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DNA synthesis (LI) and mitotic activity (MI) were studied at different times of the day in the developing mammary gland end bud of the postnatal mouse. Both LI and MI displayed a clear diurnal variation. Animals studied at the middle of the 12-hr dark cycle showed a LI 2.1-2.6 times that of the tissue from the middle of the 12-hr light cycle. Intermediate values were obtained in tissue sampled midway between these extremes. Mitotic activity was studied 4 hr after the peak and trough of LI and exhibited a corresponding variation. These diurnal vdle of the 12-hr dark cycle showed a LI 2.1-2.6 times that of the tissue from the middle of the 12-hr light cycle. Intermediate values were obtained in tissue sampled midway between these extremes. Mitotic activity was studied 4 hr after the peak and trough of LI and exhibited a corresponding variation. These diurnal vdle of the 12-hr dark cycle showed a LI 2.1-2.6 times that of the tissue from the middle of the 12-hr light cycle. Intermediate values were obtained in tissue sampled midway between these extremes. Mitotic activity was studied 4 hr after the peak and trough of LI and exhibited a corresponding variation. These diurnal variations may be related to cyclic changes in secretion of pituitary hormones or other peptide growth factors, but variation between individual end buds in the same animal is more likely a result of morphogenetic interactions at the tissue level.
Berger et al. (Sat,) studied this question.