Mammary epithelial cells (MEpiC) lie within the alveolar lumen of the breast lobules, which together with mammary ducts and adipose tissues, form a complex network in the mammary gland. MEpiC respond to various growth factors and hormonal cues and undergo changes in growth, invasion, and differentiation during pre- and postnatal stages, puberty, and pregnancy. Aberrant levels of hormones and extracellular matrix composition, and other genetic factors have been shown to induce uncontrolled proliferation of MEpiC, resulting in breast cancer development. Understanding the cellular properties of MEpiC will help identify the disease mechanisms in breast cancer and the new targets for therapeutic development.

Fibroblasts are mesenchymal cells derived from the embryonic mesoderm. Mammary fibroblasts (MF) synthesize components of the stromal extracellular matrix of the mammary gland. More importantly, such stromal extracellular matrix regulates the proliferation and differentiation of mammary epithelial cells by influencing their gene expression. In addition, previous studies have shown that mammary fibroblasts play a role in recruiting macrophages for immune surveillance which, in concert with extracellular matrix turnover and angiogenesis, is associated with a supportive mammary microenvironment for tumor cell progression. These results suggest that MF play a stimulatory functional role in breast cancer development and invasion.