Midgut

Origin: Endodermal

1. Cranial limb – forms jejunum and ileum
2. Caudal limb – forms ascending colon and 2/3 of transverse colon
3. Cecal swelling – forms cecum, appendix and part of ascending colon

• Intestinal loop elongates rapidly and leaves the small abdominal cavity and enters umbilical cord – physiological umbilical hernia (6th to 10th week)
• The elongating loop rotates 270 degrees anticlockwise around axis of superior mesenteric artery (seen in the diagram)
• Therefore upper part of small intestine lies behind colon
• 10th week, abdominal cavity enlarges and :

1. Jejunum reenters to left side
2. Ileum reenters to right side
3. Cecal swelling reenters below liver

• Cecal swelling elongates downwards to right iliac fossa – forms right colic flexure and ascending colon
• Vitellointestinal duct obliterated

Congenital anomalies:

1. Remnant of vitelline duct:

• Meckel’s diverticulum – proximal part near ileum remains patent
• Vitelline fistula – whole vitelline duct remains open
• Vitelline cyst – Middle part remains open

2. Omphalocele/ Congenital umbilical hernia – Failure of reduction of physiological hernia due to defect in abdominal wall muscles development

3. Gastroschisis

4. Atresia/ stenosis of any part of primitive intestinal loop – bowel obstruction

5. Abdominal rotation of intestinal loop  – 90 degrees only or clockwise rotation

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Hindgut

Origin: Endodermal

Derivatives: Left 1/3 transverse colon, descending colon, sigmoid colon, rectum, upper 1/2 anal canal (Lower 1/2 anal canal, proctodeum – ectodermal)

• Lower end of hindgut dilates to form cloaca
• And then continues as allantois to umbilicus
• Below cloaca is cloacal membrane
• Which is bilaminar: outer ectoderm, inner endoderm
• Between hindgut and allantois is a urorectal septum, which grows caudally and divides the cloaca and cloacal membrane  into:

1. Primitive urogenital sinus (ventrally) – Urogenital membrane
2. Rectoanal canal (dorsally) – anal membrane

• Opposite rectoanal canal, ectodermal depression called proctodeum forms
• Anal membrane ruptures:

1. Proctodeum – lower 1/2 anal canal
2. Rectoanal canal – rectum and upper 1/2 anal canal

Congenital anomalies:

1. Imperforate anus – anal membrane fails to rupture
2. Atresia of rectum – Proctodeum fails to develop
3. Stenosis of rectum – incomplete canalization
4. Recto – vaginal fistula, recto – urinary fistula, recto – urethral fistula – incomplete division of cloaca
5. Anal atresia, anal stenosis
6. Ectopic anus