Development of the middle ear in neonatal chinchillas. I. Birth to 13 days

Measures of middle-ear function in humans show large differences among neon ates, infants, and adults. In contrast, hearing sensitivity is essentially mature at birth. Hypotheses that have been proposed to explain the developm ental changes in middle-ear function include: (i) contaminating effects of the immature neonatal ear-canal wall and iii) persistent fetal tissue in th e ear canal, tympanic membrane (TM), and middle-ear space. To better unders tand the relationships between middle-ear function, hearing sensitivity and the structure of the middle ear, 30 chinchillas, aged 1-14 days, were stud ied. Middle-ear function was assessed by multifrequency tympanometry with p robe tunes ranging from 226 to 2,000 Hz. Hearing sensitivity was measured b y auditory brainstem response using clicks and 1, 2, 4, and 8 kHz tone burs ts. Structural characteristics were analyzed from temporal bone histologic preparations. At all frequencies, the acoustic admittance of the neonatal c ar is very low and tympanometric patterns are complex and irregular, compar ed to adult animals. The admittance is essentially constant from 1 to 14 da ys, indicating that developmental changes occur over a much wider age span than that investigated here. Hearing sensitivity of the chinchilla appears to be mature at birth. Histologic analysis indicated that there were no age -related changes in TM thickness, TM diameter, distance from TM to promonto ry. and stapes footplate diameter. There were small increases in bone thick ness, middle-ear area, mastoid bulla area, and in the perimeters of the mid dle ear and mastoid bulla. There were no significant amounts of loose mesen chyme or other fetal tissue in the middle-ear space.