ABSTRACT
The embryonic foldings during the fourth week incorporate a portion of the yolk sac inside, creating a tubular embryo. The primitive gut thus formed is bounded cranially by the oropharyngeal (stomodeal) membrane and caudally by the cloacal membrane. Its cranial part, associated with the pericardial cavity and the septum transversum, is known as the foregut. Its part connected to the yolk sac is the midgut, and its caudal part is the hindgut. The primitive gut tube undergoes epithelial proliferation (which occludes the lumen), elongation and rotation. Later recanalization, cellular differentiation and functional maturation complete its development. The foregut includes the pharynx, the esophagus, the stomach and the proximal duodenum. The epithelial diverticula from the foregut give rise to the thyroid gland, the respiratory primordium (trachea and bronchial tree), the biliary primordium and the pancreatic buds. The midgut includes the distal duodenum and the remaining small intestines, cecum, ascending colon and proximal half of the transverse colon. The hindgut includes the distal half of the transverse colon, the descending colon, the sigmoid colon, the rectum and the cloaca. The allantois opens into the cloaca. The primitive gut suspends from the posterior body wall by the dorsal mesentery, which undergoes positional changes caused by gut rotation and divides the peritoneal cavity into the lesser and greater sacs. Some of the developmental anomalies related to the gastrointestinal tract are discussed at the end of this chapter.
