Effects of estrogen on arthritis development in the collagen-induced arthritis model. DBA/1 mice were ovariectomized, subjected to collagen-induced arthritis (CIA) and treated with 17β-estradiol (E2; 0.83 μg/day) or placebo. Arthritis development was macroscopically assessed every second or third day by a blinded examiner, yielding a maximum of 12 points per mouse (single experiment with n = 14 mice/group). (A) Severity of arthritis is expressed as median and interquartile range. The area under the curve was calculated for each treatment group and differences between groups were analyzed by Mann–Whitney test. CII, Collagen type II. (B) Incidence of arthritis is presented as Kaplan-Meier curves and analyzed by log-rank test. (C) Microscopic synovitis and erosions on bone and cartilage were assessed in paw sections stained with hematoxylin and eosin, from placebo- or E2-treated mice with CIA, terminated on day 49. Data are median with interquartile range, Mann–Whitney test, n = 5 or 6 mice/group in a single experiment. Representative sections from each treatment group are shown in (D). Original magnification, ×20. *P <0.05, ***P <0.001.