The genetically mutant pallid mouse (which possesses specific otolith defects) and the manganese-supplemented (Mn) pallid mouse (in which otolith defects were prevented) were studied using light and transmission and scanning electronmicroscopy. Untreated pallid mice which showed head-tilting, circling and ataxia also had swimming difficulty. In these animals, the otolith defects were the only consistent findings. With one exception, no pronounced anomalies were found in the sensory epithelium, nerve fibers and ganglion cells. Only a few vestibular melanocytes, in which the melanin granules failed to fully mature, were observed in the pallid mouse. Melanization in vestibular melanocytes appeared to be improved by Mn-supplementation. The genetic basis for involvement of Mn in otolith development was discussed. This animal model provides opportunities for basic research concerning the possible prevention of genetic disorders involving the inner ear.
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