Features of the normal breast. (a) Electron micrograph of a ductule of the breast. (b) Section of lobules of the breast showing a relationship with collagenous and fatty stroma. Reprinted with permission from the American Association for Cancer Research . (c) A simplified cartoon of reported potential interactions between three cell types in the stroma and the epithelium of the breast. CSF, colony-stimulating factor; ER, estrogen receptor; IGF1, insulin-like growth factor 1; PR, progesterone receptor; PTH, parathyroid hormone; TDLU, terminal duct lobular unit.