Digestive System Development

Digestive System Development

• The digestive system develops through a series of stages, including the formation of the primitive gut tube.
• Cranial closure of the oropharyngeal membrane and caudal closure of the cloacal membrane occur.
• Cephalocaudal and lateral folding of the embryo initiate the digestive system's development.
• The endoderm-lined yolk sac cavity becomes incorporated into the embryo to form the primitive gut.
• The primitive gut tube includes the foregut, midgut, and hindgut.
• The yolk sac and allantois are other parts of the endoderm-lined cavity; they remain outside the embryo.
• The foregut and hindgut are blind-ending tubes; the midgut is temporarily connected to the yolk sac by the vitelline duct or yolk stalk.
• The epithelial lining of the digestive tract, glands (liver, pancreas), muscle, connective tissue, and peritoneum form the gut wall.
• Recanalization of the gut tube occurs in the sixth and ninth weeks as the endodermal lining completely occludes the lumen; re-opening then occurs.
• Incomplete or abnormal recanalization can cause duplication of the lumen or stenosis of the gut tube.

Molecular Regulation of Gut Tube Development

• Differentiation is guided by lateral folding, with the endoderm expressing sonic hedgehog (SHH) expression.
• Interactions between the endoderm (epithelium) and mesoderm drive the differentiation of various gut regions and their derivatives.
• HOX genes and mesodermal factors drive stabilization, with HOX 9-13 genes playing a role in the process.
• SHH expression plays a critical role in the process.

Mesenteries

• Mesenteries are double layers of peritoneum that surround and connect the primitive gut to the body wall.
• Peritoneal ligaments attach organs to each other or to the body wall.
• The dorsal mesentery gives rise to the dorsal mesogastrium (greater omentum) and the dorsal mesoduodenum, which eventually give rise to the mesentery proper, and dorsal mesocolon.
• The gastrosplenic and splenorenal ligaments develop from the dorsal mesogastrium.
• The ventral mesentery gives rise to the lesser omentum and falciform ligament.
• Rotation of the stomach and other gut movements largely shape the mesenteries' final placement and form.

Foregut

• The foregut gives rise to the esophagus, stomach, proximal part of the duodenum, the opening of the bile duct, and the liver and pancreas.

Development of Stomach

• The stomach develops from the caudal foregut in the 4th week.
• It initially appears as a dilation in the gut tube, oriented in the median plane.
• The stomach rotates around its longitudinal and anteroposterior axes, which leads to the formation of the greater and lesser curvatures.
• Its rotation changes its position in the abdominal cavity.
• The formation of the omental bursa is related to the rotation movement and position changes in the stomach.

Development of Esophagus

• The tracheoesophageal septum, a diverticulum from the ventral wall, separates the respiratory primordium from the esophagus.
• Descent of the heart and lungs results in elongation.
• The esophagus's muscular coat develops from surrounding splanchnic mesenchyme.

Development of Duodenum

• The duodenum develops from the terminal part of the foregut and the cephalic part of the midgut.
• Rotation of the duodenum puts it in a retroperitoneal position.
• Recanalization involves obliteration of the duodenum's lumen, followed by its reopening.

Development of Liver and Gallbladder

• The liver primordium arises as an outgrowth of the endoderm epithelium in the middle of the 3rd week (liver bud).
• Its cells penetrate the septum transversum, which is mesodermal in origin.
• These cells connect to the vitelline and umbilical veins, and form hepatic sinusoids, which differentiate into liver cells and form the biliary ducts.
• The liver and gallbladder originate from the endoderm.

Development of Pancreas

• The pancreas develops as an outgrowth of the duodenum in the midgut region, forming two buds (dorsal and ventral).
• During rotational movements, these buds fuse together to form the pancreas's head, body, and tail segments.
• The main pancreatic duct (Wirsung) and accessory pancreatic duct (Santorini) arise from the respective buds.
• Pancreatic parenchyma is derived from endoderm.
• The islets of Langerhans form in the 3rd month and secrete insulin in the 10th week.
• Connective tissue is derived from mesoderm.

Anomalies

• Abnormalities in the development of the digestive system can lead to conditions like congenital pyloric stenosis, esophageal atresia, tracheoesophageal fistula, biliary atresia, and annular pancreas.
• Other anomalies include: omphalocele, gastroschisis, and umbilical hernia.

Midgut

• The midgut originates from Cephalic limb and produces Distal part, jejunum, part of ileum.
• The caudal limb produces Lower part of ileum, cecum, appendix, and proximal two-thirds of the transverse colon.
• The midgut is suspended from the dorsal wall by a short mesentery and communicates with the yolk sac through the vitelline duct or yolk stalk.
• The superior mesenteric artery supplies the midgut.
• Rotation of the midgut around the superior mesenteric artery leads to the elongation of the small intestine and the development of loops in the jejunum and ileum while the large intestine does not participate in looping.
• Physiological umbilical hernia is the protrusion of the developing midgut into the umbilical cord during early fetal life, followed by returning to the abdominal cavity.
• Rotational anomalies cause malrotations.

Hindgut

• The hindgut gives rise to the distal third of the transverse colon, the descending colon, the sigmoid colon, the rectum, and the upper part of the anal canal.
• The hindgut forms the cloaca in combination with the allantois in the early embryo. This eventually separates into the urogenital sinus and the anorectal canal by developing structures.

Ileal (Meckel's) Diverticulum

• The ileal diverticulum is a small outpouching of the ileum and a remnant of the vitelline duct.
• It contains small patches of pancreatic and gastric tissues, which may cause problems like inflammation and bleeding.