Growth pattern of the middle ear in the gray short-tailed opossum, Monodelphis domestica

We studied the mass growth trajectories of middle ear ossicles and tympanic membrane and oval window area in 19 specimens of postnatal ages 30–180 days of the gray short-tailed opossum Monodelphis domestica. We weighed the skull mass and the mass of the three middle ear ossicles with appropriate balances. Using a binocular microscope provided with a grid, we measured the length of malleus and incus, as well as the longest axis and the one perpendicular to it on both the tympanic membrane and the stapes footplate. The size variation was studied with least squares regression analyses between various measurements. The incus and stapes change little in mass after 40 days of postnatal life, while the malleus does, reaching maximum mass at around 100 PND (postnatal days). This modularity in growth trajectory is in contrast with the shared evolutionary origin of malleus and incus from branchial arch 1. The maturation of the middle – and as indicated by previous work, that of the inner ear – is coupled with the improvement of hearing sensitivity at low and high frequencies after the initial onset of hearing at 29 PND.