Qualitative assessment of postnatal maturation of the organ of Corti in two rat strains.

The radial width of three rows of cuticular plates of outer hair cells (width of the OHC triad), the distribution of inner hair cells along the organ of Corti, and postnatal maturation of the middle ear were examined in rats of the Srague-Dawley and Lewis strains aged 0-24 days and two months. The width of the OHC triad is a good indication of the course of maturation of the reticular lamina and the organ of Corti in different regions of the cochlea. The width of the OHC triad in adult rats decreases continuously from the apex toward the base, while in newborn pups the trend is just the reverse. Thus, there is a region of the cochlea in which the width of the OHC triad is the same, or changes insignificantly, from birth to adulthood. This particular region (where the process of postnatal maturation of the organ of Corti begins?) corresponds to the region of maximum density of inner hair cells and tonotopically to the region of 'best hearing'. The width of the OHC triad will reach its adult values earlier in the basal than in the apical regions. The time-course of middle ear maturation correlates well with the time-course of maturation of the reticular lamina. The parameters under study are in many respects strain-specific.