Measures of middle-ear function in humans show large differences among neonates, infants, and adults. In contrast, hearing sensitivity is essentially mature at birth. Hypotheses that have been proposed to explain the developmental changes in middle-ear function include: (i) contaminating effects of the immature neonatal ear-canal wall and (ii) persistent fetal tissue in the ear canal, tympanic membrane (TM), and middle-ear space. To better understand the relationships between middle-ear function, hearing sensitivity and the structure of the middle ear, 30 chinchillas, aged 1-14 days, were studied. Middle-ear function was assessed by multifrequency tympanometry with probe tones ranging from 226 to 2,000 Hz. Hearing sensitivity was measured by auditory brainstem response using clicks and 1, 2, 4, and 8 kHz tone bursts. Structural characteristics were analyzed from temporal bone histologic preparations. At all frequencies, the acoustic admittance of the neonatal car is very low and tympanometric patterns are complex and irregular, compared to adult animals. The admittance is essentially constant from 1 to 14 days, 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 promontory, and stapes footplate diameter. There were small increases in bone thickness, middle-ear area, mastoid bulla area, and in the perimeters of the middle ear and mastoid bulla. There were no significant amounts of loose mesenchyme or other fetal tissue in the middle-ear space.
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