Diurnal changes in circulating myeloid progenitor cells in man.

To examine the diurnal changes in circulating myeloid progenitor cells (CFU-Cs), 15 normal human volunteers were studied. Total and differential peripheral blood leukocyte counts and CFU-C levels, using the in vitro agar culture method, were obtained at 8 AM, 11 AM, 3 PM, and 8 AM the following morning (in nine subjects) or only at 8 AM and 3 PM (in six subjects). On the average, an approximate twofold rise in circulating CFU C levels (per ml of blood) was found at 3 PM as compared to the 8 AM levels (P 0.002), and a smaller rise at 11 AM was found than at 8 AM levels (P 0.03). The ratios of CFU-C per ml of blood to mononuclear cells per ml of blood and neutrophils per ml of blood, respectively, were significantly higher at 3 PM than at 8 AM (p 0.002 and 0.004, respectively). The rise in CFU-C levels is usually significantly higher when compared to that in peripheral leukocyte counts.