The digestive system is derived from the endorerm and develops from the primitive gut, which is a derivative of the yolk sac. At its proximal and distal ends, however, the digestive system receives contribution from the ectoderm. At the proximal end the ectoderm-lined stomodeum makes a substantial contribution to the oral cavity. At the distal end of the digestive tube the ectoderm-lined proctodeum contributes to the formation of the terminal one-third of the anal canal.
Due to folding of the embryonic disc during the 4th week of development, the dorsal part of the yolk sac becomes incorporated into the embryo as a tubular passage called primitive gut. This tube consists of an inner lining of endoderm, which is covered externally by a layer of splanchnic meso-derm. The primitive gut is divided into
three parts called foregut, midgut and hindgut. The foregut lies within the head fold, whereas the hindgut is located within the tail fold. The middle part of the primitive gut, called midgut, is flanked on its two sides by the lateral body folds and remains temporarily connected with the main yolk sac by means of a tubular stalk called yolk stalk or vitelline duct. The primitive gut is initially closed at its cranial end by the buccopharyngeal (oropharyngeal) membrane and at its caudal end by the cloacal membrane.
The endoderm of the primitive gut gives rise to the epithelial lining and associated glands of the digestive tract. The smooth muscle and connective tissue of the gut wall are derived from the splanchnic mesoderm which surrounds the endoderm. ,
The developing gut tube is supplied by ventral branches of the descending aorta. About five aortic branches supply the thoracic part of the foregut. The remainder of the gut tube is supplied by three arteries: (1) the celiac artery, which supplies the abdominal part of the foregut and its derivatives (ie, pancreas, liver and biliary tract), (2) the superior mesenteric artery that supplies the midgut and (3) the inferior mesenteric artery, which supplies the hindgut.
The primitive gut is initially a straight tube suspended throughout most of its length by a dorsal mesentery, which is derived from the splanchnic mesoderm of the gut and extends like a curtain in the midplane and provides the pathway through which blood vessels and nerves reach the gut wall. As the development advances, the primitive dorsal mesentery becomes highly modified (the details of the modifications will be described later)
During the 5th and 6th weeks of development, the endodermal epithelium of the gut tube proliferates until it completely occludes the lumen of the primitive gut. In the 7th week vacuoles appear in the tissue occluding the lumen of the gut. These vacuoles gradually merge with each other to restore the cavity within the gut. In this way this way the gut tube becomes fully recana-teed by the end of the 8th week. In-complete recanalization can lead to certain congenital anomalies which will be described later