Morphology of the endolymphatic sac in the guinea pig after an acute endolymphatic hydrops
Introduction
An endolymphatic hydrops (EH) is generally acknowledged as the fundamental histopathological substrate of Menière’s disease (Hallpike and Cairns, 1938; Kimura and Schuknecht, 1965). An EH, which is an increased endolymph volume, may give rise to a disruption of inner ear function resulting in characteristic symptoms like vertigo, hearing loss, tinnitus and a subjective aural fullness. The mechanisms of endolymph volume regulation and pathophysiology of an EH remain enigmatic. Still, a tiny organ called the endolymphatic sac (ES) is generally believed to represent one of the primary loci for endolymph volume regulation (Kimura and Schuknecht, 1965, Rask-Andersen et al., 1999). The ES’s anatomical and histological characteristics suggest an involvement in transport functions. Besides, different other functions have been ascribed to the ES: secretion of macromolecules (Thalmann, 2001), removal of waste products (Fukazawa et al., 1991), an immunological (Yan et al., 2003) and endocrine function (Qvortrup et al., 1999). Surgical ablation of the ES in the guinea pig results in a chronic EH (Kimura and Schuknecht, 1965), which has been widely investigated to gain insight into Menière’s disease. In the present study, an acute EH was created by microinjection of artificial endolymph into scala media of the cochlea. In contrast to the chronic EH, the injection-produced hydrops develops rapidly and is not disturbed by complex cascades of secondary pathological changes.
In literature, there is only one report on the ultrastructural effects of microinjection of artificial endolymph on the ES. Rask-Andersen et al. (1999) provided a new perspective on endolymph volume homeostasis. They reported that directly after induction of an acute EH, there was an almost total absence of the normal intraluminal homogeneous substance (HS). On the other hand, when native endolymph was aspirated, the amount of HS increased substantially. It was therefore suggested that the volume of fluid in the ES, and hence the volume of the entire membranous labyrinth, might be regulated by a dynamic active secretion and degradation of the lumen-expanding HS.
Intentionally, the present study was performed to gain further insight into the most conspicuous finding of Rask-Andersen et al. (1999), which was a degradation of HS in the ES after an acute EH. An additional time-sequence study might identify specific ES time related mechanisms in the inner ear coping with an acute volume stress.
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Materials and methods
Nineteen healthy female albino guinea pigs (Harlan, The Netherlands) with adequate Preyer’s reflexes and a weight of 350–450 g were used in this study. Animal care and use were approved by the Experimental Animal Committee of Groningen University, protocol No. 2910.
General anesthesia was induced by intramuscular administration of ketamine/xylazine (60/3.5 mg/kg). For maintainance of an adequate anesthesial depth, additional injections were given every hour. Muscle relaxation was obtained with
Results
The microinjections of 1.1 μl of artificial endolymph into scala media of the cochlea were successfully performed in all guinea pigs (n = 17). The mean rate of injection was 98 nl/min (91–109). In two additional experiments 2.2 μl of suspended polystyrene microspheres (molecular probes®) were injected.
The mean endocochlear potential (EP) prior to manipulation of endolymph volume was 77.2 mV (±5.1). During the period of injection the EP remained stable. The animals which were let to survive for 2
Discussion
The present results do not confirm the findings of Rask-Andersen et al. (1999), who observed dramatic changes in ES’s luminal filling immediately following endolymph volume manipulation. It was suggested that endolymph volume might be regulated by a dynamic secretion and degradation of the lumen-expanding HS. In contrast with this theory, no differences in luminal HS content or ES’s epithelia were found between injected and non-injected ears after immediate termination. Additionally, no
Acknowledgements
This study was supported by the Heinsius Houbolt Foundation and is part of the research program of our department: Communication through Hearing and Speech. The program is incorporated in the Sensory Systems Group of the Groningen Graduate School for Behavioural and Cognitive Neurosciences (BCN). We thank Wilma Hiemstra for her technical assistance in this study.
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