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with estradiol plus progesterone (37%) was significantly decreased in comparison
with the controls (100%).
Figure 7. Effect of short-term hormone treatment on mammary carcinogenesis in the activated Her-2/neu
transgenic mice. Mice treated with estradiol alone (7/21) or estradiol plus progesterone (9/24) had a significant
decrease in the incidence of mammary tumors compared with the controls (20/20) at 8 to 9 months of age (p
< 0.01).
All the control mice developed mammary cancers by 5 months of host age
with a multiplicity of 6.8 cancers per mouse. Treatment with estradiol (0.6 can-
cers per mouse) or estradiol plus progesterone (0.8 cancers per mouse) drastically
reduced mammary cancer multiplicity and also approximately doubled the mam-
mary cancer latency (Figures 8 and 9). This experiment was repeated once with
identical results. A second repeat experiment was not informative because the
control untreated mice did not develop tumors with their usual early latency of 13
to 15 weeks of host age but started to develop tumors only at 30 weeks.
Figure 8. Effect of short-term hormone treatment on mammary cancer multiplicity in the activated Her-2/
neu transgenic mice. The group of mice (n = 21) treated with estradiol alone developed a total of 13 mammary
tumors and the group of mice (n = 24) treated with estradiol plus progesterone developed a total of 20 mammary
tumors by 8 to 9 months of age; in comparison, the group of control mice (n = 20) developed a total of 136
mammary tumors (p < 0.005). Results are expressed as means ± SEM.
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