Hair cell counts in an age-graded series of rat cochleas.

Hair cells of Sprague-Dawley rats aged 2-33 months were counted in order to assess the magnitude, location and time course of cell degeneration. The mean number of hair cell places (hair cells plus phalangeal scars) was approximately 4700: 960 inner hair cell places and 3470 outer hair cell places. These numbers do not vary systematically with age. Hair cell degeneration was observed in all animals. At 31-33 months of age, animals had inner hair cell losses ranging from 1.6 to 4.2% and outer hair cell losses ranging from 2.1 to 23.3%. The loss of hair cells was greatest in the upper apex, where the 31-33-month-old animals had 3.1-9.2% inner hair cell losses and 7.4-46.8% outer hair cell losses. Outer hair cell losses were also large in the basal end, where inner hair cell losses were small. In the older animals, hair cell losses were consistently most prominent in the third row of outer hair cells. Following examination of the hair cell population, the ganglion cells in the apical region were evaluated in a number of cochleas. No significant correlation was found between the magnitude of inner hair cell and ganglion cell losses.