Rhythms in human gastrointestinal mucosa and skin.

Rhythmicity in cell proliferation is well established in the gastrointestinal tract and skin, both in rodents and humans. This is the basis for studies on the timing of both chemotherapy and radiotherapy. More recently, circadian rhythm studies of cell-cycle proteins have confirmed earlier findings based on thymidine labeling and flow cytometry. The genetic control of circadian rhythms has been elucidated recently and a possible connection between the circadian clock and the timing of cell-cycle events has been suggested. The data for gastrointestinal mucosa and skin are reviewed and the potential clinical implications of these results are discussed.