Developmental morphology of the middle ear.

The development of the murine middle ear was monitored both qualitatively and morphometrically by scanning electron microscopy from the 19th gestational day to the adult stage. At birth, the middle ear was less well developed than the inner ear. The tympanic membrane (TM) was obscured by occlusion of the external auditory canal. Ciliated cells and secretory granules were present in the middle ear epithelium already 5 days after birth (DAB). Keratin debris was discerned on the external layer of the TM 9 DAB. By 12 DAB, mesenchymal tissue had resorbed from the middle ear cavity, except around the upper part of the ossicles. The middle ear was immature at birth but developed rapidly until 12 DAB. When compared with the avian middle ear the mouse middle ear was basically similar to that of humans, although in the human the stapedial artery is vestigial whereas in the mouse it persists as an important vessel. In man, there is no orbicular apophysis and no gonial of the malleus. The hypotympanum of the human middle ear is less developed than that of the murine middle ear. The mouse external auditory canal matures postnatally until 12 DAB, while in humans its development is complete at birth.