The malleus and incus are derived from cartilage of the first pharyngeal arch, and the stapes is derived from that of the second arch (Fig. 16.9A). Although the ossicles appear during the first half of fetal life, they remain embedded in mesenchyme until the eighth month (Fig. 16.9B), when the surrounding tissue dissolves (Figs. 16.7, 16.8, and 16.9B). The endodermal epithelial lining of the primitive tympanic cavity then extends along the wall of the newly developing space. The tympanic cavity is now at least twice as large as before. When the ossicles are entirely free of surrounding mesenchyme, the endodermal epithelium connects them in a mesentery-like fashion to the wall of the cavity (Fig. 16.9B). The supporting ligaments of the ossicles develop later within these mesenteries.
Since the malleus is derived from the first pharyngeal arch, its muscle, the tensor tympani, is innervated by the mandibular branch of the trigeminal nerve. The stapedius muscle, which is attached to the stapes, is innervated by the facial nerve, the nerve to the second pharyngeal arch.
During late fetal life, the tympanic cavity expands dorsally by vacuolization of surrounding tissue to form the tympanic antrum. After birth, epithelium of the tympanic cavity invades bone of the developing mastoid process, and epithelium-lined air sacs are formed (pneumatization). Later, most of the mas-toid air sacs come in contact with the antrum and tympanic cavity. Expansion of inflammations of the middle ear into the antrum and mastoid air cells is a common complication of middle ear infections.
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