Development of Mouth and
Pharynx
The lining of the primitive gut tube is derived from the embryonic endoderm
and the supporting tissues and mesentery from the visceral layer of lateral
plate mesoderm. The amniotic cavity expands around the developing embryo to
create the body wall but leaves the endoderm/ectoderm connection at the oropharyngeal
membrane and caudally at the cloacal membrane. The early mouth, stomodeum,
is formed by the ectoderm and the early pharynx by the endoderm. The
developing pharynx is properly part of the foregut and it extends from the
oropharyngeal membrane to the respiratory diverticulum. During the
fourth week of development, the oropharyngeal membrane ruptures.
A large fold of mesenchyme located
over the eventual forehead, the frontonasal prominence, extends inferiorly
to form the superior border of the stomodeum. Two maxillary prominences are
found on each side of it, and a single, fused mandibular prominence borders
the stomodeum inferiorly. The frontonasal prominence develops two nasal
placodes that hollow out to become the nasal cavities. On either side of
the placodes are the lateral and medial nasal prominences. The
lateral prominences fuse with the maxillary prominences to create the
nasolacrimal grooves. The medial nasal prominences fuse with each other on the
midline to form the philtrum of the upper lip and also with the maxillary
prominences to create the upper lip. The medial nasal prominence also carries
the mesenchyme that will differentiate into the primary palate, which fuses
with the palatine processes of each maxilla to form the hard palate.
The pharyngeal (branchial or gill)
arches are found on both sides of the developing neck. These arches consist of
mesenchyme from neural crest cells, which form the connective tissue structures
of the face and direct the formation of the other structures thereafter. Each
arch contains a cartilage core, an aortic arch, a skeletal muscle, and a
cranial nerve.
The first pharyngeal arch, which
is supported by the Meckel cartilage, gives rise to the maxillary and
mandibular prominences, mandible, incus, malleus, and other facial bones. The
muscular mesenchyme therein is innervated by the trigeminal nerve, and it
differentiates into all the muscles innervated by that nerve.
The second pharyngeal arch is
supported by the Reichert cartilage, and it gives rise to the lesser
horn and part of the body of the hyoid bone, styloid process, and stapes. The
muscular mesenchyme in the second arch is innervated by the facial nerve, and
it differentiates into all the muscles which that nerve innervates. The second
arch, in conjunct on with the first, will also form the auricle of the
ear.
The cartilage of the third
pharyngeal arch creates the remainder of the hyoid bone. The only muscle
derived from it is the stylopharyngeus, which is innervated by the
glossopharyngeal nerve.
The fourth pharyngeal arch gives
rise to (most of) the thyroid cartilage. The vagus nerve innervates the muscle
mesenchyme of the fourth arch, which will give rise to all the pharyngeal and
palatine muscles apart from the stylopharyngeus (IX) and the tensor veli
palatini (V3). The external branch of the superior laryngeal nerve (vagus)
innervates the cricothyroid and cricopharyngeus muscles.
The fifth pharyngeal arch exists in
some animals but not in humans. However, the sixth pharyngeal arch and
its cartilages give rise to the rest of the thyroid and all the other laryngeal
cartilages. The muscle mesenchyme in this arch is innervated by the recurrent
laryngeal nerve, a branch of the vagus, which innervates the intrinsic
laryngeal muscles.
The spaces between arches on the
exterior of the neck form pharyngeal grooves, and those spaces on the
interior side form pharyngeal pouches. The first pharyngeal groove is
located between the first and second pharyngeal arches; it deepens toward the
first pharyngeal pouch until only a thin membrane separates them. The external
auditory meatus is the derivative of the first groove, and the middle
ear and auditory tube are remnants of the first pouch, with the
tympanic membrane separating them.
The second, third, and fourth
pharyngeal grooves fuse and migrate into the developing neck as they are
overgrown by mesenchyme, briefly forming a cervical cyst that dwindles
and disappears. The second pharyngeal pouch forms the bed of the palatine
tonsil as lymphocytes migrate into its hollow, medial side. A dorsal
extension of the third pouch forms the inferior parathyroid gland, and a
ventral extension forms the thymus. Similarly, a dorsal extension of the
fourth pouch forms the superior parathyroid gland and a ventral
extension to the calcitonin-producing C cells of the thyroid gland. The
tongue forms in the anterior part of the developing mouth as two lateral
lingual swellings and one medial lingual swelling, which enlarge and
fuse. The muscles developing deep to these swellings are derived from somites
innervated by the hypoglossal nerve. The lingual swellings are located in the
floor of the first arch, and their surface lining is innervated by the mandibular branch of the trigeminal nerve. The rest of the tongue forms deep to
the second, third, and fourth arches. A small diverticulum forms in the floor
of the mouth between the second and third arches. This is the foramen cecum,
and it will extend inferiorly and eventually separate from the tongue,
travel along a pathway called the thyroglossal duct, and become the thyroid
gland.
