Morphology of reconstituted mammary glands of both strains into a nude background. Mammary cells were isolated from 2-month-old BALB/c or C57BL/6 mice and 5 μl of cell suspensions were transplanted into the right or left cleared mammary fat pads, respectively, of 21-day-old Swiss nu/nu or syngeneic mice. At surgery pellets containing E2 plus Pg or plus MPA were also implanted subcutneously into the backs of the animals. One month later, animals were killed and mammary glands excised and fixed. Microscopic examination revealed no changes between C57BL/6 or BALB/c repopulated Swiss nu/nu glands, whereas C57BL/6 cells transplanted in syngeneic glands were under-developed (100×, bar: 700 μm). A representative image of the WM or the H&E stained slides of the reconstituted mammary glands in E2/Pg-treated mice is shown. Microsatellite studies confirmed the strain origin of the epithelial cells in the Swiss background. bp, base pairs; E2, 17β-estradiol; H&E, hematoxylin and eosin; LCM, laser capture microdissection; MPA, medroxyprogesterone acetate; Pg, progesterone; WM, whole mount.