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.