Mammogenesis and Breast Carcinogenesis in Virgin Female Mice of BALB/cf Substrain With the Milk Agent2

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Abstract
A morphologic study on the mammary glands of virgin female mice of the BALB/Cf substrain with the milk agent has shown that alveolar development takes place from the end of the 3d month of life until the senile involutional changes affect the end pieces of the glandular tree early in the 2d year. Small clusters of alveoli or small nodules are recognizable by the 4th month and progressively increase in number until they reach a peak at the 10th month. Some of them progress toward large nodules, but the others regress in connection with the senile changes. Large nodules, which appear by the 6th month, show a moderate numerical increase in direct relation to the age of the animal. Some of them progress to cancer, but others do not escape the senile involution. Palpable mammary tumors appear from the 7th month, but no tumors have been observed in females that live longer than 15 months. With the breast development and regression as detectors of the hormonal stimuli, it should be supposed that, in virgin females of the BALB/cf substrain, the mammogenic triad (estrogens + progesterone + prolactin) is effective on the breasts, but only from the 4th month to the end of the 1st year. After this time there is evidence of a lack, or deficiency, of hormonal stimuli, which seems responsible for the senile involutional changes, for the disappearance of nodules, and for the nonoccurrence of mammary tumors in old animals. Results obtained in this substrain cannot be generalized to other strains.

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