4.1 Gut Development (Exam 3)

• The primitive gut originates from the endoderm during cephalocaudal flexion of the embryo.
• The endoderm gives rise to the epithelial lining and glands of both the digestive and respiratory systems.
• Initial stages involve constriction of the yolk sac, delineating the intraembryonic endoderm (future digestive tube) and the extraembryonic endoderm (inner lining of the yolk sac).
• In sagittal section, the gut tube develops in 3 parts: foregut, midgut, and hindgut, which includes the cloaca.
• The cloacal membrane persists longer than the buccopharyngeal membrane.
• Portions of the gut tube and its derivatives are suspended from the dorsal and ventral body wall by mesenteries, which are double layers of peritoneum that enclose an organ and connect it to the body wall.
• Organs suspended by mesenteries are referred to as intraperitoneal.
• Those that lie against the posterior abdominal wall and are covered by peritoneum are retroperitoneal.
• The liver primordium appears in the middle of the 3rd week, as an outgrowth of the endodermal epithelium at the distal end of the foregut.
• The liver bud consists of rapidly proliferating cells that penetrates the septum transversum.
• The connection between the liver and the foregut narrows into the bile duct.
• A small ventral outgrowth from the bile duct gives rise to the gallbladder and cystic duct.
• The pancreas is formed by two buds (dorsal and ventral) that originate from the endodermal lining of the duodenum.
• When the duodenum rotates to the right and becomes C-shaped, the ventral bud comes to lie immediately below and behind the dorsal bud.
• The ventral bud forms the lower uncinate process of the pancreas and the remainder develops from the dorsal bud.
• Pancreatic islets develop during the 3rd month of fetal life, and insulin secretion begins during the 5th month.
• The primary intestinal loop grows in length and rotates around an axis formed by the superior mesenteric artery.
• Viewed from the front, the rotation is counterclockwise and amounts to about 270° when complete.
• During rotation, the jejunum continues to grow in length so the jejunum and ileum form a number of coiled loops.
• The large intestine (of the midgut) does not participate in rotation.
• Rotation occurs while the midgut tube is herneated out and returned into the body cavity.
• Retraction of the herniated loops occurs around the 10th week.
• The vitelline duct is a long narrow tube that joins the yolk sac to the digestive tube; it is a remnant of this connection at the end of the 4th week.
• As a rule the duct undergoes complete obliteration during the seventh week, but in about two per cent of cases its proximal part persists as a diverticulum from the small intestine (Meckel's diverticulum).
• Meckel's diverticulum is situated about two feet above the ileocecal junction, and may be attached by a fibrous cord to the abdominal wall at the umbilicus.
• Sometimes a narrowing of the lumen of the ileum is seen opposite the site of attachment of the duct.

The digestive system develops from the primitive gut, derived from the endoderm during embryonic flexion. The endoderm forms linings and glands for both digestive and respiratory systems. The gut tube differentiates into foregut, midgut, and hindgut, suspended by mesenteries within the body cavity.