Response of murine mammary epithelial cells to co-culture with mammary fibroblasts and to fibroblast conditioned medium. (a) Proliferation of epithelial cells cocultured with mammary fibroblasts or in the presence of conditioned medium obtained from mammary fibroblasts. Murine mammary epithelial cells were suspended in collagen type I gels and cultured alone in basal medium (EPI), over mammary fibroblasts in basal medium (CO-CULT), or in the presence of fibroblast-conditioned medium (FCM). 3H-thymidine incorporation into DNA was assayed after 3 days of culture. *P = 0.01 that proliferation is greater under coculture condition and in the presence of FCM. (b) Phase-contrast photomicrographs (i–iv) and histological sections (v,vi) showing the tubular/ductal structure of organoids of epithelial cells in collagen-gel cell culture. Mammary epithelial cells were cultured alone in basal medium, cocultured with mammary fibroblasts in basal medium (CO-CULTURE), cultured alone in the presence of fibroblast-conditioned medium (FCM), or cultured in the presence of 50 ng/ml HGF (HGF) for 3 days. ×100 (i–iv), ×400 (v,vi). (c) Effect of FCM and estrogen on proliferation of epithelial cells. Mammary epithelial cells were cultured alone in collagen type I in basal medium, in the presence of 20 nM E2, in FCM, or in FCM obtained from fibroblasts cultured in the presence of 20 nM estradiol (E2FCM); to block any effect of estradiol in the epithelial cells, 200 nM of the antiestrogen, ICI 182,780, was added to epithelial cells at the same time as E2FCM was added. In the presence of BM (i) only solid spheres were observed. Coculture with fibroblasts (ii), or treatment with FCM (iii), or HGF (iv) produced organoids with a tubulo/ductal morphology. Organoids cultured in the presence of EGF or IGF-1 (d) produced a flattened, sheet-like morphology with few or no tubules. *P = 0.01 that proliferation in the presence of FCM was greater than in basal medium or in the presence of E2. **P = 0.05 that proliferation in the presence of E2FCM was greater than with all other treatments. (d) Morphological response of mammary epithelial cells to EGF (50 ng/ml) or IGF-1 (100 ng/ml). Phase-contrast photomicrographs of epithelial cells were taken on day 3; ×100. BM, basal medium; cpm, counts per minute; E2, 17β-estradiol; EGF, epidermal growth factor; FCM, fibroblast-conditioned medium; HGF, hepatocyte growth factor; IGF-I, insulin-like growth factor I; Tdr, thymidine.