Pharyngeal Gut

PHARYNGEAL GUT represents the section of the embryonic foregut located between the oral cavity and the esophagus, which gives rise to a number of important organs. At the beginning of development, the pharyngeal gut is separated from the oral pit by the pharyngeal membrane (membrana pharyngea, s. bucco-pharyngea), in which the oral ectoderm directly adjoins the intestinal endoderm and which ruptures very early, with its fragments—the primary palatine curtains—dissolving. In a 3-4 mm embryo, the pharyngeal gut differs from other sections by its width and is flattened in the dorsoventral direction; its wall consists of a single layer of epithelium directly bordering the mesenchyme and lacks an intestinal-fibrous layer. The lateral edges of the pharyngeal gut form pocket-like depressions toward the ectoderm of the neck, the so-called pharyngeal or branchial pouches; similar depressions, but only of a smaller magnitude, are formed from the outside—the pharyngeal or branchial grooves; they grow toward each other until they touch; the membranes formed in this way, consisting of ectoderm and endoderm, rupture and give rise to clefts running across the neck—the pharyngeal or branchial clefts. The size of the pharyngeal pouches and correspondingly of the clefts decreases from front to back; their number in humans, mammals, and birds is 4 (the 5th is rudimentary), in reptiles, amphibians, and fishes 5, in elasmobranchs 6–8. The bridges between the pouches or clefts form the so-called pharyngeal or branchial arches, embracing the neck from the sides; the branchial branches of the aorta pass through them, and cartilaginous strips are formed that give rise to the visceral skeleton. At the end of the 4th week, the branchial arches move over one another, starting from the front, and the clefts fuse, with a deep cervical sinus (sinus cervicalis) forming on the neck; only in fishes do the branchial arches and their clefts remain throughout life. In the 3rd and 4th week of intrauterine life, the pharyngeal gut gives rise to a number of organs. Its anterior part, bordering on the pharyngeal membrane, takes part in the formation of the oral cavity and forms: 1) the primordium of the root of the tongue; 2) Waldeyer's pharyngeal lymphatic ring, i.e., the tonsils: lingual, palatine, and pharyngeal; in the places of their formation, the epithelium forms depressions, or crypts, around which lymphoid tissue accumulates; 3) the thyroid gland (glandula thyreoidea), which is laid down on the anterior section of the abdominal wall in the form of a tube-like protrusion of epithelium growing downward, toward the heart, and dividing into 2 lobes (see separate table, figures 1 and 2). From the epithelium lining the branchial pouches develop: 4) the thymus gland (glandula thymus) in the form of paired outgrowths of the 3rd and partly 4th pouches, located closer to their ventral side; 5) the parathyroid glands (gl. parathyreoideae), or epithelial bodies, growing out of the epithelium of the same pouches, but only closer to the dorsal side, and finally 6) the postbranchial bodies (corpuscula postbranchialia) from the epithelial depression corresponding to the 5th rudimentary pouch. Along the midline of the ventral side, behind the primordium of the thyroid gland, are laid down 7) the respiratory organs: the larynx, trachea, and lungs, in the form of an unpaired outgrowth dividing into 2 vesicles (see also Gills, Branchial arches, clefts, and corresponding organs).