Digestive System

Digestive System • Primitive gut (PG) develops I0 from endoderm • PG  digestive system • Dorsal part of the yolk sac is incorporated into embryo as the PG due to formation of head, tail & lateral folds during 4th week • Endoderm gives rise to most of epithelium of the digestive tract & parenchyma of its associated glands • Liver & pancreas • Ectoderm of stomodeum (primitive mouth) epithelium at the superior end of digestive tract • Ectoderm of proctodeum (anal pit) epithelium at the inferior end of the digestive tract

Digestive tract • Connective tissue & muscles in the wall of the digestive tract are derived from splanchnic mesenchyme that surrounds the endodermal primitive gut (PG) • PG is divided into four parts • Pharynx • Foregut • Midgut • hindgut

Pharynx • Supported by pharyngeal/branchial arches • Series of paired sacculations in lateral walls  Pharyngeal pouches • By the end of the 4th week five pairs have formed

Pharyngeal Pouches • First pharyngeal pouch • Enlarges & develops into a tubotympanic recess • This will become the auditory tube & tympanic cavity (middle ear) • Second pharyngeal pouch • Remains small & is largely obliterated as palatine tonsil develops • Endodermal cells form surface epithelium of tonsil & lining of its crypts • Mesenchyme around this pouch differentiates into lymphoid tissue

Pharyngeal Pouches (cont.) • Third pharyngeal pouch • Dorsal endoderm inferior parathyroid glands • Ventral endoderm unite to formthymus • Both glands lose connection with pharynx & migrate in a caudal & medial direction • Fourth pharyngeal pouch • Dorsal endoderm  superiorparathyroid glands • Fifth pharyngeal pouch (part of fouth) • Ultimobranchial body which incorporates into thyroid gland • These cells give rise to parafollicular/C cells & secrete calcitonin

Foregut • Lies caudal to pharyx & extends as far back as the liver outgrowth • At about 4 weeks a small diverticulum appears in ventral wall at caudal border • Tracheobonchiole/respiratory diverticulum • Gradually separates from foregut dividing foregut • Dorsal esophagus • Ventral respiratory primordium

Foregut • Esophagus • Tube extending from pharynx to stomach • Initially short but elongates rapidly keeping pace with differentiating neck & descending heart & lungs

Foregut • Stomach • Fusiform dilatation in 4th week of development • During the following weeks appearance & position changes (descends) • Increases in length • Dorsal border grows faster than ventral wall + 90o CW rotation along its long axis • Posterior faces left, Anterior faces right • Convex greater curvature (GC)  lies on left • Concave lesser curvature (LC)  lies on right • Epithelium differentiates into gastric glands & surrounding mesenchyme • Three incomplete layers of the muscular coat

Foregut • Duodenum • Develops early in 4th week from caudal part of foregut & cephalic part of midgut • Junction of two parts directly distal to origin of liver bud • Takes form of C shape loop & rotates to right as stomach rotates • Duodenal rotation along with rapid growth of head of the pancreas causes swing to left side of abdomen • During 2nd month the lumen is obliterated by cell proliferation but recanalized shortly after

Foregut • Liver & gallbladder • Liver primordium appears in middle of 3rd week as hepatic diverticulum (HD) or liver bud at distal end of foregut • Hepatic diverticulum rapidly enlarges due to cell proliferation dividing into a large & small part • Large cranial part  liver primordium • Small caudal part  gallbladder primordium • Hepatic cells continue to divide, the connection between the HD & duodenum narrows  bile duct

Liver (cont.) • Epithelial hepatic cell cords intermingle with vitelline & umbilical veins  Hepatic sinusoids • Liver grows rapidly & fills most of abdominal cavity • Initially right & left lobe are = size • Right lobe becomes larger & subdivides into caudate & quadrant lobes • At 9th week 10% of fetal body weight • At full term 5% of fetal body weight

Gallbladder & Pancreas • Gallbladder primordium • Distal end expands  gallbladder • Proximal part stays narrow  cystic duct • Pancreas formed by two buds originating from endodermal lining of duodenum • Buds arise on opposite sides at end of 4th week • Dorsal pancreas  forms most of the gland • Pushes out from dorsal wall slightly above hepatic diverticulum • Ventral pancreas  head & uncinate process • Develops near entry of bile duct • Main pancreatic duct • Formed by distal part of dorsal duct + entire ventral duct

Midgut • In a 4-5 week old embryo midgut is suspended from dorsal abdominal wall by a short mesentery & communicates with the yolk sac via vitelline duct • Will give rise to: • Part of duodenum & rest of the small intestine • Distal to where the bile duct enters • Ceacum • Appendix • Ascending colon • Proximal 2/3 of transverse colon

Midgut (cont.) • All structures of midgut supplied by superior mesenteric artery • Development is characterized by a rapid growth in the length of the gut  formation of primary intestinal loop • Cranial limb • Part of duodenum, jejunum, part of ileum • Caudal limb • Balance of ileum to proximal 2/3 of transverse colon • Vitelline duct • At junction of cranial & caudal limb • If it persists in adult  Meckel’s diverticulum

Midgut (cont.) • As the intestinal loop elongates rapidly & liver enlarges, the abdominal cavity is temporally too small causing intestinal loop to project into the umbilical cord  physiological umbilical hernia (starting around the 6th week) • By about the end of the 3rd month the herniated loops begin to return to the abdominal cavity

Hindgut • Gives rise to: • distal 1/3 of transverse colon • the descending colon • the sigmoid colon • the rectum • upper part of anal canal • All hindgut derivatives are supplied by the inferior mesenteric artery • The terminal portion = cloaca • Cloacal membrane • lined with endodermal cells internally • Lined with ectodermal cells externally

Congenital Malformations • Branchial Anomalies • Esophageal Atresia • Esophageal stenosis • Short esophagus • Intestinal stenosis & atresia • Omphalocoele • Umbilical hernia • Meckel’s Diverticulum

Branchial Anomalies • Most of the abnormalities of the branchial region are represented by remnants of the branchial structures that normally disappear • Branchial fistula • An abnormal opening on the side of the neck • Usually the result of persistance of parts of 2nd branchial groove & 2nd branchial pouch • Agenesis of thyroid & parathyroid • DiGeorge Syndrome • Failure of 3rd & 4th pharyngeal pouches to differentiate • May be due to teratogen action • No known genetic cause

Esophageal Atresia • Usually occurs with tracheo-esophageal fistula (TEF) • Several types • Lower esophagus communicates with back of trachea & upper esophagus ends in a blind pouch ( most common) • Discontinuous esophagus with no tracheo-esophageal fistula (rare) • Possibly genetic • Possible maternal vitamin A deficiency

Congenital Esophageal Stenosis • Rarer than esophageal atresia with TEF • Little known etiology • Symptoms • Regurgitation of food • Failure to gain weight • Dilation of esophagus above level of obstruction may press against the trachea & bronchi  produce wheezing sounds

Congenital Malformations (cont.) • Short esophagus • Causes partial thoracic stomach • Small portion of stomach above diaphragm • Gastroesophageal incompetence is a common cause of vomiting (often projectile) during infancy • Intestinal stenosis & atresia • Occurs commonly in duodenum & ileum • May be due to interruption of blood supply to a loop of fetal intestine • Results in necrosis, which later becomes fibrous cord connecting proximal & distal ends

Congenital Malformations (cont.) • Omphalocoele • Results from failure of intestine to return to abdomen during 10th week • Hernia may contain a single loop of intestine or most of the intestine • Herniated mass is covered by a thin transparent membrane composed of peritoneum & amnion • Associated with chromosome abnormalities & other severe defects • Umbilical hernia • Differs from omphalocoele • herniated mass is covered by skin & subcutaneous tissue

Congenital Malformations (cont.) • Gastroschisis • Herniation of abdominal contents through the body wall directly into the amniotic cavity • Defect occurs lateral to umbilicus, usually on the right, through a region weakened by regression of the right umbilical vein which normally disappears • Viscera are not covered by peritoneum or amnion • Bowel may be damaged by exposure to amniotic fluid • 1 in 10,000 births (frequency is ) • May be related to cocaine use

Congenital Malformations (cont.) • Meckel’s Diverticulum • Ileal diverticulum is common • Higher frequency among males • Represents remnant of the vitelline duct • Appears as finger-like pouch arising from ileum • 3-6 cm long & may remain connected to umbilicus by a fibrous cord or fistula • Wall of diverticulum contains all tissue layers of the ileum • May include gastric & pancreatic tissue • Gastric secretion often produces ulceration

Digestive System