Postnatal development of the middle ear in New Zealand White rabbits: Ossicles and tympanic ring

We studied the postnatal development of the middle ear (ME) in New Zealand White rabbits. Bullae were scanned using a desktop X-ray microtomograph and 3D models of the ME ossicles as well as the tympanic ring (TR) were prepared. In 0, 1, 2 days old rabbits the ossification process was incomplete. We can therefore present quantitative data obtained from older rabbits (ages: 4–180 days) and a qualitative description at the earlier ages. For a number of the measured parameters an exponential curve could be fitted to the data, and the time constant (at which 63% of the final value was obtained) was calculated. The length of the manubrium increased rapidly in a period of about 15 postnatal days, from 1.73 mm to 4.08 mm. The distance between the tip of the malleus and the TR increased rapidly until day 30, from nearly 0 to 1.40 mm. The increase of the surface area within the TR was small as compared to inter-specimen variance, but the ratio [tympanic membrane area]/[TR area] clearly increased (from 1.00 to 1.11), with a time constant of 8.3 days. The area of the stapes footplate (FP) increased rapidly in about 15 days (from 0.72 mm2 to 1.49 mm2, time constant 4.8 days). The TR was nearly developed at birth whereas the stapes footplate was quite underdeveloped. The distance between the tip of the malleus and the incudomallear rotation axis increased rapidly until day 20 and varied between 3.47 mm and 5.00 mm. The distance between the tip of the incus and the rotation axis increased until day 133, from 1.39 mm to 1.69 mm. Our study shows that in rabbits the ME is underdeveloped at birth and that the functional geometry develops over the same time course as the ability to hear. The conical shape of the tympanic membrane (TM) is formed by retraction and growth of the manubrium, mainly during the first 40 days after birth.

Research highlights
► High resolution 3D morphology of developing middle ear ossicles.
► Middle ear ossicles are only partly developed at birth.
► Conical shape of eardrum is formed by retraction and growth of manubrium.
► Time constants of postnatal middle ear growth is in the order of days.