Cell interactions in initiation of mammary epithelial proliferation by oestradiol and progesterone in prepubertal heifers

Abstract

ABSTRACT The acute temporal effects of exogenous oestradiol (0·1 mg/kg per day), progesterone (0·25 mg/kg per day) or both together on the proliferative response of epithelial cells, fibroblasts, adipocytes and endothelial cells in the mammary tissue of prepubertal cross-bred heifers were determined. Mammary biopsies were taken immediately before, then 24, 48 and 96 h after the initiation of daily administration of hormones to three heifers per treatment group. Incorporation of [ 3 H]thymidine into explants prepared from biopsies was evaluated after a 1-h incubation by measuring trichloroacetic acid (TCA)-insoluble radioactivity in explant homogenates as well as by quantitative histoautoradiography. Incorporation expressed as d.p.m./mg tissue or d.p.m./μg DNA was increased ( P ≤ 0·05) approximately 11-fold by 96 h in oestradiol-treated heifers. Progesterone-treated animals were unresponsive and heifers treated with both hormones were intermediate in response compared with oestradiol-treated heifers. Autoradiographic data for ductal or terminal duct epithelial cells showed similar dramatic increases in labelling by 24 h with further increased ( P < 0·01) labelling by 96 h (5·1% vs 0·1%) in heifers given oestradiol. As with incorporation, tissue from progesterone-treated heifers showed no time or treatment response compared with pretreatment biopsies and tissue from heifers given both showed intermediate responses, i.e. significantly increased labelling by 96 h compared with pretreatment ( P ≤ 0·05) but less labelling ( P < 0·05) than oestradiol-treated heifers. A proliferative response of epithelial cells in oestradiol-treated heifers occurred prior to the response of fibroblasts adjacent to epithelial cells (≤ 50 μm). Interestingly, labelling of endothelial cells was also markedly increased by 48 h in oestradiol-treated heifers. Non-adjacent fibroblasts (≥ 100 μm from ductal epithelial cells) and adipocytes showed no time or treatment response. These data demonstrate that exogenous oestradiol markedly and acutely stimulates proliferation of mammary ducts, while progesterone has no effect and may decrease the effectiveness of oestradiol. In contrast to mice, mammary epithelial response to oestradiol in prepubertal heifers is not first exhibited after a proliferative response of stromal cells. This suggests that the cell-type interactions involved in the control of mammogenesis are dissimilar in ruminants and rodents. Journal of Endocrinology (1993) 136, 149–157