Circannual variations of circadian periodicity in murine colony-forming cells

The circadian and circannual rhythms of committed stem cells (CFUc) were investigated by using the agar colony technique. There was a marked circadian rhythmicity, with variations between maximum and minimum colony yields of 2-5.5 times; such variations could also be seen throughout the year. A phase shift was observed for the plating efficiency during the year, with the maximum (acrophase) found at different times of the day. At specific times the plating efficiency varied considerably from month to month, with almost no variations at 4:00 P.M. to highly significant variations at other times, e.g., 4:00 A.M. Such changes in proliferation should be taken into consideration especially in long-term studies of hematopoiesis.