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The gut tube is the primitive precursor of both the respiratory and digestive systems. Its formation is initiated by ventrally directed invagination of the anterior intestinal portal (AIP) and the caudal intestinal portal (CIP), respectively situated at the anterior and posterior ends of the endoderm. The invaginations migrate towards each other as the endoderm behind them fuses at the midline to form tubes that eventually meet and fuse at the umbilicus. A lumen is formed by day E9.0, as a result of dorso-lateral cell migration. Lateral plate-derived splanchnic (visceral) mesoderm is then recruited to the endoderm and forms the outer layer of the gut tube. This layer later undergoes smooth muscle differentiation, which alters its shape. Tube formation is complete by E9.5 and coincides with the beginning of development of the enteric nervous system.
The endodermal epithelium lines the lumen of the tube and later undergoes regional specification along the anterior–posterior (A-P) axis, during which subsequent differentiation and functional specialization of the various organs occurs along the A-P axis of the gastrointestinal tract.
The gut tube is subdivided into the foregut, midgut and hindgut regions, each of which gives rise to specific organs at predetermined times in embryonic development. The foregut and hindgut are derived from the AIP and CIP, respectively, while the midgut is composed of tissue originating from both the AIP and CIP. As development progresses, the endoderm at various places along the foregut forms hollow buds that grow into the mesoderm and form specific organs which remain connected to the gastrointestinal tract via ducts. The foregut gives rise to the pharynx, esophagus, stomach, liver, pancreas and lung and part of the duodenum. The midgut gives rise to the remainder of the duodenum, the jejunum, the ileum, and portions of the large intestine (cecum, appendix, ascending colon, and most of the transverse colon). The hindgut gives rise to the remainder of the large intestine, excluding a portion of the anal canal.