Gut Embryology Study Handouts

Gut Embryology The gut is an endoderm-derived structure. At approximately the sixteenth day of human development, the embryo begins to fold ventrally (with the embryo's ventral surface becoming concave) in two directions: the sides of embryo fold in on each other and the head and tail fold toward one another. Primitive gut is formed entirely from endoderm when the dorsal part of the yolk sac incorporates into the embryo during the process of folding. • Endoderm of primitive gut will produce the epithelial lining and glands of the GI tract • Mesoderm is either somatic or splanchnic − Somatic mesoderm – forms parietal peritoneum − Splanchnic mesoderm – forms visceral peritoneum and smooth muscle Abnormalities include – Oesophageal atresia and tracheoesophageal Formation of the oesophagus: fistula. Food into lungs During week 4, the foregut develops a small respiratory diverticulum [1] on ventral surface due. adjacent to the pharyngeal gut. This tracheobronchial diverticulum subsequently elongates + separates gradually from the dorsal foregut [2] through the formation of the oesophagotracheal septum [3] to become the primitive respiratory tract. Embryology of the foregut Foregut: distal oesophagus, stomach, proximal duodenum, liver, larynx to alveoli, + superior part of pancreas Formation of the stomach: The stomach appears as a fusiform dilatation of the foregut [1] in the 4th week of development. It appearance and position change greatly because of different rates of growth in various regions of its wall [2] and changes in position of surrounding organs. Positional changes are easily explained by its rotation around a longitudinal and an anteroposterior axis. The dorsal wall of the stomach grows faster than the ventral wall. The differential growth will form the greater and lesser curvatures of the stomach 2 The left and right vagus nerves flank the left and right sides of developing stomach respectively 1 The developing stomach is attached to the body walls by the dorsal and ventral mesenteries The stomach rotates 90 degrees clockwise around its longitudinal axis [3] (produces a lesser sac space) 3 After re-orientation, the left After re-orientation, the greater vagus nerve is now located curvature (embryonic dorsal on the anterior side of side of the stomach) now faces stomach and the right vagus to left of body, and lesser Formation ofthethe duodenum: nerve is located on the curvature (the embryonic ventral posterior side→left The and duodenum rotate about a ventrodorsal axis (A-P axis) pulling the and right side of stomach the stomach) faces to the anterior vagal trunks right end of the stomach up. In the 8th week these rotations pull the duodenum into a C-shaped position. The postnatal positions of stomach and duodenum are attained Formation of the greater and lesser omentum: − The dorsal mesentery has thinned and now hangs from the greater curvature. The greater omentum → later fuse to form one thick sheet ~4 layers peritoneum − The ventral mesentery is now attached to the developing liver and has formed the lesser omentum Formation of the Peritoneal cavity (Lesser and greater sacs): The development of the omenta and rotations of foregut structures produce distinct spaces of the peritoneal cavity: − The space anterior to the stomach is called the greater sac. − The space posterior to the stomach is called the lesser sac / omental bursa The greater and lesser sacs communicate via a small opening (located near the hilum of the liver) called the epiploic foramen