DEVELOPMENT AND FATE OF THE PRIMITIVE PHARYNX, BRANCHIAL ARCHES, AND THE TONGUE
These two lectures will (1) review the fate of the primitive foregut, and (2) describe the development of the primitive pharynx. An appreciation of the development of the pharynx and the pharyngeal arches that surround it will help you to understand the organization of the head and neck, and the brain and cranial nerves.
Review and Summary of the Fate of the Foregut, Midgut and Hindgut
During the fourth week of development, the head and tail folding brings about an incorporation of a portion of the primitive yolk sac into the embryo the foregut and hindgut. Between these two regions is the midgut which is in open communication with the ventrally located yolk sac. As we discussed earlier this communication will normally disappear after the sixth week of development.
A. The foregut will give rise to part of the tongue, the pharynx, esophagus, stomach, proximal part of the duodenum as well as the thyroid gland, thymus, parathyroid glands, liver, pancreas, and respiratory passageway from the larynx through the alveoli of the lungs.
B. The midgut develops into the remaining small intestine, the appendix, and the ascending colon and the first 2/3 of the transverse colon.
C. The hindgut terminates caudally as a blind cloaca and the cloaca is continuous with the allantois. The urorectal septum divides the cloaca into a dorsal anorectal canal and a ventral urogenital sinus. The hindgut gives rise to the first 1/3 of the transverse colon, the descending colon, the sigmoid colon, the rectum and the upper 2/3 of the anal canal.
The Primitive Pharynx and Pharyngeal Arches and their Derivatives
During the fourth week of embryonic development, a series of 5 bar-like ridges appear on the ventrolateral surface of the head and neck region. The ridges are called pharyngeal (branchial) arches. The arches are covered by ectoderm. The ectoderm between the arches form clefts (grooves) called pharyngeal (branchial) clefts (grooves). The arches are bordered medially by the pharynx which is lined by endoderm. Medially each of the pharyngeal arches is separated by a pharyngeal pouch. These pouches approach the corresponding branchial cleft. The approximation of the ectoderm of the pharyngeal cleft with the endoderm of the pharyngeal pouch forms the pharyngeal membrane. The grooves and pouches are named (numbered) the same as the preceding arch.
Each arch contains a cartilaginous core, an aortic arch, and a definite cranial nerve. Each cranial nerve will supply the structures that develop from the mesenchyme of the arch.
The first branchial arch is divided into a maxillary and mandibular process. As discussed in the last lecture the first arch will give rise to the face. The first (hyomandibular) pharyngeal groove gives rise to the external auditory canal; the mesenchyme of first and second arches which are located on either side of this pharyngeal groove will also give rise to the external ear. The second (hyoid) arch enlarges and grows so that by the 6th week it will overlap the 3rd, 4th and 6th arches and covers them. The space between the 2nd arch and the other three arches is called the cervical sinus. The cervical sinus is lined by ectoderm. It can at a later time, enlarge and form cysts that are called cervical cysts. Tracts of epithelial cells can remain after development of the second pharyngeal arch. On occasion, a fistula opens to both the external surface and the pharynx connecting these structures and these are called complete branchial fistulas. However, they can open externally on the anterolateral surface of the neck or internally to the pharynx by way of a ruptured pharyngeal membrane. If they do they are called incomplete branchial fistulas.
The ectoderm of the first arch which surrounds the stomodeum forms the epithelium lining the buccal cavity. This epithelium will also give rise to the salivary glands, enamel of the teeth, epithelium of the body of the tongue. The external surface ectoderm of the first arch gives rise to the epithelium over the maxilla and mandible and to some epithelium of the auricle; the first branchial groove forms the lining of the external auditory meatus and the external surface of the tympanic membrane.
Now if you understand that the nerve that innervates the first arch is the 5th cranial nerve, then you will be able to see that those structures that develop from the first arch ectoderm are innervated by the 5th cranial nerve.
The ectodermal covering of the 2nd arch forms the epithelium of part of the auricle and external auditory canal, and some of the epithelium behind the ear. Since the second arch is innervated by the 7th cranial nerve, its derivatives are innervated by the facial nerve.
The ectoderm of the 3rd and 4th arches is mostly covered by the 2nd arch ectoderm. What remains can be found around the external ear (innervated by cranial nerve 9), and the external auditory meatus, external tympanic membrane and back of the ear (innervated by cranial nerve 10).
Endodermal Derivatives of the Arches
The tongue arises in the region where the stomodeum and primitive pharynx meet. It is seen initially as a proliferation of mesenchyme. The stomodeum is lined by ectoderm and therefore the lateral llingual swellings, which give rise to much of the body of the tongue, is lined by ectoderm. The tuberculum impar, at the caudal level of the first arch, forms part of the body and is covered by endoderm. The root of the tongue develops from a primitive swelling - the hypobranchial eminence (copula) - at the levels of the second, third and fourth pharyngeal arches. At the level of the fourth arch, an epiglottic swelling arises which lies cephalic to the laryngotracheal groove.
The thyroid gland is a diverticulum that originates at the level of the first pouch. It arises from the floor of the pharynx and migrates caudally to a position ventral and inferior to the larynx. This diverticulum forms a right and left lobe with an isthmus of thyroid tissue between. During development, the thyroid gland continues to retain a connection with the pharyngeal lumen. This connection is known as the thyroglossal duct (because it connects the thyroid with that part of the pharyngeal floor which is organizing into the tongue). Ordinarily, the thyroglossal duct closes off, leaving only an enlarged pit on the tongue (the foramen cecum) to mark its point of origin. Retention of all or part of the duct may give rise to cysts known as thyroglossal duct cysts. Retention and enlargement of that portion of the thyroglossal duct in contact with the thyroid results in the formation of a pyramidal lobe.
The first pharyngeal pouch is drawn out into the auditory (Eustachian) tube, the tympanic cavity of the middle ear, the internal lining of the ear drum and the lining of the mastoid air cells. The 2nd pharyngeal pouch forms the wall of the pharynx at the level of the palatine tonsils. The 3rd pharyngeal pouches give rise to two important diverticula. From the dorsal part of the 3rd pharyngeal pouch arises parathyroid III which becomes the inferior parathyroid gland. From the ventral part of the pouch arises thymus III which becomes the thymus. During the 7th week, both of these diverticula separate from the pouch and move caudally. The thymus may leave thymic tissue along its path as it descends. Occasionally parathyroid III or accessory parathyroid tissue formed from either the 3rd or 4th pharyngeal pouches will be carried into the mediastinum by the migrating thymus.
The 4th pharyngeal pouch forms in the wall of the pharynx at the level of the laryngeal aditus. Parathyroid IV arises from the dorsal portion of the 4th pharyngeal pouch; it separates from the pouch and migrates caudally, but ultimately becomes the superior parathyroid since it does not migrate as far caudally as parathyroid III. A small amount of thymic tissue called thymus IV may also arise from the ventral portion of the 4th pouch.
The 5th pharyngeal pouches, which may appear as pouches off of the 4th pouches, give rise to the paired ultimobranchial bodies. These lose their attachment to the pharynx and become incorporated into the thyroid which has moved caudally. The ultimobranchial bodies give rise to the parafollicular cells of the thyroid. These cells produce calcitonin.
Innervation of the Arches
The maxillary and mandibular processes of the first pharyngeal arch contain the maxillary and mandibular divisions, respectively, of the fifth (trigeminal) cranial nerve. The second arch contains the seventh (facial) cranial nerve. The third arch contains the 9th (glosopharyngeal) nerve. The fourth arch contains the superior laryngeal, and palatal branches of the vagus nerves. The fifth (6th) arch contains the recurrent laryngeal nerve of the vagus. Neural crest material gives rise to the parasympathetic ganglia. Neural crest cells along with ectodermal placodes give rise to the sensory ganglia of the head and neck.
Mesodermal Derivatives of the Arches
The mesoderm of the arches gives rise to skeletal muscles, aortic arches, and the connective and supportive tissues.
The skeletal muscles that develop from branchial arch mesoderm are skeletal muscles, but they do not arise from somites. For this reason, the nerves supplying the muscles arising from these arches have been designated functionally as special visceral efferent. Each of the muscles arising from the arches is listed in the table at the end of this summary. The innervation of each of these muscles is easily determined by remembering which cranial nerve passes in each arch.
The cartilages that develop in the pharyngeal arches serve the embryo as a temporary support. Some remain as cartilages, some degenerate after bone is laid down intramembranously next to cartilage, and others are replaced by endochondral bone formation. Much of the head cartilages develop from mesenchyme of neural crest origin. Mesenchyme of the mandibular process differentiates into Meckel's cartilage. Portions of this cartilage further differentiate into the malleus and incus ear ossicles and the sphenomandibular ligament. The mandibular bone develops intramembransously from other mesoderm of the mandibular process at a later stage of fetal development. The maxillary process mesoderm ultimately forms the maxilla. Further differentiation gives rise to the stapes of the ear, the styloid ligament, and lesser cornua and part of the body of the hyoid bone. The third arch forms the greater cornua and rest of the body of the hyoid bone, whereas the fourth and fifth (6th) arch mesoderm develop into the cartilage of the larynx.
The first and second aortic arches passing in the first two pharyngeal arches degenerate. The third aortic arch passes through the pharyngeal arch and persists as the stem of the internal carotid artery (and part of the common carotid artery). The fourth aortic arch in the fourth pharyngeal arch persists on the right as the proximal subclavian artery. On the left it develops into a portion of the arch of the aorta. The sixth aortic arch supplies the small fifth (6th) pharyngeal arch and survives as the pulmonary arteries and ductus arteriosus. To view an outline of the fates of the pharyngeal region, click here.
Cervical Fistulas and Cysts - Incomplete fusion or remnants of the walls of the cervical sinus (second branchial groove). The internal openings of fistulas are at the sites of the pharyngeal membranes; the external openings are along the anterior border of the sternocleidomastoid muscle.
Bifid Tongue - A midline split in the anterior two-thirds of the tongue due to improper fusion of the lateral lingual swellings.
Thyroglossal Duct Cyst - A cyst of remnants of the thyroglossal duct. In removing this, a portion of the body of the hyoid bone is often removed, since the hyoid bone grows around the developmental path of this structure.