<P> Enteroendocrine cells do not form glands but are spread throughout the digestive tract . They exert their autocrine and paracrine actions that integrate gastrointestinal function . </P> <P> The gastrointestinal hormones can be divided into three main groups based upon their chemical structure . </P> <Ul> <Li> Gastrin--cholecystokinin family: gastrin and cholecystokinin </Li> <Li> Secretin family: secretin, glucagon, vasoactive intestinal peptide and gastric inhibitory peptide </Li> <Li> Somatostatin family </Li> <Li> Motilin family </Li> <Li> Substance P . </Li> </Ul> <Li> Gastrin--cholecystokinin family: gastrin and cholecystokinin </Li>

Hormones produced by the thymus play a role in the