PR expression in mammary glands treated with progestins for 2 months: immunohistochemical studies. Control mice received blank silastic pellets. For both (a) PR-A and (b) PR-B, a representative image of four to six histologic slides per group is shown. The number of stained cells is expressed as the percentage of the number of stained cells per epithelial cells in 400× fields of every slide and a minimum of 1,500 epithelial cells/slide were counted (bar: 80 μm). The right panels show the quantification in control (filled columns), MPA-treated (dotted columns), and Pg-treated (empty columns) mice (mean ± standard error). PR-A expression decreased significantly in progestin-treated BALB/c mice, whereas PR-B expression increased in progestin-treated mice of both strains. *P < 0.05, **P < 0.01, ***P < 0.001 control versus treated mice. (c) Quantification (mean ± standard error) of the percentage of positive stained cells for ER-α, ER-β, PR-A, and PR-B in control BALB/c or C57BL/6 female mice shown in Figures 3 and 4a,b. BALB/c mice show higher levels of ER-α and PR-A than C57BL/6 female mice. In control groups, animals were killed at diestrus. ***P < 0.001, **P < 0.01. ER, estrogen receptor; MPA, medroxyprogesterone acetate; Pg, progesterone; PR, progesterone receptor.