174. The Vestibulocochlear System: The Middle Ear

  1. The tympanic cavity of the middle ear is of entodermal origin and is derived from the first pharyngeal pouch, an outpocketing of the pharynx, which grows laterally in the direction of the external acoustic meatus. The pouch, lined with epithelium of entodermal origin, appears in embryos at about week 4.
    1. THE POUCH GROWS RAPIDLY in a lateral direction and at about the end of month 6 of gestation, its external wall comes into contact with the floor of the first ectodermal cleft (deep end of the external auditory meatus)
      1. A thin mesodermal plate persists between the ectodermal and entodermal epithelial structures, and the combination of all 3 layers forms the tympanic membrane or eardrum
    2. THE DISTAL PART of the pharyngeal pouch, the tubotympanic recess, widens and gives rise to the primitive tympanic cavity. The proximal portion remaining narrows and forms the auditory or Eustachian tube
      1. The tube's pharyngeal orifice is surrounded by a considerable amount of lymphoid tissue, the tubal or pharyngeal tonsil (adenoids)
        1. Nasal inflammations associated with the tubal tonsillar swelling often result in occlusion of the tube and inflammation of the tympanic cavity resulting in otitis media (seen in young children quite frequently)
    3. DURING LATE FETAL LIFE, the tympanic cavity expands dorsally and posteriorly to form the tympanic antrum
      1. Its walls are covered with epithelium of entodermal origin
    4. AFTER BIRTH, the bone of the developing mastoid process is invaded by the epithelium of the tympanic cavity and epithelial-lined air sacs are created (pneumatization)
      1. Later, most of the mastoid air cells contact the antrum and the tympanic cavity
  2. The ossicles of the middle ear
    1. BY THE END OFWEEK 7, the mesenchyme above the primitive tympanic cavity demonstrates a number of condensations caused by proliferation of the dorsal tips of pharyngeal arches I and II
      1. The condensations become the cartilaginous precursors of the auditory ossicles, the malleus, the incus, and the stapes
        1. The malleus and incus are derived from pharyngeal arch I (Meckel's cartilage) and the stapes is derived from arch II (Reichert's cartilage)
    2. THE OSSICLES appear during the first half of fetal life but remain embedded in mesenchyme until month 8, when the surrounding tissue dissolves
    3. THE ENTODERMAL LINING of the primitive tympanic cavity gradually extends along the wall of the newly developed space, and the tympanic cavity becomes almost twice as large as originally
    4. WHEN THE OSSICLES are entirely free from the surrounding mesenchyme, the entodermal epithelium connects them in a mesenterylike manner to the cavity wall. The supporting ligaments of the ossicles develop in these mesenteries
    5. SINCE THE MALLEUS IS DERIVED from pharyngeal arch I, its muscle, the tensor tympani, is innervated by the mandibular branch of the trigeminal (V) nerve
    6. SINCE THE STAPES is of pharyngeal arch II, its muscle, the stapedius, is innervated by the facial (VII) nerve
  3. Further development: at birth, the cavities of the middle ear fill with air via the eustachian tube. With the ossicles, they form the system used for transmitting vibrations to the inner ear
    1. THE PART OF THE OSSEOUS LABYRINTH opposite the stapes remains thin and becomes the oval window (fenestra vestibuli) which opens into the vestibule; below this, another thinning of the bony labyrinth forms the round window (fenestra cochleae) which opens into the scala tympani. Both are closed by membranes

the vestibulocochlear system:  the middle ear: image #1