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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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- • 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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