Intestinal epithelial regeneration: active versus reserve stem cells and plasticity mechanisms.

The gastrointestinal system is arguably one of the most complicated developmental systems in a multicellular organism, as it carries out at least four major functions: digestion of food, absorption of nutrients, excretion of hormones, and defense against pathogens. Anatomically, the fetal gut has a tubular structure with an outer layer of smooth muscle derived from lateral splanchnic mesoderm and an inner lining of epithelium derived from the definitive endoderm. During morphogenesis of the gut tube, the definitive endoderm transforms into a primitive gut tube with a foregut, midgut, and hindgut. During the course of further development, the midgut gives rise to the small and proximal large intestine and the hindgut gives rise to the distal large intestine and rectum. The small intestine is subdivided into three parts: duodenum, jejunum, and ileum, whereas the large intestine is subdivided into the cecum, colon, and rectum.