Postnatal growth of cochlear spiral in Mongolian gerbil.

Measurements were made of cochlear dimensions in an age-graded series of Mongolian gerbils. The radii of the cochlear spiral at five locations and the length of the modiolus were determined from in situ photographs of mid-modiolar sections. The anterior to posterior dimensions of the auditory bulla and of the inferior colliculus were also measured to provide a general context for inner ear growth functions. At birth in this species, the cochlear capsule is still growing, and, although the basal turn is closer to the final adult proportion than more apical turns, all radii approach adult dimensions together at 9-10 days after birth (DAB). Comparison of the morphological development of gerbil otic capsule with the change in cochlear tonotopic mapping observed during physiological development shows that growth is complete before the onset of measurable electrical responses at 12-14 DAB.