Identifying Mammary Epithelial Stem and Progenitor Cells

The adult mouse mammary gland is composed of a branched ductal epithelium, which is embedded in a fatty stroma, termed the fat pad, comprising largely adipocytes but also fibroblasts, macrophages, and endothelial cells (Daniel and Silberstein 1987; Sakakura 1991). The various branches of the mammary tree are connected to a primary duct that terminates at the skin surface through the nipple. The mammary epithelium initially develops during embryogenesis from the ectoderm by cell migration and proliferation thus forming mammary buds, which subsequently sprout a rudimentary ductal tree that is present at birth (Sakakura 1987). Further mammary gland development is completed postnatally in defined stages (puberty, pregnancy, lactation, and involution), which are coupled to the sexual maturation and reproductive status of the animal (Fig. 12.1) (Hennighausen and Robinson 1998).