The role of the ER in the suppressive effect of 17β-estradiol on CFU-OB self-renewal. (a) ICI 182,780 blocks the effect of 17β-estradiol. Marrow cell cultures were established in collagen gels (7.5 × 10⁶ per gel) in the absence or presence of 50 nM ICI 182,780. The cultures were maintained for 6 days without (Veh) or with 1 nM 17β-estradiol and then assayed for CFU-OB number. Assay of freshly isolated cells indicated that there were 281 ± 17 CFU-OBs per 7.5 × 10⁶ cells used to establish each culture. Thus, there was a 5.4-fold increase in CFU-OBs in cultures maintained in vehicle in the absence of ICI 182,780. Bars represent the mean number (± SEM) of CFU-OBs per gel. ^AP < 0.05 treatment versus vehicle as determined by mixed-effects ANOVA. (b) Lack of effect of 17β-estradiol on CFU-OBs from ERα^–/– mice. Marrow cells were obtained from ERα^+/+ or ERα^–/– mice, and collagen gel cultures were established using 8 × 10⁶ cells per gel. Cultures were maintained in the absence or presence of 10 nM 17β-estradiol for 5 days and then assayed for CFU-OBs. Assay of freshly isolated cells from ERα^+/+ or ERα^–/– mice indicated that there were 248 ± 56 or 384 ± 56 CFU-OBs, respectively, per 8 × 10⁶ cells used to establish each culture. Thus, there was a 7.3-fold (ERα^+/+) or 6.5-fold (ERα^–/–) increase in CFU-OBs in cultures maintained in vehicle. Bars represent the mean number (± SEM) of progenitors. ^AP < 0.05 treatment versus vehicle as determined by mixed-effects ANOVA.