GI System Embryology

• The lecture covers the embryology of the gastrointestinal (GI) system, focusing on the development, structure, and potential abnormalities.
• It begins by outlining the intended learning outcomes, which include assessing the embryological origins of the GI system and its divisions.
• An additional learning outcome is describing the microstructure and vasculature.
• Correlating tissue growth and rotational events that shape the foregut, midgut, and hindgut are another desired learning outcome.
• Determining various GI genetic anomalies related to embryological formation makes the last learning outcome.

Primitive Gut

• The primitive gut forms through cephalocaudal and lateral folding of the embryo.
• It is derived from the endoderm.
• The four sections of the primitive gut are the pharyngeal gut (pharynx), foregut, midgut, and hindgut.

Sections of the Gut

• The foregut extends from the distal esophagus to the proximal half of the second part of the duodenum.
• The midgut ranges from the distal half of the second part of the duodenum to the proximal two-thirds of the transverse colon.
• The hindgut extends from the distal one-third of the transverse colon to the rectum.

Molecular Regulation of Gut Tube Development

• Retinoic acid is highest in the colon and causes transcription in different areas of the gut tube.
• Sonic hedgehog (SHH) expression determines the type of structure that forms from the gut tube, such as the stomach and duodenum.
• Establishes HOX genes which help form mid/hind gut regions (small intestine, cecum, colon and cloaca).

Mesenteries

• Mesenteries connect organs to the body wall and consist of double layers of the peritoneum.
• Organs can be intraperitoneal or retroperitoneal.

Foregut: Esophagus

• The esophagus develops at four weeks from the dorsal tracheoesophageal septum.
• Esophageal abnormalities, such as esophageal atresia or fistula, can occur.

Foregut: Stomach

• During the fourth week, the stomach undergoes longitudinal rotation and rotation along the anteroposterior axis.
• This rotation forms the greater and lesser curvatures.
• The stomach rotates 90 degrees clockwise.

Foregut: Omenta

• Omenta are fused peritoneal folds that connect the stomach and duodenum with other abdominal organs.
• The greater and lesser omentum are key structures.

Foregut: Duodenum, Liver, Gallbladder, and Pancreas

• The duodenum is the terminal part of the foregut and the cephalic midgut, with a C-shaped structure.
• Liver and gallbladder development begins in the third week as the liver bud.
• Liver cords form the parenchyma and biliary ducts; Kupffer cells also form.
• The pancreas forms from dorsal and ventral buds from the endodermal duodenum, which form the definitive pancreas.

Midgut

• The midgut begins distal to the bile duct in the duodenum and extends to the proximal two-thirds of the transverse colon.
• Rapid elongation with the primary intestinal loop causes physiological umbilical hernia to accommodate organs in the extraembryonic cavity.
• The midgut rotates in a counter-clockwise direction 270 degrees.

Hindgut

• The hindgut gives rise to the distal third of the transverse colon, the descending colon, the sigmoid, the rectum, and the upper part of the anal canal.
• Rectoanal fistulas and Congenital Megacolon/Hirschsprung Disease are potential clinical correlations.
• The hindgut also forms the lining of the bladder and urethra.
• The cloaca becomes the anorectal canal, and the allantois becomes the urogenital sinus.

Vasculature

• The foregut is supplied by the celiac artery.
• The midgut is supplied by the superior mesenteric artery.
• The hindgut is supplied by the inferior mesenteric artery.

Gut Defects: Atresia

• Gut defects include ventral wall closure defects like gastroschisis/omphalocele.
• Failure of the GI tract "part" to develop leads to Esophageal Atresia/Duodenal A./Jejunal A.
• Other defects include peristalsis failure (Hirschsprung Disease), narrowed passage (Pyloric Stenosis), lack of passage (Meconeum Ileus), twisting defect (Volvulus), and patent passage (Vitelline Fistula).

Key Points

• The epithelium of the digestive system and the parenchyma of its derivatives originate from the endoderm.
• Connective tissue, muscular components, and peritoneal components originate in the mesoderm.
• The gut system extends from the oropharyngeal membrane to the cloacal membrane and is divided into the pharyngeal gut, foregut, midgut, and hindgut.

Short Reflection for Embryo SGD

• Gut microbiome and its understanding based on the Journal is another key topic.