Mammary epithelial stem cells: our current understanding.

It has recently been shown that the progeny from a single cell may comprise the epithelial population of a fully developed lactating mammary outgrowth in mice. Serial transplantation of epithelial fragments from this clonally derived gland demonstrates that the subsequently generated outgrowths are also comprised of progeny from the original antecedent. Similarly, genetic analysis of contiguous portions of individual human mammary ducts within the same breast indicates their clonal derivation. These observations support the concept that multipotent tissue-specific epithelial stem cells are present among the parenchymal cells of the mammary gland. Here, we present the developing evidence for the presence of stem cells in virtually every renewing mammalian tissue as well as some classically considered to consist only of differentiated cells. Further, we review the present morphologic and biologic evidence for stem cells and lineage-limited progenitor cells in human and rodent mammary epithelium. Although a number of selective markers are known for various lineage-limited hematopoietic cells and their progeny, our understanding of the biology of the precursor cells for mammary epithelium is just beginning. Our purpose here is to develop further interest in the clarification of these issues in the biology of the mammary gland.