In general, postembryonic production of inner ear vestibular hair cells (HCs) is believed to occur in all nonmammalian vertebrates. However, no study on this topic has been published on reptiles and, consequently, it is not known whether this also applies to these vertebrates. Therefore, the present study applied stereological methods in order to estimate the total number of HCs in turtles of varying sizes. The findings are that in prehatchlings the utricular macula (UM) contains~4000 HCs as compared to~5000 in juveniles,~8000 in medium-sized turtles, and 12,000 in large, sexually mature turtles. Scanning electron microscopy (SEM) reveals that presumably newly generated HCs with small surface areas and thin stereovilli are found in all regions of the UM. Furthermore, it reveals that utricular HCs can be classified as belonging to a specific region from the morphology of their apical structure. Striolar HCs have a large free oval-to-ovoid surface, a hair bundle with numerous stereovilli, and a short kinocilium. Rampary and cotillary HCs have smaller and slimmer free surfaces, comparatively fewer stereovilli, but much longer kinocilia. In conclusion, the current study demonstrates that postembryonic production of HCs does occur in reptiles and thereby supports the general view that this is a common trait in all nonmammalian vertebrates.

The total cell number was found to be 143 000 in subjects older than gestational week 16. The number of hair cells and supporting cells did not change between the 16th gestational week and 15 years and was 36 000 and 107 000, respectively. In the youngest specimen (10th and 12th gestational week) there was a statistically significant lower total number of cells (62 000) and a lower hair cell:supporting cell ratio, resulting in a mean number of 13 000 hair cells and 49 000 supporting cells.

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The aim of the present study is to estimate the total number of the sensory hair cells (chalice innervated and bouton innervated) and supporting cells in the mouse utricular sensory epithelium at two different time points after systemic kanamycin treatment. Mice were given two daily subcutaneous injections of kanamycin (600 or 900 mg/kg) for 15 consecutive days and allowed to survive either 1 or 3 weeks after end of treatment. Cell numbers were estimated using a physical fractionator. Paraffin-embedded tissue was immunohistochemically stained for active caspase-3 in order to detect apoptosis. There was no change in hair cell or supporting cell number after treatment with kanamycin and the survival time had no effect. Although no positive staining for caspase-3 was seen, hair cells with swollen chalices and dark stained nuclei were observed in the sensory epithelium of the treated animals, indicating some effect of the treatment. In conclusion, the dosing regime and survival times studied here are not sufficient to induce hair cell loss in the mouse utricle.