Small Intestine; Development

The small intestine is a complex epithelial-lined tubular organ that grows to ∼6 m in length in the adult human. It is a heterogeneous tissue that exhibits regional differences in morphology and function along the horizontal axis (from duodenum to ileum) and along the vertical axis (from crypt to villus tip). Its epithelium is derived from the primitive endoderm, and connective tissue, muscle, and hematopoietic components are mesodermal in origin. The duodenum, up to the opening for the common bile duct (ampulla of Vater), is derived from the foregut (which also gives rise to the liver, lungs, and pancreas). The midgut gives rise to the duodenum beyond the ampulla, and includes the jejunum, ileum, and large bowel, up to and including the proximal transverse colon.