Abstract
VARIOUS renewal systems of mammalian cells exhibit diurnal variations in proliferative activity as evidenced by rhythmical fluctuations in mitotic index and labelling index (percentage of cells labelled) after ‘flash-labelling’ with tritiated thymidine. In other tissues, it is not possible to demonstrate such variations1–4. In general, it appears that diurnal variations in proliferative activity are found in cell systems with a low rate of cell renewal but not if the cell renewal rate is high. In tumours, diurnal variations in mitotic index have been reported5, but in other investigations such fluctuations have not been demonstrated1,6. In some animal tumours, the specific activity of deoxyribonucleic acid (DNA) after tritiated thymidine labelling is higher at night than during the day7, thus indicating diurnal changes in rate of DNA-synthesis per cell, changes in fraction of total cells in DNA-synthesis, or both.
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