Middle ear development. III: Morphometric changes in the conducting apparatus of the Mongolian gerbil.

Middle-ear structural ontogeny was examined in 12 age groups of Mongolian gerbils between 2 and 42 days after birth. Measurements of tympanic membrane surface area; depth of the tympanic membrane cone; the lengths of the malleus and incus long processes; and stapes footplate, annular space, and oval window areas were obtained using video micrographs and computer digitization techniques. The incus long process matured first at 3.5 days after birth, while the pars flaccida surface area was the last middle-ear variable studied to reach adult size (26 days after birth). The incus long process increased its length by 30% from 0.5 mm to 0.65 mm. The malleus long process, however, demonstrated much more relative growth (47%). Pars tensa area expanded from 6.35 mm2 at two days after birth to its adult size of 16.9 mm2 and the stapes footplate expanded by 50%. The developmental changes observed in middle-ear anatomy are then discussed with regard to their contribution to the functional maturation of both the middle ear and more central auditory function.