EMBRYOLOGY AND DEVELOPMENT
INITIAL STAGES OF DEVELOPMENT
Beginning at week 4, the tubotympanic sulcus develops as an extension of the endodermal epithelium of the first pharyngeal (branchial) pouch and eventually forms the middle ear canal and the eustachian tube. The tubotympanic recess has elongated and constricted to form the primordial tympanic cavity and eustachian tube by week 8. Simultaneously, the expanding end of the tubotympanic sulcus comes into proximity with the medial aspect of the ectodermal first pharyngeal cleft, the primordial external auditory canal. Although intimately related, the 2 linings remain separated by a layer of mesenchyme known as the pharyngeal membrane. This trilaminar relationship develops into the adult tympanic membrane, which comprises the outer cutaneous, middle fibrous, and inner mucosal layers. As the middle ear cavity expands, the tympanic sinus is created by the pneumatization of already ossified temporal bone. By 9 months, pneumatization of the tympanum and epitympanum is virtually complete. At the same time, the mastoid antrum is formed by the growth of the tympanic cavity into the mastoid portion of the temporal bone. The attachment of the sternocleidomastoid on the temporal bone promotes the formation of the mastoid process. Although the development of the mastoid air cells begins in fetal life, full maturation does not occur until age 2 years.
Early in development, the middle ear cavity is filled with loose mesenchyme that spans the gap between the primordial tympanic membrane and oval window. However, during the last 2 months of pregnancy, this mesenchyme is systematically reabsorbed, leaving the nearly mature ossicles suspended in the middle ear cavity. Beginning sometime between weeks 4 and 7, a condensation of neural crest ectoderm embedded within the mesenchyme begins to form the ossicles. Meckel cartilage, which is derived from the first pharyngeal (branchial) arch, gives rise to the head of the malleus and the body of the incus (ossicle portions above the tympanic membrane). The remainder of Meckel cartilage develops into the mandible and sphenomandibular ligament (Meckel ligament). The first pharyngeal arch is also associated with the mandibular division of the trigeminal nerve, the muscles of mastication, the tensor tympani muscle, and the tensor veli palatini muscle. The second pharyngeal arch gives rise to Reichert cartilage, which eventually forms the manubrium of the malleus, the long process of the incus, stapes superstructure, and the tympanic portion of the stapes footplate (ossicle portions below upper limit tympanic membrane). The vestibular portion of the stapes footplate derives from the otic capsule. The facial nerve, the muscles of facial expression, the stapedius muscle, the upper portion of the hyoid bone, and the stylohyoid ligament are also derived from the second pharyngeal arch mesoderm. It is important to note that although the pharyngeal arches are mesenchymal, the ossicles are derived from neuroectoderm that is embedded within the mesenchyme. This partly explains the association between ossicular malformations and disorders of neuroectoderm.