Digestive Tract Development

Development of Digestive Tract

• The digestive tract forms from the primitive gut
• The primitive gut is derived from the endodermal yolk sac
• Digestive tract development starts at the 4th week of Intrauterine Life (IUL)
• The cephalocaudal and lateral folding of the embryo incorporates endoderm into the embryonic disc, forming the primitive gut tube
• The gut tube extends from the buccopharyngeal membrane (cranial) to the cloacal membrane (caudal)
• The midgut is temporarily connected to the yolk sac via the vitello-intestinal duct/yolk stalk
• The foregut is cranial to this connection and the hindgut is caudal

Germ Layer Contribution

• The endoderm becomes the mucosal epithelium, mucosal glands, and submucosal glands
• The splanchnic/visceral mesoderm forms the muscular walls of the digestive tract including the lamina propria, muscularis mucosae, submucosa, muscularis externa, and adventitia/serosa, also forms the mesenteries
• The somatic mesoderm gives rise to the parietal peritoneum
• The neural crest forms neurons and nerves of submucosal and myenteric plexuses

Foregut Derivatives

• The foregut derivatives are the primitive pharynx and respiratory system
• The tracheoesophageal septum divides the foregut into the esophagus and trachea
• Other derivatives:
  • Esophagus
  • Stomach
  • 1st part of the duodenum
  • Derivatives of pharyngeal pouches
  • Liver
  • Pancreas
  • Gallbladder

Esophagus Development

• Develops between the pharynx and stomach
• The foregut develops a median laryngotracheal groove
• The groove enlarges to form a diverticulum
• The tracheoesophageal fold develops and divides the tube: ventrally into the trachea, and dorsally into the esophagus
• Initially, the esophageal lumen is occluded by endodermal cells
• Later, these cells break down, and the esophagus recanalizes

Stomach Development

• Fusiform dilatation of the foregut distal to the esophagus occurs at the 4th week of IUL
• This dilatation presents 2 borders: ventral and dorsal
• It also presents 2 surfaces: right and left
• The dilatation has 2 ends: upper (cardiac) and lower (pyloric)
• The dorsal border grows more, forming the greater curvature, while the ventral border forms the lesser curvature

Stomach Rotation

• Rotation occurs along the longitudinal axis
• It initially rotates 90 degrees so that:
  • The left surface becomes the anterior surface
  • The right surface becomes the posterior surface
• During the anteroposterior axis rotation, the cephalic and caudal ends of the stomach lie in the midline
• After rotation:
  • The cardiac end moves from the midline to the left and slightly downward
  • The pyloric end moves from the midline to the right and slightly upward

Duodenum Development

• Develops from two sources:
  • Proximal half (1st & 2nd part up to opening of bile duct) from the foregut
  • Distal half (2nd part below the opening of the bile duct with 3rd & 4th part) from the midgut

Midgut Derivatives

• Lower half of the duodenum
• Small intestine (jejunum, ileum, caecum, appendix)
• Ascending colon
• Right 2/3rd of the transverse colon
• The midgut elongates to form a U-shaped primary intestinal loop
• It is suspended dorsally in the midline by a double-layered dorsal mesentery
• The midgut communicates ventrally with the yolk sac via the vitelline duct/yolk stalk

Physiological Hernia

• The midgut loop elongates in the 6th week of IUL
• The midgut loop and liver grow simultaneously and cannot be accommodated by the abdominal cavity
• The midgut loop herniates through the umbilical opening into the extraembryonic celom
• This herniation of the loop through the umbilical opening is physiological hernia

Reduction of Hernia

• By the end of the 10th week, the abdominal cavity increases in size, and the loops re-enter the abdominal cavity
• By the 12th week, all loops settle inside the abdominal cavity

Midgut Rotation

• After formation, the loop lies outside the abdominal cavity
• The loop has: prearterial/proximal/cranial and postarterial/distal/caudal segments
• Before returning to the abdominal cavity, it undergoes 90 degree anticlockwise direction
• The cranial segment moves to the right, and the caudal segment moves to the left
• The cranial segment elongates and forms the jejunum and ileum, which lie on the right side and outside the abdominal cavity

Cecum and Appendix

• The caudal segment gives rise to the cecal bud (cecum and appendix)
• Coils of the jejunum and ileum take another 2nd 90 degree rotation when going back into the abdominal cavity
• The caudal segment returns to the abdominal cavity after the 3rd 90 degree anti-clockwise rotation
• Now, the cecum & appendix lie on the right side of the abdominal just below the liver
• The descent of the cecum takes place later
• This rotation is important for the definitive relations of the intestines

Hindgut Derivatives

• Left 1/3rd of the transverse colon
• Descending colon
• Sigmoid colon
• Rectum
• Upper part of the anal canal
• The cloaca is the dilated part of the hindgut, which is closed by the cloacal membrane
• During the 7th week, the cloacal membrane disappears, giving rise to two openings with the urorectal septum:
  • Urogenital sinus
  • Anal opening
• The tip of the urorectal septum later forms the perineal body

Urinary Bladder, Rectum & Anal Canal Development

• The urogenital sinus; the broad ventral part of cloaca, which gives rise to the urinary bladder and urethra
• The anal opening; the narrow dorsal part of the cloaca, called the primitive rectum, which gives rise to the rectum and the upper part of the anal canal

Blood Supply

• Foregut: Celiac artery
• Midgut: Superior mesenteric artery
• Hindgut: Inferior mesenteric artery

Anal Canal Development

• It develops from 2 sources:
  • Hindgut develops the upper half from the primitive rectum
  • Proctodeum develops the lower half from the anal pit (proctodeum)
• The proctodeum is an ectodermal depression below the anal membrane
• Initially, this anal membrane separates the hindgut and proctodeum
• The later membrane rupture allows the two parts to communicate
• The site of the anal membrane termed as the Pectinate line

Congenital Umbilical Hernia

• Omphalocele is a herniation of bowel loops through the umbilical opening that fails to return to the abdominal cavity
• Gastroschisis is a herniation of bowel loops through the umbilical opening when the umbilicus fails to close properly (weak anterior abdominal wall)
• Meckel’s diverticulum is a failure or incomplete obliteration of the vitello-intestinal duct

Rectal Fistula

• An abnormal connection between two hollow viscera

Congenital Megacolon (Hirschsprung’s Disease)

• A segment of the colon is dilated