Biological clocks and the digestive system

Circadian rhythms play a major role in regulating the digestive systems of many organisms. Cell proliferation, migration, differentiation, and even st ructure vary as a function of time of day in many different digestive organ s (i.e., stomach, gut, liver, and pancreas) and cell types, resulting in re gionally specific temporal variations in protein and gene expression. Feedi ng and light set the hands of the digestive clock(s). However, the clockwor k has a genetic basis. During the last 10 years, new developments have emer ged in our understanding of how cells keep time. Surprisingly, clock genes in mammals are expressed not only in specialized time keepers in the brain, but also in peripheral organs, suggesting that the ability to keep time ma y also belong to cells within the digestive system. This article reviews se veral classic examples of circadian variation in the digestive system, with an emphasis on rhythms in cell proliferation, function, and structure. It also briefly summarizes several new ideas about how cells in the brain and possibly the digestive system keep time.