--- module: 086-02 language: en chapter: 86 title: "Organogenesis, Congenital Anomalies, and Teratology" module_title: "Gastrointestinal, urogenital, diaphragmatic, and musculoskeletal organogenesis" source_sha256: dbfe0054cb413e933b7465de4f7e1b15b3c887aa2ae8356051b0c3feffe33a84 --- # Gut, urogenital, diaphragm, and musculoskeletal organogenesis ## Principles and gut germ layers ### Epithelial-mesenchymal interaction, ducts, rotations ### Identify tube, fusion plane, route, remodelling step ### Folding forms foregut, midgut, hindgut ### Endoderm: epithelium and glandular parenchyma ### Splanchnic mesoderm: muscle, stroma, vessels ### Neural crest: enteric ganglia ## Foregut: oesophagus, stomach, duodenum ### Oesophagus lengthens as heart and lungs descend #### Partition error: fistula or atresia #### Failed recanalisation: stenosis ### Faster dorsal wall forms greater curvature ### Longitudinal rotation: left side anterior #### Explains vagal trunk positions ### Pylorus moves up and to the right ### Dorsal mesogastrium: greater omentum, spleen ### Duodenum C-shaped, secondarily retroperitoneal #### Failed recanalisation: stenosis or atresia #### Bile-duct level changes vomiting ## Liver, biliary tree, and pancreas ### Liver bud grows into septum transversum #### Endoderm: hepatocytes, biliary epithelium #### Mesoderm: stroma, sinusoids, haematopoiesis ### Duct remodelling errors: atresia, cystic dilation ### Dorsal and ventral pancreatic buds #### Duodenal rotation brings ventral bud to fuse #### Ventral bud: uncinate process, part of head ### Fusion failure: pancreas divisum ### Bifid ventral tissue: annular pancreas ## Midgut rotation and body wall ### Vitelline duct links midgut to yolk sac ### Physiological herniation into umbilical cord ### About 270 degrees around superior mesenteric artery #### Ninety while herniated, 180 on return ### Caudal limb: distal ileum to transverse colon ### Caecum descends from subhepatic position ### Malrotation: abnormal mesenteric base #### Volvulus or obstructing bands ### Vitelline remnants: ileal diverticulum, fistula #### Ectopic gastric tissue may bleed ### Omphalocele: failed return, membrane-covered ### Gastroschisis: paraumbilical, uncovered bowel ## Hindgut, cloaca, and anal canal ### Urorectal septum divides cloaca #### Anorectal canal and urogenital sinus #### Partition errors: malformations and fistulas ### Pectinate line: endoderm above, ectoderm below #### Inferior mesenteric versus internal iliac supply ### Failed crest migration: distal aganglionic bowel #### Functional obstruction, proximal dilation ## Diaphragm ### Septum transversum and pleuroperitoneal membranes ### Oesophageal mesentery and body-wall muscle ### Phrenic supply follows cervical myoblasts ### Posterolateral defect: viscera enter thorax #### Impairs lung development ## Kidney and bladder ### Pronephros, mesonephros, metanephros ### Ureteric bud invades metanephric mesenchyme #### Bud: ureter, pelvis, calyces, collecting ducts #### Mesenchyme: nephrons to distal tubule #### Failed induction: renal agenesis ### Apparent ascent and medial hilar rotation #### Successive arterial branches from aorta #### Horseshoe kidney arrested beneath IMA ### Trigone incorporates mesonephric-duct tissue ### Urachal remnants: fistula, sinus, cyst ## Genital development ### Indifferent gonad and primordial germ cells ### Sertoli anti-Müllerian hormone regresses ducts ### Leydig testosterone, local dihydrotestosterone ### No testis signal: paramesonephric ducts persist #### Uterine tubes, uterus, upper vagina #### Fusion errors: bicornuate or septate uterus ### Incomplete urethral-fold fusion: hypospadias ## Axial skeleton and limbs ### Sclerotomes resegment into vertebrae ### Myotomes split into epaxial and hypaxial ### Apical ectodermal ridge: proximodistal growth ### Zone of polarising activity: digit axis ### Upper limb rotates laterally, lower medially ### Programmed cell death separates digits