The Acinar Pattern in the Mammary Glands of Virgin Mice at Different Ages2

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Abstract
The left third gland of each virgin mouse 6 months old was removed by biopsy and stained to compare with the right third gland removed later after the animal was killed. The purpose of the experiment was to see if there were differences in the architecture of the glands at different ages. The mice used were strain C3HeB/FeJ, (C3HeB/FeJ × RIII/J)F1 hybrids, strain RIII/AnJ, and (C3H/J × RIII/AnJ)F1 hybrids. For comparison, the arrangement of acini along the ducts was arbitrarily graded from 1, indicating many single acini, to 4, indicating profuse acinar development. None of the glands were composed of an almost naked duct system. The largest group consisted of 230 (C3HeB × RIII/J)F1 hybrids. About 20 of these hybrids from 7–20 months of age were killed each month. Fifty percent of the right third glands, removed at autopsy, showed the same degree of acinar development as the left third gland showed at 6 months of age; 26% revealed an increase in acini and 24% a decrease. The RIII/An virgins and F1 hybrids of C3H and RIII/An mice were not killed until they developed mammary tumors. There was not much correlation between the degree of acinar development of the mammary glands at autopsy and the age when the tumors occurred. This may have been due partly to the fact that the number of acini in the glands may increase or decrease as the animal grows older.