Digestive Embryology
Table of Contents

Development of Gut

 Gut tube: the gastrointestinal tube derived from the dorsal aspect of the yolk sac after incorporation into the embryonic body.
- wall: three layers of inner epithelial lining, middle muscular layer and outer mesenchymal tissue.
- lumen: the tube extends cranially and caudally and the primitive mouth is derived from ectoderm of the stomodeum and anal pit from proctodeum.
 Foregut: the first section of the GI tract composed of pharynx, lower respiratory tract, esophagus, stomach, liver, biliary apparatus, pancreas and half of the duodenum.
 Development of esophagus:
- part of the developing foregut immediately caudally to the pharynx
- separated from trachea ventrally by the tracheoesophageal septum
- initially short but elongate rapidly. Proliferation of endodermal epithelium can block the lumen but apoptosis at the end of the embryonic period recanalize the tube.
 Clinical significance of esophagus:
- oesophageal atresia: incomplete portioning of the tracheoesophageal tube by the septum can cause blockage to the oesophagus tube and a fistula connection between the two.
- Congenital hiatal hernia: failure of oesophagus to elongate enough causing part of the stomach to develop above the diaphragm and become strangulated by the diaphragmatic oesophageal orifice.
 Development of stomach:
- 4th week, dilation of foregut indicate primordial stomach
- Enlarged and broadens ventral-dorsally. Dorsal is faster than ventral hence forming the greater curvature and the other lesser curvature.
- Rotation of the stomach 90o clockwise with ventral and dorsal border becoming right and left side respectively. The cranial end tilts inferiorly and caudal end superiorly.
- The stomach is suspended from dorsal wall by the dorsal mesentery (mesogastrium). During rotation, it is carried to the left forming the greater omentum while the ventral mesentery forms the lesser omentum
 Clinical significance of stomach:
- congenital hypertrophic pyloric stenosis: thickening of the pylorus by the hypertrophy of circular and longitudinal muscles cause severe stenosis of pyloric canal (genetic defect). This leads to obstruction of the passage of food and consequently projectile vomiting of stomach content.
 Development of duodenum:
- Develops from the caudal part of the foregut to the proximal part of the midgut. Join bile duct distally
- grows rapidly and forms C-shape loop that project ventrally
- undergoes same method of proliferation and recanalization
 Midgut: the second part of the gut tube consisting of distal half of duodenum, jejunum, ileum, caecum, appendix. Ascending colon, and proximal two third of transverse colon
- midgut loop: the midgut elongates and forms a U-shaped tube ventrally
- physiological herniation: migration of the midgut tube into the proximal part of the umbilical cord because there is not enough room in the abdomen for the rapidly growing midgut.
- Rapid growth of cranial end to form small intestine and caudal end to form large intestine
- The midgut then rotates counterclockwise 90o within the umbilical cord around superior mesentery artery. During reduction of the hernia, the small intestine returns first and large intestine spin 180o anti-clockwise.
 Clinical significance of midgut:
- meckel’s diverticulum: an ileal diverticulum representing the remnant of the yolk sac stalk. It contains gastric and pancreatic tissue and acid secretion can cause ulceration and bleeding.
- non-rotation of midgut: midgut does not rotate as it re-enters the abdominal and caudal limb returns first and small intestine on the right side. Tangling of stomach can obstruct mesentery artery.
- umbilical hernia: the midgut herniates in the umbilical cord that fails to adequately close and no reduction. Result is a protruding mass of small intestine and omentum
 Hindgut: last part of the gut tube made up of distal third of the transverse colon, descending colon, sigmoid colon, rectum and upper two third of anal canal
- descending urorectal septum divides the rectum and urethra.
 Clinical significance of hindgut:
- megacolon: a segment of the colon is dilated because the lack of autonomic ganglions distal to the dilated segment. Dilation is result from failure of peristalsis in the aganglionic segment and prevents intestinal movements. Abnormality due to failure of neural crest cells to migration into the walls of colon
- rectal atresia: the anal canal and rectum are separated due to abnormal recanalization and lack of blood supply
- imperforated anus: failure of anal membrane to rupture. Undesolved leaving a web of membrane called congenital web.

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