Growth and turnover of rat otoconia as revealed by labeling with tetracycline

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Abstract
Background: The formation process of otoconia remains controversial, and the turnover rate of mammalian otoconia has not been determined. Methods: Tetracycline was administered as a tracer for calcium, and the growth and turnover of rat otoconia were examined by fluoresence microscopy. Results: Exposure of rats to tetracycline from 15.5 gestational day to 3-day postpartum resulted in incorporation of the drug into central portions of all otoconia, in both maculae sacculi and utriculi. When postnatal rats were injected subcutaneously with tetracycline, uptake of the drug into otoconia depended on the age of rats at injection. Apparent fluorescence was emitted from the periphery of all otoconia when tetracycline was injected into 7-day-old or younger rats. However, very little or no fluorescence was observed when this reagent was administered 10 days after birth. No fluorescence was detected when rats of 12 days of age or older were given this antibiotic. Otoconia that had been labeled with tetracycline during gestation were monitored during subsequent development, and it was found that all otoconia retained labelling in their central portions for at least 12 months. No otoconia that were not labeled with tetracycline were found. Conclusions: These findings indicate that: (1) all otoconia grow synchronously during late gestation and the neonatal period (up to about 10 days after birth) by accretion, (2) no new otoconia are formed subsequently, and (3) essentially no turnover of otoconia occurs and probably no turnover of calcium takes place under normal conditions once otoconia have formed.

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