Development and Anatomy of the Enteroendocrine System in Humans.

The gastrointestinal (GI) tract exhibits an enormous surface area that consists mostly of absorptive enterocytes. Enteroendocrine cells (EECs) are found scattered along the GI tract between absorptive enterocytes and other secretory cells, and comprise around 1% of the epithelial cell population. Interestingly, they develop from the same crypt stem cell as the other absorptive or secretory cells of the gut. EECs differentiate along the crypt villus axis and are renewed every 4-6 days, and hence possess a high plasticity. They constitute the largest endocrine system in the human body by secreting multiple peptide hormones to control, for example, postprandial digestion, insulin homeostasis, food intake, and gut motility. For this purpose, most EECs exhibit luminal sensors that detect the GI tract content. Thereafter, they may act either in a classical endocrine fashion, or by paracrine effects on nearby neural and immune cells. This creates a pivotal role for EECs to influence the GI immune system and the enteric nervous system. In this chapter, the anatomical characteristics, development, differentiation and maturation of EECs are described, and their important biological potential illustrated as part of the gut interacting sensory system.