Mammary glands from BALB/c or C57BL/6 mice treated with E2 or E2 plus Pg. Mice were implanted subcutaneously with silastic pellets containing E2 (20 μg) or Pg (20 mg) + E2 (20 μg). Representative H&E stained sections and WM are shown. The arrow points to ductal dilatations, typical of E2 action in BALB/c mice. All WM pictures were taken at the same magnification. Magnification: 100×, bar: 1.1 μm.P < 0.05, BALB/c versus C57BL/6. E2, 17β-estradiol; H&E, hematoxylin and eosin; N, number of structures/slide; Pg, progesterone; WM, whole mount.