Expression and localization of RANKL and RANK protein within the mammary gland. (a,b) Representative images of RANKL and RANK immunohistochemical staining in control and conjugated equine estrogens plus medroxyprogesterone acetate (CEE + MPA) groups. There is no detectable staining for RANKL in the control group but RANKL protein is clearly evident exclusively within the luminal epithelial cells of the lobuloalveolar and ductal epithelium in the CEE + MPA group. RANKL was not detected within myoepithelial, stromal, or infiltrating immune cells. RANK expression was observed in both ductal and lobuloalveolar structures in all groups. RANK protein was predominately observed in basal cells in close proximity to the basement membrane and also in some epithelial cells, which extended from the basement matrix/basal layer to the lumen. The observed staining of intraluminal secretory material using the RANK antibodies was due to non-specific binding to proteinaceous material in serum (data not shown).