Ultradian biorhythms of cell proliferation

We studied circadian and ultradian rhythms of mitotic index of epithelial cells of the esophagus, intestine, and dorsal surface of the tongue, and thymocytes from mice maintained at normal and inverted light regimens. It was shown that in normal animals, the period of ultradian oscillations was shorter during the active circadian phase compared to the passive phase. Photoinversion changed the correlation between ultradian oscillations of the mitotic index and its circadian phases, which was differently pronounced in various tissues. Thus, changes in daily illumination serve as the time indicator for circadian rhythms of cell proliferation, but not for ultradian oscillations of mitotic activity.