Chapter 50. The Pharyngeal Clefts and Pouches - Review of Medical Embryology Book

The pharyngeal clefts are seen in the 5-week embryo and in development almost completely disappear. The second, third, and fourth clefts are overlapped by the development of arch II and form a space lined by squamous epithelium, the so-called cervical sinus. This, too, disappears during extension of the cervical flexure

THE FIRST PHARYNGEAL CLEFT is the only one to persist, but only partly, to form the epithelium of the external auditory meatus and part of the tympanic membrane
AT THE EXTERNAL ORIFICE OF THE AUDITORY MEATUS, swellings arising from the mandibular (I) and hyoid (II) arches participate in the formation of the external ear
THE CERVICAL SINUS sometimes persists in vestigial form and forms a branchial cyst. If it communicates only with the outside, it forms a pharyngeal fistula, which is harmless. If it opens to both the interior and exterior, it forms a pharyngocutaneous fistula, which allows saliva to run out during mastication

The pharyngeal pouches are balloonlike diverticulae of the pharyngeal entoderm that line the inside of the branchial arches. They develop in a craniocaudal sequence between the arches, e.g., pouch 1 lies between arches I and There are 4 well-developed pairs of pouches; the fifth pair is rudimentary or absent. The pouch entoderm reaches the branchial groove ectoderm to form the double-layer branchial membranes that separate them. The arches enclose the primitive pharynx within which develop the important T structures: the tongue, tonsils, tube (eustachian), thyroid, thymus, and parathyroids

THE FIRST PHARYNGEAL POUCH elongates as the tubotympanic recess and appears between the external and internal ear, enveloping the middle ear bones. The distal portion reaches the first branchial groove and forms the tympanic cavity and mastoid antrum. The remainder forms the eustachian tube, which opens into the pharynx

Fusion of the ectoderm and the entodermal layers forms the tympanic membrane or eardrum

THE SECOND PHARYNGEAL POUCH does not elongate as much as the first. It forms the tonsillar fossa. At its extremity, the entodermal epithelium swells and invades the surrounding mesenchyme to form the palatine tonsil, which develops in situ. During months 3 to 5, the tonsil is gradually infiltrated by lymphatic tissue and eventually forms lymph nodules
THE THIRD PHARYNGEAL POUCH is the area from which the primordia for the thymus gland is formed. These paired symmetric primordia, originally from a long ventral wing which proliferates and obliterates its cavities, migrate to form a single median gland found in the anterior portion of the upper thoracic region

The inferior parathyroid glands arise from the dorsal border or wing of this pouch and later migrate toward the posterior inferior end of the lateral lobe of the thyroid gland
Both parathyroids and thymus primordia lose pharyngeal connections and migrate caudally and then separate from each other. Growth and development of the thymus are not completed by birth, but the gland is relatively large during the perinatal period and grows until puberty, at which time it begins to diminish progressively and finally atrophies

THE FOURTH PHARYNGEAL POUCH also gives rise to a thyroid primordium, but in humans, this regresses. The superior parathyroid glands arise from the dorsal border or wing of this pouch and later migrate to reach the superior end of the lateral thyroid lobe on its posterior side. Its pharyngeal connection degenerates
THE FIFTH PHARYNGEAL POUCH forms the ultimobranchial body, which is thought to participate in formation of the thyroid gland. Cells of this body are thought to disseminate and give rise to parafollicular or C cells of the thyroid gland (store and secrete calcitonin) involved in normal calcium level regulation in the body fluids. These cells are apparently of neural crest origin which have migrated here