Auricle

Week 6: Hillocks of His form from condensations of the first and second branchial arches. The first three hillocks are attributed to the first arch, and the second three hillocks are attributed to the second arch ( Table 3.1 ).

Week 20: Adult configuration is achieved.

The auricle continues to grow and reaches 85% of adult size by 5 years of age.

External Auditory Canal

Week 8: Ectoderm of the first pharyngeal groove (branchial cleft) invaginates. Epithelial cells then grow as a solid core (meatal plug) toward the middle ear. The core begins to dissolve at ~21 weeks to create the external auditory canal (EAC). The lateral epithelium forms the skin of the bony EAC, while the medial epithelium forms the lateral surface of the tympanic cavity—particularly, the lateral layer of the tympanic membrane (TM). The EAC may not reach final adult size and shape until the early teenage years.

Tympanic Membrane

The tympanic membrane (TM) forms as the ectoderm of the first pharyngeal groove (EAC) medializes, thins the mesenchyme of the first branchial arch (fibrous middle layer of the TM or the “tympanic ring”), and abuts the endoderm of the first pharyngeal pouch (tubotympanic recess or middle ear). The tympanic notch (notch of Rivinus) lacks the fibrous middle layer.

Middle Ear

Week 3: Endoderm from the first pharyngeal pouch forms the tubotympanic recess. Pneumatization begins at 10 weeks to form the middle ear and auditory tube (eustachian tube, ET).

Week 4: Mesenchyme from the first and second pharyngeal arches fuses and begins to form the malleus and incus. The first pharyngeal arch cartilage is also referred to as Meckel′s cartilage, and the second pharyngeal arch cartilage is also referred to as Reichert′s cartilage. The cartilaginous ossicles form at adult size and shape by week 16. Subsequently, ossification occurs by endochondral bone formation ( Table 3.2 ). The second pharyngeal arch cartilage also goes on to form the stapes blastema between weeks 4 and 5, which gives rise to the stapes suprastructure. The footplate is of otic capsule origin.

In general, the second pharyngeal arch cartilage forms the ossicular portions seen through the TM on otoscopy (manubrium of the malleus, long process of the incus, stapes superstructure). The first pharyngeal arch cartilage forms the ossicular portions superior to the TM (head of the malleus, body and short process of the incus). In addition, the tensor tympani is formed from the first pharyngeal arch, while the stapedial muscle is formed from the second pharyngeal arch.

Inner Ear

Week 3: The otic placode forms from ectoderm of the first pharyngeal groove. It invaginates and is completely encircled in mesoderm and termed the otocyst by week 4. The pars superior (semicircular canals and utricle) develops prior to the pars inferior (saccule and cochlea). The membranous labyrinth is complete by week 15 or 16. Ossification occurs between weeks 20 and 25 ( Fig. 3.1 ).