The size and location of cytoplasmic lipid droplets (CLDs) changes upon secretory activation. Mammary glands were isolated from FVB mice on pregnancy (P) days (a) 12, (b) 16, and (c) 18, and (d) day 2 of lactation (L2). Tissues were fixed in neutral-buffered formalin, stained with anti-adipophilin (ADRP) antibody and Alexa Fluor 594 conjugated secondary antibody to outline the cytoplasmic lipid droplets (appearing in red), Alexa Fluor 488-conjugated wheat germ agglutinin to outline the luminal surface of the luminal space of the secretory alveoli (appearing in green), and 4',6-diamino-2-phenylindole (DAPI) to stain the nuclei of mammary epithelial cells (appearing in blue). Idealized schematic drawings, not meant to represent the micrographs shown in the top panel, illustrate the positions of the luminal space (labeled LU), nuclei (purple), and CLDs (labeled red) at pregnancy days (e) 12, (f) 16, and (g) 18, and (h) day 2 of lactation. The scale bars in (a-d) represent 10 μm. Luminal space is indicated by the letters 'Lu', and the white arrowheads indicate CLDs.