Figure 1: Tumor-derived cells differentiate into different cell lineages and contribute to the formation of a functional mammary gland. 1.0K tumorderived cells were mixed with 50K normal FVB/N mammary epithelial cells and injected into the cleared fat pad of Nu/Nu female hosts. Tissue sections of mixed mammary outgrowths and mammary tumors were stained for β- gal (green) and ERα or PR (red). A1) Mixed mammary outgrowth showing ERα expression (red) indicated by arrows only in β-gal- cells. A2) Cross section of a β-gal+ mammary hyperplasia that is ERα-. B1) Mixed mammary outgrowth showing PR expression (arrows) only in β-gal- cells. B2) Cross-section of a β-gal+ mammary hyperplasia that is PR-. C) Cross section of a mixed mammary outgrowth at Day 2 postpartum demonstrating that tumor-derived cells, determined by β-gal (green) expression, become functional secretory epithelial cells and produces the milk protein β-casein (red). Arrows indicate tumor-derived cells that are producing β-casein. L-lumen. Sections A2, B2 and C were counterstained with DAPI. Scale bars = 20μm (A, B), 10μm (C). Figure from Booth et al., 2010.