Embryonic Folding and Primitive Gut Formation

• At the end of the 3rd week, the flat embryonic disc folds and bulges into the amniotic cavity.

Types of Folding

• Cephalo-caudal folding leads to the formation of head and tail folds.
• Lateral folding leads to the formation of lateral folds.

Results of Folding

• The amniotic cavity almost completely surrounds the embryo.
• A large part of the yolk sac cavity is incorporated into the embryo's body, forming the primitive gut which is lined by endoderm.
• The part of the gut in the head fold is the foregut, in the tail fold is the hindgut, and between the lateral folds is the midgut.

Primitive Gut Tube

• Formed from the incorporation of the dorsal part of the yolk sac into the embryo, due to lateral and craniocaudal folding.
• Extends from the oropharyngeal membrane to the cloaca.
• It's divided into the foregut, midgut, and hindgut.
• The lining mucosa and glands are derived from endoderm, while other components are derived from visceral mesoderm.
• By the fifth week, the caudal part of the foregut, the midgut, and a major part of the hindgut are suspended from the abdominal wall by the dorsal mesentery.
• The dorsal mesentery extends from the lower end of the esophagus to the cloacal region of the hindgut.

Mesentery

• Defined as a double layer of peritoneum that encloses an organ and suspends it from the posterior abdominal wall.
• Intraperitoneal organs are enclosed and connected to the body wall by double layers of peritoneum.
• Retroperitoneal organs lie against the posterior body wall and are covered by peritoneum on their anterior surface only.

Dorsal Mesentery Forms

• Dorsal mesogastrium in the region of the stomach.
• Dorsal mesoduodenum in the region of the duodenum.
• Dorsal mesocolon in the region of the colon.
• Mesentery proper for the jejuna and ileal loops.

Ventral Mesentery

• Exists only in the region of the terminal part of the esophagus, the stomach, and the upper part of the duodenum.
• Growth of the liver into the mesenchyme of the septum transversum divides the ventral mesentery into the lesser omentum and the falciform ligament.
• The lesser omentum extends from the lower esophagus, stomach, and upper duodenum to the liver.
• The falciform ligament extends from the liver to the ventral body wall.

Esophagus

• The foregut divides into the esophagus dorsally and the trachea ventrally due to the tracheoesophageal folds, which fuse to form the tracheoesophageal septum.
• The esophagus initially shortens but lengthens with the descent of the heart and lungs.
• During development, the endodermal lining of the esophagus proliferates rapidly, obliterating the lumen, which recanalizes later.

Esophageal Abnormalities

• Esophageal atresia can occur if the tracheoesophageal septum deviates too far dorsally, causing the esophagus to end as a closed tube, and is sometimes associated with VATER or VACTERL syndromes.
• Esophageal stenosis, usually in the lower third, may be caused by incomplete recanalization, vascular abnormalities, or accidents that compromise blood flow.
• Congenital hiatal hernia occurs when the esophagus fails to lengthen sufficiently, pulling the stomach up into the esophageal hiatus through the diaphragm.

Stomach

• The stomach appears as a fusiform dilation of the foregut in the fourth week of development.
• The stomach rotates 90° clockwise around its longitudinal axis.
• Its left side faces anteriorly and its right side faces posteriorly.
• The left vagus nerve innervates the anterior wall and the right nerve innervates the posterior wall.
• The original posterior wall of the stomach grows faster than the anterior portion, forming the greater and lesser curvatures.
• The caudal/pyloric part moves to the right and upward, and the cephalic/cardiac portion moves to the left and slightly downward.
• The stomach's axis runs from above left to below right.