Mammary stem cells and mammopoiesis.

The isolation and characterization of mammary stem cells is fundamental to understanding mammary gland development and tissue homeostasis as well as breast oncogenesis. Recent studies have led to the prospective isolation of pluripotential stem cells from the mouse mammary gland through the identification of specific cell-surface markers and transplantation of cells into the mammary stromal microenvironment. A single cell was sufficient to reconstitute a fully developed mammary gland in vivo, indicating that combinatorial activity between independent stem cells is not essential to generate an outgrowth. Here we review the characteristics of mouse mammary stem cells, their estrogen receptor status, and the potential cellular hierarchy that exists within the mammary gland.