Labelling experiments with [H-3]thymidine demonstrate a continuous production of cells in the mechanoreceptive lateral line organs of the eel (Anguilla anguilla) and butterfly fish (Pantodon buchholzi) as well as in the electroreceptive ampullary organ of the transparent catfish (Kryptopterus bicirrhus). Shortly after [H-3]thymidine injection many cells are labelled in the middle and basal parts of the sensory organ and after a few days' survival sensory cells are also labelled. The vestibular sensory organs of selected species of fishes, amphibians, reptiles and birds also show a continuous production of cells. In the budgerigar (Melopsittacus undulatus) labelled cells are found in the basal and middle layer of the sensory epithelium a few hours after injection with [H-3]thymidine. A few days after the injection labelled cells are found in non-calyceal hair cells. After one month the calyceal cells are also labelled. Similar experiments with the bat Pipistrellus nathusii and with normal and gentamicin-treated mice (Mus musculus) show no labelled cells in the inner ear sensory epithelia. The lateral line organs and vestibular epithelia of non-mammalian vertebrates all contain a small number of dark cells with the characteristics of apoptotic cells. Macrophages and inclusions in some cells, thought to be remnants of apoptotic cells, are occasionally seen. Fixation at different osmolarities has little effect on the number of dark cells. It is suggested that the continually produced cells replace apoptotic dying cells.
