A critical period during postnatal auditory development of mice.

CBA/J mice were given a 50 dB unilateral conductive hearing loss by removal of the left cartilaginous external auditory meatus. When the conductive loss extended from 12 to 24 days after birth, the left globular cells and left large spherical cells of the ventral cochlear nucleus were significantly smaller (P less than 0.01) than comparable cells on the right side. Right medial nucleus of the trapezoid body cells and right inferior colliculus cells were significantly smaller (P less than 0.01) than comparable left side cells. These same effects were found with conductive losses of 4-24, 4-45, and 4-90 days after birth. There were no significant differences (P greater than 0.05) between right and left cell sizes with conductive losses of 4-12 or 24-45 days after birth. There were significant right/left cell size differences (P less than 0.01) when the conductive loss was 4-18, 12-18, or 18-24 days, but these differences were of lesser magnitude than when the conductive loss included the full 12-24 day period. Since normally all these neurons have adult soma size by postnatal day 12, it is evident that adequate acoustic stimulation is necessary between 12 and 24 days postnatally in order to maintain normal brainstem auditory neuronal size in CBA/J mice.