The length of the cell cycle and the thymidine index (less than 1 day and 6.5%, respectively) for growing, 7,12-dimethylbenz(a)anthracene-induced mammary adenocarcinoma in female Sprague-Dawley rats vary only slightly over a 15-fold change in tumor mass. The observed decreasing growth rate with increasing tumor mass is attributed to a change in the balance of cell production to cell loss. The induced tumor, regressed by ovariectomy and stimulated by estradiol and progesterone, has a shorter doubling time, a 1-day cell cycle time, and a 11% thymidine index. The proliferative fraction of hormone-responsive tumors is modified by hormonal intervention. A transplanted mammary adenocarcinoma, derived from a 7,12-dimethylbenz(a)anthracene-induced tumor in Fischer rats, has the shortest doubling time, a slightly shorter cell cycle time, and the highest thymidine index (15%). Computer analysis by two mathematical models for the percentage labeled mitoses data is presented for comparison.


The research upon which this publication is based was performed pursuant to Contract PH43-NCI-66-71, General Laboratories and Clinics, and Contracts NIH-NCI-71-2098 and PH43-NCI-66-29, Division of Cancer Treatment, National Cancer Institute, NIH.

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