Nat. Rev. Gastroenterol. Hepatol. doi:10.1038/nrgastro.2017.76 Figure 3 The origins, regions and layers of the major segments of the gastrointestinal tract Figure 3 | The origins, regions and layers of the major segments of the gastrointestinal tract. The gastrointestinal (GI) tract begins as a simple and uniform embryonic tube that differentiates along the anterior–posterior axis to form complex organs of the foregut, midgut and hindgut regions. GI tissues that arise from the foregut include the oesophagus, stomach and the proximal portion of the duodenum. The midgut develops into the posterior portion of the duodenum, the jejunum, ileum, ascending colon and two-thirds of the transverse colon. The distal one-third of the transverse colon and the rectum are derived from the hindgut of the embryonic tube. The epithelium of GI tissues, with its tissue-specific structures (such as gastric pits, villi and crypts), is derived from the endoderm; however, in the tissues of the mouth and anus, the epithelium is derived from the ectoderm. The basement membrane of GI tissue arises from both the endodermal and the mesodermal germ layers. The remaining histological layers of GI tissues, including the lamina propria of the mucosa layer, the submucosa, muscularis externa, adventitia and serosa, are mesodermal in origin. In most GI tissues, such as the stomach, duodenum, jejunum and ileum, the muscularis externa is bound by the underlying serosa; however, in tissues of the thoracic oesophagus, ascending colon, descending colon and the rectum, the muscularis externa is bound by adventitia. Hussey, G. S. et al. (2017) The extracellular matrix of the gastrointestinal tract: a regenerative medicine platform Nat. Rev. Gastroenterol. Hepatol. doi:10.1038/nrgastro.2017.76