Development of the Digestive System

DEVELOPMENT OF THE DIGESTIVE SYSTEM 0. Introduction The digestive system develops from the primitive intestine. The endodermis is at the origin of the epithelia (which line the lumen of the different segments of the digestive tract), glandular derivatives and lymphoid organs; the intra-embryonic mesenchyme located around the primitive intestine is at the origin of the muscular and connective constituents of the walls. The primitive intestine is made up of three parts which can be distinguished as follows: The anterior primitive intestine, in its cephalic part, called pharyngeal intestine, lines the lumen of the gill region. It is at the origin of the oral cavity and the pharynx with their glandular and lymphoid derivatives as well as the respiratory diverticulum. In its caudal part, the primitive anterior intestine is the origin of the esophagus, the stomach, part of the duodenum, the liver and the bile ducts and the pancreas. The middle primitive intestine is the origin of the terminal part of the duodenum, jejunum and ileum, cecum, appendix and a large part of the colon (ascending segment and 2/3 of the transverse colon). The posterior primitive intestine is the origin of the rest of the colon (1/3 distal to the transverse colon, descending colon, sigmoid colon), rectum and anal canal by division of the cloaca region. 1. AT THE END OF DELIMITATION OF THE EMBRYO 1.1. Formation of the primitive intestine The primitive intestine is isolated, during demarcation, at the fourth week of development, by tubulation of the roof of the rectocele. It remains in communication in its middle part with the yolk sac through a narrow channel, the yolk canal, enclosed in the umbilical cord. 1.2. Setting up the mesos During the delimitation of the embryo, at the same time as the primitive intestine is individualized, the joining of the lateral blades of the splanchnic mesoblast on the midline leads to the formation of a thin partition within the embryo. This mesoblastic partition, initially dorso-ventral, contains the primitive intestine and maintains it suspended within the intra-embryonic coelom (future pleuroperitoneal cavity) while ensuring its vascularization via the arterial branches of the dorsal aorta which circulate there. This thin partition takes the generic name of meso, although with specific names depending on the levels (mesogastric at the level of the stomach, mesentery at the level of the ileum, mesocolon at the level of the colon, etc.). The dorsal meso, present all along the digestive tract, connects the primitive intestine to the dorsal side of the embryo; the ventral meso, only present at the level of the abdominal segment of the primitive anterior intestine, connects this segment to the ventral side of the embryo. These mesos will evolve to follow the transformations and changes in position of the digestive tract, part of which remains intraperitoneal while other segments will become retroperitoneal. 1.3. Vascularization of the primary intestine The primitive intestine is vascularized by branches coming from the ventral surface of the dorsal aorta: 5 branches ensure the vascularization of the thoracic portion of the primitive intestine and 3 that of the portion abdominal; they travel through the mesos and approach the different segments of the digestive tract through their peripheral layer. The territories irrigated by these last three branches define three zones of the primitive intestine at the origin of most of the definitive digestive tract: the abdominal portion of the anterior intestine, vascularized by the Coeliac trunk, the midgut, irrigated by the superior mesenteric artery and the posterior intestine dependent on the artery inferior mesenteric.