Murine progesterone receptor expression in proliferating mammary epithelial cells during normal pubertal development and adult estrous cycle. Association with eralpha and erbeta status.

The ovarian steroids estrogen and progesterone are important in directing the normal growth and development of the mouse mammary gland. Previously, we have demonstrated that the majority of proliferating mammary epithelial cells do not express estrogen receptor-alpha (ERalpha). In this study we examined the relationship between progesterone receptor (PR) expression and proliferation in mammary epithelial cells using simultaneous immunohistochemistry for progesterone receptor (PR) and tritiated thymidine [(3)H]-Tdr) autoradiography. Results showed that the majority (>80%) of mammary epithelial cells labeled with [(3)H]-Tdr were PR-positive in the terminal end buds (TEBs) of pubertal mice and the ducts of pubertal and adult mice. Whereas the majority of mammary epithelial cells were also PR-positive, the basal cell population, which comprises the minority of mammary epithelial cells in the mammary ducts, was predominantly PR-negative. Nevertheless, the PR-positive phenotype remained the major proliferating cell type in the basal population. These findings suggest that the progesterone signaling pathway is involved in the proliferation of basal cell populations, potentially directing formation of tertiary side branching during pubertal development and alveolar bud formation in adult glands. A proportion of the basal cells exhibited weak expression of ERbeta, suggesting that the role of ERbeta in mediating normal estrogen-induced responses should be further studied. (J Histochem Cytochem: 47:1323-1330, 1999)