Fig. 1. Reconstituted basement membrane (rBM) allows partial maintenance of ER protein levels in primary mammary epithelial cells, but has no effect in primary fibroblasts. (A) Morphology of (a) freshly isolated mammary organoids derived from mouse mammary glands (time 0); (b) primary epithelial cells grown for 2 days on plastic; or (c) on top rBM. Bar, 100 µm. (B) Representative western blots showing ER and ß-casein protein expression in cell extracts derived from primary mammary epithelial cells grown in the presence of lactogenic hormones for 10 days on plastic (a) or on top of rBM (b). Total E-cadherin levels do not change in these experiments and were used for loading control. Quantification was by densitometry of ER protein levels from mammary epithelial cells grown on plastic (solid line) or on top of rBM (dotted line) in the presence (c) or absence (d) of lactogenic hormones. ER was expressed in relative units (day of isolation of the organoids, time 0, corresponds to 1). Results are mean and s.e.m. of five different primary culture preparations using a pool of bilateral 4th inguinal mammary glands from ten nulliparous Balb/c mice in each preparation. *P<0.05 vs. plastic. (C) Morphology of mammary fibroblasts grown for 6 days on plastic (a) or on top of rBM (b). Bar, 100 µm. (D) ER protein expression in epithelial cells (ep) and fibroblasts just after isolation from the gland (t0), and in fibroblasts grown for 6 days on plastic (pl) or on top of rBM (rBM).