MIDDLE-EAR DEVELOPMENT-VI - STRUCTURAL MATURATION OF THE RAT CONDUCTING APPARATUS

Background: The contribution of middle-ear development to the overall development of hearing has not been explored in great detail. This pre sentation describes the maturation of conductive elements in the rat m iddle ear, and provides the basis on which future studies of middle-ea r functional development will follow. Methods: The middle-ear apparatu s was examined at nine different ages (between 1 and 80 days postpartu m) in Long Evans rats, At each age elements of the conducting apparatu s were observed with either light or scanning electron microscopy (SEM ), and quantitative measurements were made from video enhanced photomi crographs. Tympanic membrane area and cone depth, the length of the ma lleus and incus arms, ossicular weight, stapes foot plate and oval win dow areas, and bulla volume were all measured. Development of the area and lever ratios were derived from these measurements. The data were fitted to exponential equations and the time in days required to reach 90% of the adult level determined. Results: The pars tensa achieved 9 0% of total area by 17 days. The oval window achieved the 90% criterio n by 13 days, while the area ratio was within 10% of its adult size by 8 days. The ossicles took between 26 and 34 days, while bulla volume took 59 days to reach the 90% level. Conclusions: Middle-ear growth wa s very orderly and systematic in the data reported. When maturation of the area ratio was considered against development of the endocochlear potential or the round window compound action potential, it was clear that the growth of this important aspect of the middle ear preceded t he onset of cochlear function. (C) 1994 Wiley-Liss, Inc.