Circadian rhythms of peripheral blood leukocytes in aging mice

The effect of aging on white blood cell numbers was studied with special reference to the lability in blood cell physiology. Thus, the total numbers of leukocytes, polymorphonuclear granulocytes (PMNs) and lymphocytes were investigated in C3H mice, 3-month-old mice vs. mice aged 16, 21 and 26 months. Strong circadian rhythms were observed in the different cell classes, while there were insignificant seasonal changes. Even the oldest mice had significant variations, especially in the total leukocyte numbers and the PMNs. Subtle age-related shifts in the rhythmicity patterns were indicated. When comparing the 24-h means, there was a shift between the different white blood cell classes. A permanent reduction of the lymphocyte number was observed from the age of 16 months, while the number of PMNs steadily increased in the aging mice. The present results may explain inconsistencies among previous studies, demonstrating the significance of a chronobiological approach.