Hormone-dependent mammary tumors were induced in virgin GR mice by treatment with estrone and progesterone. Discontinuation of hormonal treatment was followed by regression of the tumor. This response to hormone treatment was also observed in the first transplant generation in inbred syngeneic hosts, but after several transplantations the tumor growth became hormone independent.

The hormone dependence of the primary tumors and tumors after a single transplantation was demonstrated by growth curves. Furthermore, the cell proliferation kinetics has been investigated in a growing as well as in a regressing hormone-dependent tumor after a single transplantation from the same primary tumor. The experimental data consist of growth curves, percentage-labeled mitoses curves, and labeling indices. Since these data do not contain information concerning the localization of the cell loss in the cell cycle, they were analyzed by a computer method based on three mathematical models differing in respect to the mode of cell loss. All three models gave approximately the same estimates of the cell kinetic parameters in the growing as well as the regressing tumor.

The results for the growing, hormone-dependent tumor showed a growth fraction of 62%, a cell production rate of 3.4%/hr, and a cell loss rate of 2.3%/hr.

Regression of the tumor after hormonal deprivation was accompanied by a decrease in growth fraction to 18% and a decrease in the cell production rate to 0.9%/hr, while the cell loss rate was unchanged at 2.8%/hr. Furthermore, the discontinuation of hormonal treatment introduced an increase in the mean transit time of the cell cycle, particularly in the mean transit time of the G1 phase.

The results might indicate that estrone and progesterone treatment stimulated growth of hormone-dependent GR mouse mammary tumors mainly by an increase of growth fraction and cell production rate.

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