marked signs of inner ear malfunction which could indicate that the secretory activity in the sac might be closely related to the regulation of inner ear fluid homeostasis and that functional disturbances in this system may lead to disorders of inner ear function.

There have been several reports of perisaccular fibrosis of the endolymphatic sac in some autopsied cases of Meniere's disease. We attempted to initiate perisaccular fibrosis of the sac in guinea pigs by injecting a small amount of 10% silver nitrate solution that cauterizes the tissue readily to produce scar, ie, fibrosis. Ninety-nine guinea pigs out of 108 (92%) that were treated with silver nitrate showed the endolymphatic hydrops in varied extent, from slight to profound. The extent of hydrops might depend on the extent of destruction of the sac following the cauterization. The lumen of the sac was sometimes occupied with dense fibrous connective tissue with deposition of black-silver particles in it. The silver nitrate injection method is an easy way to produce endolymphatic hydrops in guinea pigs.

The acute and chronic effects of treatment with ethacrynic acid (EA) and furosemide (FU) on the structure of the murine endolymphatic sac were studied by means of light and transmission electron microscopy. The animals were treated with loop diuretics in doses which are known to cause morphological alterations of the stria vascularis and a significant reduction of the endocochlear potential. A single intravenous injection of either EA or FU resulted in immediate morphological changes such as an increase in cytoplasmic contents of endoplasmic reticulum and more prominent Golgi structures of the light cells. These cells developed membrane-bound granules and a smooth tubular network in the apical cytoplasm. These findings together with the appearance of a precipitate on the luminal aspect of the cell membrane suggested secretory activity. Ten days after daily intraperitoneal injections with loop diuretics in subtoxic doses, the epithelial cytoarchitecture of the endolymphatic sac was altered, with pronounced veiling of the light cells by the dark cells. It is concluded that the changes in the endolymphatic sac epithelium after treatment with ototoxic diuretics may not be a result of a primary toxic effect on the sac per se, but rather be secondary to alterations in fluid and ion homeostasis in the rest of the inner ear.

The effects of hyperosmolar substances on the ultrastructure of the endolymphatic sac were studied in mice. Fifteen minutes after intravenous injection of urea or glycerol, subcellular changes in the endolymphatic sac were observed. These consisted of the occurrence of abundant cytoplasmic granules with a floccular or lamellar material, or both, in the light epithelial cells. Similar material was also present in the lumen of the endolymphatic sac, suggesting a common source and increased secretory activity. Mannitol caused similar changes, though less pronounced. The possibility that the alterations in the fine structure of the endolymphatic sac may be associated with a reduction in the hydrostatic fluid pressure in the rest of the labyrinth is discussed.

Intravenously injected kanamycin sulfate (KM) was located in the endolymphatic sac (ES) of guinea pigs by an immunohistologic method. Kanamycin sulfate had passed through the capillaries in the subepithelial layer and reached the epithelial layer as early as 1 minute after injection. The amount of KM in the epithelial layer gradually increased with time (until 30 minutes after injection). The KM also accumulated in floating cells in the ES. These results tend to indicate that the ES is rather easily permeable to systemically administered drugs, which readily get into the regional endolymph. In conclusion, much more attention should be paid to the ES and drug ototoxicity associated with drug therapy in the management of patients with inner ear disturbances.