Postnatal mammary ductal growth: three-dimensional imaging of cell proliferation, effects of estrogen treatment, and expression of steroid receptors in prepubertal calves.

Cows may provide insights into mammary development that are not easily obtained using mouse models. Mammary growth in control and estrogen-treated calves was investigated to evaluate general patterns of proliferation and relationship to estrogen receptor (ER) expression. After in vivo labeling with bromodeoxyuridine (BrdU), serial histological sections of mammary tissue were used to generate three-dimensional reconstructions. BrdU-labeled cells were present throughout the highly branched terminal ducts. ER and progesterone receptors (PR) were colocalized in nuclei of ductal epithelial cells. However, basal cells and epithelial cells that were located in the central region of epithelial cords and those that lined the lumen of patent ducts were ER- and PR-negative, as were stromal cells. Cells along the basal portion of the epithelium were not myoepithelial. ER in mammary epithelial cells but not stromal cells is analogous to patterns in human breast but contrasts with localization in murine mammary gland. After estrogen stimulation, 99% of BrdU-labeled (and Ki67-labeled) epithelial cells were ER-negative. Data suggest that proliferation in response to estrogen treatment was initiated within ER-positive epithelial cells of the developing mammary gland and the signal was propagated in paracrine fashion to stromal elements and ER-negative epithelial cells.