Cytometry and Time-Dependent Variations in Peripheral Blood and Bone Marrow Cells: A Literature Review and Relevance to the Chronotherapy of Cancer

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Abstract
By flow cytometry of individual cells, multiple cell properties can be analyzed. Such parameters may be important in relation to cytotoxic treatment of cancer. For example, DNA measurements will answer questions regarding cell kinetics. Myelosuppression is the major dose-limiting toxicity during cancer treatment. Therefore, the study of cell cycle parameters in bone marrow cells is highly relevant. However, inattention to the existence and potential importance of biological rhythms may introduce artifacts and misleading results. The literature of rhythms in hematology is reviewed. Time-dependent variations in hematological variables have been extensively studied and rhythms have been described for all kinds of blood cells. Also the numbers of hemopoietic stem cells in the bone marrow undergo circadian variations. Our group has shown how such variations change with aging in mice. The relevance of time sequence studies in aging research of hemopoiesis was clearly demonstrated. In animal studies using cytometry, our group has demonstrated extensive circadian variations in cell cycle distribution of bone marrow cells, especially the DNA synthesis (S-phase). In humans a few and rather small time sequence studies of the bone marrow have been performed, so far. In this overview the clinical implications of circadian rhythms of S-phase variations measured by flow cytometry of human bone marrow cells are discussed. Male volunteers were examined every 4 h around-the-clock. The data indicated a lower proliferative activity during night, suggesting the possibility of reducing the bone marrow toxicity to cancer treatment when taking these time-dependent variations into consideration.