Circadian and circannual variations of cell cycle distribution in the mouse bone marrow

The cell cycle distribution of bone marrow cells from the femurs of female C3H mice has been investigated by flow cytometry according to the time of the day and month of the year. Both circadian and seasonal variations were found for the different cell cycle phases as well as the total cell numbers per femur. Both the mesor, the acrophase and the amplitude of the S, G2 and (G1 + G0) phases varied significantly in some months, while in other months only insignificant rhythms were found. The relative cell cycle distribution only partly reflected variations in the total numbers of proliferating cells, since the total cell number per femur was also variable. The total numbers of cells in DNA synthesis seem to be higher in the first part of the year, indicating increased cell proliferation during winter and spring. In this period the acrophases of DNA synthesis and G2 were in the morning, while the second half of the year showed the peak later in the day. In general, hemopoietic cell proliferation seems to constitute a labile equilibrium with rapidly changing activities.