Gastrointestinal Embryological Development
The primitive gut tube forms during week four of gestation. It derives from the incorporation of the dorsal part of the definitive yolk sac into the embryo due to embryonic folding. The primitive gut is divided into the foregut, midgut, and hindgut.
The esophagus, stomach, liver, gallbladder and bile duct, pancreas, and upper duodenum derive from the foregut. All of these structures receive their blood supply from the celiac trunk (except for some portions of the esophagus, which are supplied by other branches of the aorta).
The first step in the development of the esophagus is the formation of the laryngotracheal diverticulum in the ventral wall of the primitive foregut during week four. The distal end of the laryngotracheal diverticulum enlarges to form the lung bud. It is initially in open communication with the foregut, but eventually they become separated by folds of mesoderm, the tracheoesophageal folds. When the tracheoesophageal folds fuse in the midline to form the tracheoesophageal septum, the primitive foregut is divided into the laryngotracheal tube ventrally and the esophagus dorsally. The esophagus is initially short, but lengthens with descent of the heart and lungs. During development, the endodermal lining of the esophagus proliferates and obliterates its lumen; later, recanalization (reopening) occurs.
The primitive stomach appears from a dilatation in the foregut in week four. Its dorsal part grows faster than ventral part. The stomach rotates so that its original left surface faces ventrally and its dorsal curvature extends to the left. This 90-degree clockwise rotation yields the greater and the lesser curvature of the definitive stomach. In particular, the rotation causes: (1) the dorsal mesentery to be carried on the left, forming the greater omentum; (2) the left vagus nerve to innervate the ventral surface of the stomach, and the right vagus nerve to innervate the dorsal surface.
The liver arises (fourth week) as a ventral diverticulum (pocket)of the foregut. The endodermal lining of the foregut arises into mesoderm to form the hepatic diverticulum. Cells from the hepatic diverticulum grow into the septum trasversum (a part of the future diaphragm), forming the hepatic cords. These cords arrange themselves around vitelline and umbilical veins forming the hepatic sinusoids. The proximal part of the hepatic diverticulum gives rise to the common bile duct, cystic duct, gallbladder, and hepatic ducts. The epithelial lining of gallbladder and extra hepatic bile duct proliferates and obliterates the lumen (opening). Later recanalization (reopening) occurs. The primitive liver initially bulges into the abdominal cavity, stretching the septum trasversum to form the falciform ligament and the lesser omentum. The lesser omentum can be divided into the hepatogastric ligament and the hepatoduodenal ligament; it contains the bile duct, portal vein, and hepatic artery. The falciform ligament contains the left umbilical vein.
The ventral pancreatic bud and the dorsal pancreatic bud arise from the endodermal lining of foregut. Both of these endodermal tubes grow into surrounding mesoderm forming the acinar cells and ducts of definitive pancreas (esocrine portion) and the pancreatic (Wirsung) duct. Some cells remain accumulated and isolated into mesoderm to form the islet cells of endocrine pancreas. Connective tissue and vascular components of the definitive pancreas are derived from the surrounding mesoderm. Because of the 90-degree clockwise rotation of the duodenum, ventral and dorsal buds fuse to form the definitive adult pancreas. The uncinate process and a portion of the head of the pancreas originate from ventral bud. The remaining portion of the head, body, and tail of the pancreas originate from the dorsal bud. The upper duodenum originates from the caudal-most part of duodenum.
The lower duodenum, jejunum, ileum, cecum, appendix, ascending colon, and the proximal two-thirds of the ascending colon all derive from the midgut. All of these structures receive their blood supply from the superior mesenteric artery.
The lower duodenum develops from the most cranial part of the midgut. During development, endodermal lining of the duodenum proliferates rapidly and obliterates the lumen; later, recanalization occurs. The midgut also forms a U-shaped loop that herniates (bulges) through the primitive umbilical ring into the extra embryonic coelom. The midgut loop consists of the cranial limb, that forms the jejunum and the upper part of the ileum, and the caudal limb, that forms the cecum and the appendix, the lower part of the ileum, the ascending colon, and the proximal two third of the transverse colon. The midgut loop rotates about 270 degrees counterclockwise around the superior mesenteric artery to return into the abdominal cavity.
The distal end of the transverse colon, the descending colon, the sigmoid colon, rectum, and the upper anal canal all form from the hindgut. All of these structures receive their blood supply from the inferior mesenteric artery.
The cranial end of hind gut develops into the distal one-third of the transverse colon, descending colon, and sigmoid colon. The terminal end of the hindgut is named the cloaca. The cloacal membrane is formed where the cloaca meets the ectoderm of the proctodeum. The cloaca is separated by the urorectal septum into the upper anal canal and the urogenital sinus. The cloacal membrane is portioned by the urorectal septum into the anal membrane and the urogenital membrane.
The upper anal canal also develops from the hindgut. The lower anal canal develops from the invagination of surface ectoderm due to the proliferation of mesoderm surrounding anal canal (proctodeum). The hindgut and proctodeum are involved in the embryologic formation of the entire anal canal.
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