Normal Embryology and Anatomy

In the normal embryo, the abdominal portion of the foregut is visibly divided into the esophagus, stomach and proximal duodenum by the fifth week (Larsen 2001). During the fifth week, the stomach starts out as a straight tube but since there is differential growth of its dorsal and ventral wall, the greater curvature will become more elongated than the lesser curvature. Continued differential expansion of the superior part of the greater curvature gives rise to the fundus and cardiac incisure by the end of the seventh week.

During the seventh and eighth weeks, the stomach will rotate 90° around a craniocaudad axis. The former anterior portion will form the right side of the stomach and the former posterior portion becomes the left side of the stomach.

The stomach is relatively fixed in the abdominal cavity at the esophagogastric junction and at the pyloroduodenal junction. Furthermore, there are four different ligaments which will fix the stom ach: the gastrohepatic ligament, the gastrosplenic ligament, the gastrocolic ligament and the gastro-phrenic ligament.

The stomach is classically divided into the fundus, being the upper most portion, the body, which is the largest part, the antrum, at the distal end, and the pylorus.

Due to the above mentioned rotations of the stomach, the presumptive duodenum will bend into a C shape and will be displaced to the right until it lies against the dorsal body wall and becomes secondarily retroperitoneal. A system of digestive glands develops from endodermal buds of the duodenum, including the liver parenchyma, the gall bladder and their ducts (derived from the hepatic diverticulum), as well as the pancreatic parenchyma and its ducts (derived from the fusion of the dorsal and ventral pancreatic ducts) (Fig. 3.1).

The duodenum is typically divided into four portions. The first portion is relatively mobile in contrast to the rest of the duodenum which is fixed in the retroperitoneal cavity. The second descending portion of the duodenum is in close contact with the pancreas and receives the common bile duct and duct of Wirsung in its midportion. The third portion is horizontally oriented and is ventrally crossed by the superior mesenteric artery (SMA) and superior mesenteric vein (SMV). The fourth ascending portion will turn ventrally to the duodenojejunal junction, which is additionally held in place by the ligament of Treitz.

The stomach and proximal duodenum are derived from the embryonic foregut and will therefore receive their blood supply from the celiac axis. Part of the third portion and the entire fourth portion of the duodenum depend on the superior mesenteric artery because of their midgut origin.

The gastrointestinal (GI) tract also starts out as a short, straight tube (Larsen 2001). This tube will then elongate and will become fixed in the abdominal cavity by a very complicated process. The proximal duodenojejunal loop and the distal cecocolic loop are able to pull adjacent bowel along with them. Before the tenth week of embryonic life, both loops will independently rotate 270° in a counterclockwise direction around the SMA and return to the abdominal cavity. This results in the actual position of the duodenum with the duodenojejunal junction at the left side in the upper abdomen. On the other hand, the cecum will finally lie in the right lower quadrant and the transverse colon anterior to the SMA. When the process of rotation is finalized, the bowel is fixed in this position by the mesentery. The normal mesentery extends from the left upper quadrant at the ligament of Treitz to the cecum in the right lower quadrant and is broad based.

The bolus from the esophagus will arrive in the stomach, where digestion takes place until ingredients are sufficiently broken down to be passed into the intestine for further digestion and absorption. The gastric wall consists of the mucosa with the muscularis mucosae, submucosa, muscularis externa and serosa. The mucus produced by the mucous surface cells and mucous cells of the glands protects the stomach lining from damage due to hydrochloric acid and pepsin secreted by the gastric glands. The rugae or longitudinal folds of the empty stomach are consistent with the mucosa and

Esophagus

Stomach

Esophagus

Stomach

Lung bud

Liver

Ventral 1 n Dorsal } Pancreas

22 days

Lung bud

Esophagus

Liver

Ventral 1 n Dorsal } Pancreas

Esophagus

Trachea

5-6 weeks

Trachea

22 days

5-6 weeks

Fig. 3.1. Normal embryology and anatomy of the foregut and organs of digestion submucosa. The muscularis externa of the stomach consists of three smooth muscle layers: an inner oblique, a middle circular and an outer longitudinal. These muscle layers are responsible for the 'gastric motility' in receiving large volumes of ingested food and fluids during a meal, in mixing the ingested food with gastric juices and in propelling the gastric contents into the small intestine.

The wall of the duodenum is, like the gastric wall, divided into the mucosa, submucosa, muscu-laris externa and serosa. The main function of the duodenum is the digestion of nutrients (Ross and Romrell 1989).

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