Dev Of Dig System

Question 1

🧬SYSTEMIC EMBRYOLOGY

Field: Embryology
Topic: Embryology of the Digestive system
For: Medical students and Anatomy learners
By: Anatomist Vitalis

The digestive system begins to form in the 3rd to 4th week of intrauterine life and continues to mature until birth and beyond. It originates primarily from the endoderm, which gives rise to the epithelial lining of the entire gastrointestinal tract (GIT), while the splanchnic mesoderm contributes to the smooth muscles, connective tissues, and blood vessels. The enteric nervous system (ENS) derives from neural crest cells.

🧫 Germ Layer Contributions:
1. Endoderm → Epithelium of the gut and accessory organs (liver, pancreas, etc.)
2. Mesoderm (Splanchnic) → Smooth muscle, connective tissue, blood vessels
3. Ectoderm (Neural Crest) → Enteric nervous system (Meissner’s & Auerbach’s plexuses)

🔁 Primary Gut Tube Division
After lateral folding of the embryo, the yolk sac is incorporated into the embryo forming a closed tube – the primitive gut tube – which is divided into: Foregut, Midgut and Hindgut
🧠 Mnemonic: “Forget My Hat”
F – Foregut
M – Midgut
H – Hindgut

🧪 Developmental Timeline & Derivatives
📍Foregut (Weeks 4–6)
Arterial Supply: Celiac artery
🔹 Derivatives: Esophagus, Stomach, Liver, Gallbladder, Pancreas, Upper duodenum (up to the bile duct) and Lower respiratory tract (lung buds).
🔹 Developmental Highlights:
1. Esophagus lengthens with the descent of the heart and lungs.
2. Stomach rotates 90° clockwise around its longitudinal axis, forming the lesser and greater curvatures.
3. Liver and gallbladder arise from a hepatic diverticulum.
4. Pancreas forms from two buds (dorsal and ventral) that fuse.
🧠 Mnemonic: “SHe PELLS”
S – Stomach
He – Hepatic system (liver)
P – Pancreas
E – Esophagus
L – Lungs
L – Liver
S – Spleen (from mesoderm, but associated)

📍Midgut (Weeks 5–10)
Arterial Supply: Superior mesenteric artery (SMA)
🔹 Derivatives: Lower duodenum, Jejunum, Ileum, Cecum, Appendix, Ascending colon and Proximal 2/3 of transverse colon.
🔹 Physiological Herniation:
1. At week 6, the rapidly growing midgut herniates into the umbilical cord.
2. It rotates 270° counterclockwise around the SMA axis, then returns to the abdomen by week 10.
🧠 Mnemonic: “DJ CACTI”
D – Duodenum (lower)
J – Jejunum
C – Cecum
A – Appendix
C – Colon (ascending)
T – Transverse colon (proximal 2/3)
I – Ileum

📍Hindgut (Weeks 4–7)
Arterial Supply: Inferior mesenteric artery (IMA)
🔹 Derivatives: Distal 1/3 of transverse colon, Descending colon, Sigmoid colon, Rectum, Upper part of anal canal and Epithelium of the urinary bladder and urethra (via cloaca).
🔹 Cloacal Partitioning:
The cloaca divides into the urogenital sinus and the anorectal canal by the urorectal septum.
🧠 Mnemonic: “Cool Dirty ASS-RU”
C – Colon (descending)
D – Distal transverse colon
A – Anal canal (upper part)
S – Sigmoid colon
S – Septum (urorectal)
R – Rectum
U – Urethra (epithelial lining)

👶 Anomalies & Clinical Correlations
👉Esophageal atresia/tracheoesophageal fistula (TEF)
1. Incomplete separation of the trachea and esophagus.
2. Newborns choke, cough, cyanose with feeding.
👉Pyloric stenosis
Hypertrophy of pyloric muscle → projectile vomiting
👉Annular pancreas
Abnormal migration of pancreatic buds → encircle duodenum → obstruction
👉Meckel’s diverticulum
1. Remnant of vitelline duct in the ileum.
2. Rule of 2s: 2 inches, 2 feet from ileocecal valve, 2% population.
👉Malrotation of midgut
Incomplete rotation → volvulus or obstruction.
👉Hirschsprung disease (aganglionic megacolon)
Failure of neural crest cells to migrate → absence of enteric ganglia → functional obstruction
👉Imperforate anus
Failure of anal membrane to perforate.

🧠 Mnemonic for Anomalies: “EMPATH HI”
E – Esophageal atresia
M – Meckel’s diverticulum
P – Pyloric stenosis
A – Annular pancreas
T – Tracheoesophageal fistula
H – Hirschsprung disease
H – Herniation (physiologic, failure → omphalocele)
I – Imperforate anus

✅ Summary:
Start: Week 3–4
Derived from: Endoderm (epithelium), Mesoderm (support), Ectoderm (nerves)
Divisions:
1. Foregut – Supplied by Celiac artery
2. Midgut – Supplied by SMA
3. Hindgut – Supplied by IMA
Midgut rotation: 270° counterclockwise
Cloaca → Urogenital sinus & anorectal canal
Clinical concerns: TEF, Meckel’s, stenosis, malrotation, Hirschsprung, etc.

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