Notes on Development of the GIT

Introduction

The GI tract lining is endodermal in origin; in the mouth and anal canal, parts are derived from ectoderm (stomatoeum and proctodeum). After head and tail folding, the primitive gut forms from part of the yolk sac cavity. The foregut is cranial to the yolk sac communication, the hindgut is caudal, and the intervening portion is the midgut. The gut communicates with the yolk sac via a yolk stalk; the foregut/midgut boundary and hindgut boundary are formed by membranes (buccopharyngeal and cloacal) that later disappear, allowing the gut to open to the exterior at both ends. During gut formation, the dorsal aorta forms a midline artery with vitelline branches to the gut; eventually three main ventral branches persist: the coeliac (foregut), the superior mesenteric (midgut), and the inferior mesenteric (hindgut). The allantoic diverticulum extends into the hindgut. The cloaca divides into a ventral primitive urogenital sinus and a dorsal primitive rectum, separated by the urorectal septum; the cloacal membrane divides into a ventral urogenital membrane and a dorsal anal membrane. The anal membrane later degenerates, forming the anal canal; the anal pit contributes to the anal canal and the surrounding structures.

Derivatives of Foregut

Derivatives of the foregut are mainly supplied by the celiac trunk; the exception is the esophagus, where the abdominal part is supplied by the celiac trunk and the thoracic part by branches of the aorta. Foregut derivatives include: part of the floor of the mouth and the tongue, pharynx, derivatives of the pharyngeal pouches, thyroid, esophagus, stomach, and the duodenum up to the major duodenal papilla; liver and extrahepatic biliary system; and the pancreas.

Derivatives of Midgut

Derivatives of the midgut are supplied by the superior mesenteric artery. They include: the duodenum (descending part distal to the major papilla; horizontal and ascending parts), jejunum, ileum, caecum and appendix, ascending colon, and the right two-thirds of the transverse colon.

Derivatives of Hindgut

Derivatives of the hindgut are supplied by the inferior mesenteric artery. They include: the left one-third of the transverse colon, descending and pelvic colon, rectum, upper part of the anal canal, and parts of the urogenital system derived from the primitive urogenital sinus. The endoderm of foregut, midgut, and hindgut forms the epithelial lining, while smooth muscle, connective tissue, and peritoneum are derived from splanchnic mesoderm.

Derivation of individual parts of the Alimentary Tract

Oesophagus

The esophagus forms from the foregut segment between the pharynx and stomach. It elongates during neck formation as the heart, lungs, and diaphragm descend. The musculature is derived from mesenchyme surrounding the foregut: the upper two-thirds form striated muscle, and the lower one-third forms smooth muscle.

Stomach

The stomach begins as a fusiform dilation of the foregut, just distal to the esophagus. Differential growth creates the greater and lesser curvatures. The dorsal mesogastrium attaches the stomach to the posterior abdominal wall, while the ventral mesogastrium attaches to the septum transversum. The ventral mesogastrium becomes the lesser omentum; the dorsal mesogastrium is divided by the spleen into the gastrosplenic and lienorenal ligaments. The stomach rotates about its longitudinal axis by $90^b0$ (clockwise); the ventral border becomes the lesser curvature, and the dorsal border becomes the greater curvature.

Duodenum

The superior (first) part and the upper half of the descending (second) part of the duodenum arise from the foregut; the rest from the midgut. The gut forms a loop (duodenal loop) attached to the posterior abdominal wall via the mesoduodenum, which later fuses with the posterior peritoneum, making most of the duodenum retroperitoneal. The duodenal cap corresponds to the proximal part near the pylorus and is related to the ventral pancreas. Proximal duodenum is supplied by branches of the celiac trunk, distal by the superior mesenteric artery.

Jejunum and Ileum

Jejunum and most of the ileum derive from the prearterial segment of the midgut loop.

Caecum and Appendix

The caecal bud, from the postarterial segment of the midgut loop, enlarges to form the caecum; the distal part forms the appendix. In fetal life, the appendix typically arises from the apex of the caecum; later, growth of the lateral caecum shifts the appendiceal attachment to the medial side.

Ascending and Transverse Colons

The ascending colon develops from the postarterial midgut; the right two-thirds of the transverse colon from the midgut postarterial segment, while the left one-third of the transverse colon arises from the hindgut.

Descending, Rectum, and Anal Canal

The descending colon derives from the hindgut. The rectum comes from the primitive rectum (dorsal cloaca); the upper rectum is attributed to hindgut proximal to the cloaca. The anal canal is formed partly from endoderm of the primitive rectum and partly from ectoderm of the anal pit (proctodeum). The junction between endodermal and ectodermal lining is marked by the pectinate line.

Rotation of the Gut

The midgut loop initially lies outside the abdominal cavity in an extraembryonic (yolk sac) position. It consists of a prearterial (proximal) and a postarterial (distal) segment. Viewed ventrally, the loop undergoes an anticlockwise rotation of $90^b0$, bringing it to a horizontal plane with the prearterial segment on the right and the postarterial on the left. The prearterial segment lengthens to form the coils of the jejunum and ileum, which lie initially to the right of the distal loop. As the coils return to the abdomen, there is further anticlockwise rotation, and the jejunum and ileum pass behind the superior mesenteric artery to the left; the duodenum ends up behind the artery, and the coils occupy the posterior and left part of the abdomen. The prearterial segment then returns to the abdomen and rotates anticlockwise so that the transverse colon lies anterior to the SMA and the caecum moves to the right. Overall, the gut undergoes a total of $270^b0$ anticlockwise rotation.\n

Fixation of the Gut

After rotation, certain parts fuse their mesenteries with the posterior abdominal wall and become retroperitoneal: the duodenum, ascending colon, descending colon, and rectum. The remaining portions retain mesenteries (the small intestine mesentery, transverse mesocolon, and pelvic mesocolon).

Developmental Anomalies of the GIT

Congenital obstruction can arise from multiple causes: atresia (absence or replacement by fibrous tissue, or a lumen-sealing septum), stenosis (narrowing), and nondevelopment of the enteric nerve plexuses (Hirschsprung's disease) leading to megacolon. Other anomalies include abnormal thickening of the pyloric muscle (congenital pyloric stenosis). Omphalocele occurs when abdominal contents herniate through the umbilical ring and remain outside the body, covered by a membrane; gastroschisis occurs with a ventral wall defect, exposing intestines to amniotic fluid without a covering membrane. Omphaloceles are often associated with other anomalies and elevated AFP. These conditions may be associated with trisomies or Beckwith-Wiedemann syndrome.