The Enigmatic Presence of Actin Rings, Actin Cables and Ectopic Hair Cells in the Vestibular Maculae of Mice and Humans

Abstract Purpose Prior studies have shown that the utricles of mice contain a population of actin rings that reside in the lower stratum of the sensory epithelium, embedded among the nuclei of supporting cells. Although these structures display structural similarities with phagosomes, their function is not clear. The present study characterized actin rings, as well as actin cables and ectopic hair cells, in the sensory epithelia of normal, developing, and ototoxically-lesioned utricles of mice, and in utricles of adult humans. Methods Filamentous actin structures in the utricles of mice were labeled with phalloidin. Actin rings in developing, mature and lesioned utricles were characterized and quantified. Intraepithelial actin cables and ectopic hair cells in normal and developing utricles were also examined. Similar studies were conducted on utricles obtained from adult humans. Results All mouse utricles contained actin rings, whose number was unaffected by hair cell injury. Actin rings present in ototoxically-lesioned utricles did not clearly engage in the phagocytosis of hair cell debris. The lower stratum of the sensory epithelium also contained actin cables, which were associated with ‘ectopic’ hair cells. Although many ectopic hair cells appeared to be ‘impaled’ by the actin cables, those hair cells showed no clear signs of pathology or degeneration. Actin rings, actin cables and ectopic hair cells were also present in the utricles of neonatal mice, well before the full population of hair cells had been generated. Actin rings and cables were also observed in utricles of mature humans (2 males, ages 30 and 67; two females, ages 27 and 75). Conclusions Actin rings were present in mouse utricles, but were not affected by hair cell damage. Developmental data suggest that, if actin rings and cables are associated with spontaneous hair cell death, then such processes are likely to begin before the utricle has fully matured. At present, the origin and function of these structures remain unclear.