Morphological appearance of mammary glands from MMTV-BRCA1 transgenic mice. (A) Iron hematoxylin-stained whole mount preparations from the thoracic mammary gland of 13-week-old virgin nontransgenic, hemizygous, and homozygous MMTV-BRCA1, MMTV-BRCA1t340, and MMTV-BRCA1sv transgenic mice in the diestrus phase of the estrous cycle are shown. Homozygous MMTV-BRCA1 mice demonstrate additional branching and end buds compared to hemizygous and wild type controls. There is an apparent morphological difference in the mammary ductal tree among homozygous and hemizygous MMTV-BRCA1t340 mice and wild type controls. The mammary glands of both hemizygous and homozygous MMTV-BRCA1sv mice show more marked proliferation, and resemble those of a 12.5-day pregnant animal. Magnification of the whole mounts is 10X. (B) Representative sections of 13-week-old virgin mammary glands from nontransgenic, hemizygous, and homozygous MMTV-BRCA1, MMTV-BRCA1t340, and MMTV-BRCA1sv mice are shown. The mammary glands of homozygous MMTV-BRCA1 mice contain more alveoli than hemizygous, nontransgenic, or MMTV-BRCA1t340 mice. The mammary glands of both hemizygous and homozygous MMTV-BRCA1sv mice contain numerous alveoli. Photographs were taken with a 20X objective.