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Overview

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Knowledge of embryologic development facilitates the understanding of congenital malformations and many other disease processes.

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Embryonic period: first 8 weeks; fetal period: from 9th week to birth.

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Landmarks

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  • • Ectoderm → epidermis, nervous system.
  • • Mesoderm → mesenchymal tissues: muscular and connective tissue component of intestinal tract.
  • • Endoderm → GI tube to form epithelia and parenchymatous tissues of visceral/thoracic organs.
  • • Fusion zones:
    • – Cephalad endo-/ectoderm: stomatodeum.
    • – Caudad endo-/ectoderm: proctodeum.

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Developmental Details

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Weeks 2–4 (Embryonic Disc → Early Organ Layout)

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  • • Three germ layers: ectoderm, mesoderm, endoderm.
  • • Notochord: primordial axis of the embryo → axial skeleton and inductor of neural plate (→ neuroectoderm).
  • • Day 21: heart begins to beat → circulation.
  • • Craniocaudal and lateral folding of 3 layers → formation of cranial and caudal ends → head fold (stomatodeum), tail fold (proctodeum).

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Weeks 4–8 (Morphogenesis/Organogenesis)

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  • • Week 4: formation of primordial gut: foregut, midgut, hindgut → cloacal membrane.
  • • Formation of cloaca → urinary, genital, and rectal tracts empty through the same opening.
  • • Week 5: migration of neural crest cells along spinal cord → sympathetic ganglia; proximal to distal migration of neural crest cells to internal organs.
  • • Week 6: formation of levator ani.
  • • Week 7: fusion of urorectal septum with cloacal membrane (perineal body) → partitioning of cloaca into dorsal and ventral part; division of cloacal sphincter into posterior (external anal sphincter) and anterior (bulbocavernosus, transverse perinei muscles) parts.
  • • Week 8: rupture of anal membrane → communication of hindgut with amniotic cavity; formation of internal anal sphincter (hindgut).

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Weeks 9–12 (Tissue and Organ Differentiation)

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  • • Midgut → small intestine including most of duodenum, colon from cecum to splenic flexure: arterial supply by SMA.
    • – Week 9: physiologic umbilical herniation → 90-degree counter-clockwise rotation.
    • – Week 10: return of intestines to the abdomen → 180-degree counter-clockwise rotation, 90-degree horizontal rotation of duodenum/pancreas to the right.
  • • Hindgut → colorectum from splenic flexure to anus, bladder, most of the urethra: arterial supply by IMA.
    • – Formation of anal canal: proctodeum = fusion zone between hindgut (proximal) and ectoderm (distal) → dentate line, separate blood supply above/below.

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Clinical Focus and Pathology

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  • • Remnant of primitive streak → sacrococcygeal teratoma.
  • • Remnants of notochord → chordoma.
  • • Intestinal malrotations → incomplete rotation, incomplete fixation (eg, mobile cecum), volvulus.
  • • Incomplete return of intestines → omphalocele, umbilical hernia.
  • • Persistent yolk stalk → Meckel diverticulum, omphaloenteric fistula.
  • • Failure of recanalization → intestinal duplication.
  • • Failure of neural crest cells to migrate to distal bowel → Hirschsprung disease.
  • • Abnormal partitioning of urorectal septum → anorectal malformations.

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Cross-Reference

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