The primordial gut is first recognizable during the 4th week after conception and is formed from the dorsal part of the umbilical vesicle (yolk sac) by embryonic infolding. All three germ cell layers contribute to formation of the gastrointestinal tract. The luminal surface (mucosa) contains cells derived from the endoderm, the wall is composed of mesodermal tissues (splanchnic mesenchyme), and the enteric nervous system originates from the ectoderm.¹ Several genes, including Hox, ParaHox, BMP (bone morphogenetic protein), CDX1, CDX2, and Shh and Ihh (Sonic hedgehog and Indian hedgehog), influence patterning along anteroposterior, dorsoventral, leftright and radial axes; cell migration; and interactions between the endodermal and mesodermal components of the gastrointestinal tract.², ³ and ⁴

Much of the development of the large intestine is completed during the 4th through the 14th weeks after fertilization (Table 1.1). The proximal large intestine, including the cecum, appendix, ascending colon, and proximal one-half to two-thirds of the transverse colon, is derived from the midgut, which also includes the entire small intestine distal to the ampulla of Vater. The entire midgut receives its vascular supply from the superior mesenteric (midgut) artery. The distal transverse, descending, and sigmoid colon; rectum; and superior two-thirds of the anal canal are derived from the hindgut and are supplied by the inferior mesenteric artery.¹

The midgut forms a U-shaped loop as it grows. This loop is divided into cranial and caudal limbs by its connection to the omphalomesenteric duct. It herniates into the proximal umbilical cord during the 6th to 10th weeks due to a lack of intra-abdominal space resulting from growth of the liver and kidneys. The loop rotates 90 degrees counterclockwise around the superior mesenteric artery during this time. The cranial limb, which is destined to become the jejunum and ileum, grows more rapidly than does the caudal limb. An elevation on the antimesenteric side of the caudal limb becomes the cecal swelling during the 6th week. Its apex narrows into a diverticulum that develops into the vermiform appendix over the next 4 weeks. The distal limb rotates an additional 180 degrees as the midgut returns to the abdominal cavity during the 10th to 11th weeks. The ascending colon and descending colon become fixed by fusion of their mesenteries to the parietal peritoneum of the posterior body wall, while the transverse colon remains suspended by its mesentery in the peritoneal cavity.¹,²

The terminal portion of the hindgut expands to form the cloaca, which ends blindly at the cloacal membrane in early development. Ingrowth of the urorectal septum between the 4th and 7th weeks gives rise to the rectum and proximal anal canal dorsally, as well as the urogenital sinus ventrally. The cloacal membrane ruptures at the end of the 8th week and provides communication between the fetal gastrointestinal tract and the amniotic fluid.¹,²

The mucosal epithelial lining of the primitive gut is a uniform single cuboidal layer in early development. The gut lumen fills with a solid growth of epithelium from 6 to 8 weeks and recanalizes via apoptosis to form a hollow lumen.² The lumen is lined by a simple, polarized columnar epithelium by the 9th week. Goblet cells first appear in gut mucosa during the 11th and 12th weeks.⁵ Mesodermal tissues simultaneously grow toward the lumen to form the lamina propria. Subsequent epithelial differentiation is dependent upon cell signaling between endoderm and mesoderm, which continues throughout postnatal life. Proliferation, differentiation, and apoptosis of colonic crypt epithelium are largely controlled by Wnt signaling, but other factors,

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