Marker evolution during the development of the rat mammary gland: stem cells identified by markers and the role of myoepithelial cells.

Using monoclonal antibodies and other immunological reagents we have identified characteristic markers for various epithelial cell types within the rat mammary gland. We have followed the evolution of cell types from the emergence of mammary ducts from the epidermis in the fetus to adulthood. Throughout mammary development some cells retain a group of markers which characterize the early stages of development. We have previously suggested that these cells are the stem cells for mammary development. In the adult, these cells are present in end buds and in the myoepithelial layer of ducts. We suggest that the myoepithelial layer, which we propose should be called the "basal" layer, contains several cell types, of which two are pluripotent. It contains the stem cells for mammary development, which also are present in end buds, and a precursor of ductules and alveoli. In the ducts, basal cells are probably also the precursor of luminal cells. We propose a scheme of mammary development.