170. The Vestibulocochlear System: The External Ear and The Eardrum (tympanic Membrane)

  1. Introduction: development of the ear, the complex organ of hearing and balance, involves all 3 embryonic germ layers
    1. ECTODERM is the origin of the internal and external ears
    2. ENTODERM takes part in the formation of the middle ear
    3. MESODERM plays a role in the formation of all 3 parts of the ear
  2. Ontogenesis and phylogenesis: the internal ear is the first to appear. Only the internal ear is seen in fish. The middle and external ears first appear in amphibians
  3. The external ear
    1. THE EXTERNAL EAR IS DERIVED from the dorsal part of the first branchial (pharyngeal) groove and the external covering of arches I and II that border it
      1. The first branchial groove gives rise to a massive cellular cord or meatal plug by proliferation of the surface ectoderm. The cord reaches the tympanic cavity and in month 7, hollows out, to form the external auditory meatus
      2. Bone is developed in "membrane" around the inner part of the canal forming the tympanic ring, which expands after birth to form the bony external meatus
      3. Hairs and ceruminous glands are developed as ingrowths of the lining epithelium
      4. The nerve supply of the canal and adjacent parts of the eardrum is mainly from the mandibular nerve via its auriculotemporal branch. The posterior part of the canal and adjoining region of the drum are supplied by the vagus (X) nerve
  4. The eardrum, or tympanic membrane
      1. The ectodermal epithelial lining at the bottom of the external auditory meatus which forms its outer coat
      2. The entoderma1 epithelial lining of the tympanic cavity which forms its inner coat
      3. An intermediate layer of loose connective tissue containing the handle of the malleus and the chorda tympani (VII) nerve
    2. THE MAJOR PORTION OF THE EARDRUM is firmly attached to the handle of the malleus and is formed only after dissolution of the mesenchyme surrounding the ossicles
      1. The handle of the malleus and the chorda tympani nerve are trapped between the ectoderm of the meatal plug and the entoderm of the tympanic cavity and lie in the thin mesoderm between the two
    3. THE REMAINDER OF THE EARDRUM forms the separation between the external auditory meatus and the original tubotympanic recess
      1. The meatal plug, arising as a solid ingrowth of surface ectoderm, comes into contact with the lateral wall and the adjacent part of the floor of the tubotympanic recess, accounting for the obliquity of the drum in the adult
  5. The auricle or pinna is formed by the coalescence of a number of mesenchymal proliferations that form small tubercles or hillocks which appear around the upper portion of the branchial groove at about day 40 of gestation (in about a I3 mm embryo)
      1. The anterior tubercles are derived from the mandibular side of the first branchial groove, and the posterior tubercles from the hyoid side
      2. There are 3 tubercles seen on each side of the external auditory meatus
      3. The hillocks fuse and are gradually formed into the definitive auricle
        1. The first hillock forms the tragus; the second, the crus of the helix; the third, the helix; the fourth, the anthelix, the fifth, the antitragus; and the sixth, the lower part of the helix and lobule
      4. The development of the external ear is completed about month 4 (135 mm)
      5. Developmental abnormalities of the auricle are not uncommon

the vestibulocochlear system:  the external ear and the eardrum (tympanic membrane): image #1