Quantification of the process of hair cell loss and recovery in the chinchilla crista ampullaris after gentamicin treatment

The degree of ototoxic drug sensitivity and hair cell repair was determined in the chinchilla horizontal crista ampullaris after intraotic administration of gentamicin. Histological evaluation was made of 22 cristae ampullaris from one normal and six post-treatment (PT) animal groups killed at 1, 4, 7, 14, 28, and 56 days. New hair cell production was quantified, using the dissector technique. Transmission electron microscopy was used to investigate the ultrastructural characteristics of the hair cells in the regenerated epithelium. At 1 day PT, type I and II hair cells presented cytoplasmic vacuolization, swollen nerve calyces and 20% of type I and 18% of type II hair cells were lost. At 4 days PT, 95% of type I hair cells and 14% of type II hair cells had disappeared. In addition, most of the type II hair cells showed clumping of nuclear material. Nerve fibers were not found in the sensory epithelium, but were still observed below the basal lamina. Supporting cells appeared unaffected, maintaining their location in the crista. At 1 and 4 days PT, the damage to hair cells was more pronounced in the central region of the crista ampullaris. The degree of ototoxic damage at 7 days was similar to that of 14 days: no type I hair cells were present and most of the type II hair cells had disappeared; supporting cell nuclei began to occupy the apical part of the sensory epithelium and most of the nerve fibers had retracted. Quantitatively, 87 and 93% of type II hair cells were lost at 7 and 14 days PT, respectively. Initial signs of hair cell recovery began at 28 days PT; immature type II-like hair cells appeared, supporting cell nuclei began to align at the base of the sensory epithelium and nerve fibers penetrating the basal lamina were observed. No type I hair cells were found, but 40% of the normal number of type II hair cells were present. Hair cells appeared to regenerate in the peripheral areas of the cristae ampullaris first. At 56 days PT, an increase in the number of mature type II hair cells was present, supporting cells were aligned at the base of the epithelium, and more nerve fibers appeared to penetrate the basal lamina to the sensory epithelium. Although type I hair cells were absent from the epithelium 55% of the normal number of type II hair cells were present. At this time, more regenerated hair cells were located in the center of the cristae ampullaris as compared to the periphery. At the transmission electron microscopic level, type II hair cells at different stages of maturation were observed. Some exhibited mature stereocilia, a cuticular plate, and terminal endings with synaptic specialization opposing these hair cells. In conclusion, type I hair cells were more sensitive than type II hair cells to gentamicin intoxication (as they disappeared as early as 4 days PT). After 56 days PT, the number of type II hair cells reached 55% of normal. No type I hair cells had regenerated at this time. These results demonstrate quantitatively the differential ototoxic sensitivity and regenerative capacity of hair cells.