HUMAN EMBRYOLOGY

Chapter 24 Development of Digestive and Respiratory Systems 1. Primordium -- The primitive gut

1.1 Foregut: pharynx → proximal duodenum, liver, pancreas, lower respiratory tract and lungs; 1.2 Midgut: distal duodenum → right 2/3 of transverse colon; 1.3 Hindgut: left 1/3 of transverse colon → proximal anal canal, urogenital sinus.

2. Development of Midgut 2.1 Formation of midgut loop cephalic limb Midgut elongates → midgut loop → vitelline duct at apex caudal limb

2.2 Physiological umbilical herniation and rotation Midgut loop → umbilical coelom, rotates 90° counterclockwise Cranial limb → intestinal coils Cecal swelling appears in caudal limb Vitelline duct → obliterated

2.3 Return and rotation of intestine 1) Intestine → abdomen(small intestine first,cecal swelling last) 180° counterclockwise rotation. 2) Cecal swelling: right upper abdomen → right iliac fossa → ascending colon, cecum and appendix.

3.1 Terminal part → cloaca with allantois connected, cloacal membrane 3. Derivation of Hindgut 3.2 Partitioning of cloaca urorectal septum 1)Cloaca anorectal canal → rectum, upper part of anal canal urogenital sinus

2) Cloacal membrane → urogenital membrane anal mambrane → ruptures (week 8) 3) Mesenchyme around anal membrane → anal pit (ectoderm) → lower part of anal canal

4. Development of Liver & Gallbladder 4.1 Primordium: ventral wall of caudal end of foregut → hepatic diverticulum

4.2 Process Hepatic diverticulum → large cephalic limb → cords of liver cells, intrahepatic biliary apparatus, and hepatic duct small caudal limb → gallbladder and cystic duct.

5. Development of Pancreas 5.1 Primordia: endoderm of caudal part of foregut → dorsal and ventral pancreatic buds → acini, ducts, pancreas islets

5.2 Process 1) Duodenum rotates to right → Dorsal bud → left; Ventral bud → right, dorsally and to left, below and behind dorsal bud. 2) Two buds fuse: Ventral bud → lower part of pancreas head; Dorsal bud → upper part of head, body, tail

3) Duct system Distal part of dorsal duct and entire ventral duct → main pancreatic duct; Proximal part of dorsal duct → obliterated, or accessory pancreatic duct.

6. Development of Lower Respiratory Tract and Lungs 6.1 Primordium: Ventral wall of foregut → laryngotracheal groove → laryngotracheal diverticulum

6.2 Process Distal end of diverticulum → right and left lung buds → bronchi and lungs; Undivided part → trachea; Opening of diverticulum → larynx.

7. Congenital Malformations 7.1 Atresia and stenosis of gut Improper recanalization of the gut lumen; Esophagus, duodenum, common bile duct are most frequently involved.

7.2 Meckel's (ileal) diverticulum Persistence of proximal portion of vitelline duct → outpocketing of the ileum; Located cm from the ileocecal valve.

7.3 Umbilical fistula Persistence of entire vitelline duct → canal between umbilicus and ileum. Fetal discharge may be found at the umbilicus.

7.4 Congenital umbilical hernia Failure of intestine returning or incomplete closure of umbilical coelom; Intestines remain in umbilical ceolom; Covered by amnion, but not by skin.

7.5 Imperforate anus Failure of anal membrane to perforate, or failure of anal pit to develop.

7.6 Rectal atresia Deviation of urorectal septum in dorsal direction; Probably with rectal fistula of various types.

7.7 Abnormal rotation of intestinal loop Left-side colon: nonrotation when returning of intestinal loop; cecum and colon return first and settle on left side of abdominal cavity. Reversed rotation: midgut loop rotates 180° clockwise; transverse colon passes behind duodenum.

7.8 Congenital megacolon Due to an absence of parasympathetic ganglia in the bowel wall. Sigmoid is involved in most cases.

7.9 Annular pancreas Migration of two portions of ventral pancreatic bud in opposite directions, and surrounds the duodenum. May cause duodenal obstruction.

7.10 Tracheoesophageal fistula Hypoplasia of tracheoesophageal septum; Fistula found between trachea and esophagus; Usually accompanied by esophageal atresia.

7.11 Hyaline membrane disease (respiratory distress syndrome) Hypoplasia of type Ⅱ alveolar cells → insufficient surfactant → surface tension↑ → alveoli collapse → difficult breath; Atrophied alveoli, lined with hyaline membrane made up of plasma proteins; Usually seen in premature infants and accounts for about 20% of deaths among newborns.

SUMMARY 1.Derivatives of fore-, mid- & hind-gut; 2.Primordia of the liver, pancreas, & respiratory system; 3. Developmental features of the midgut, and partitioning of the cloaca. 4. Congenital Malformations of the digestive, and respiratory systems.