Stem Cells and Tissue Homeostasis in Mammary Glands

Abstract

Two important issues face us in the coming decade regarding tissue maintenance, aging and neoplasia. Both require our understanding the role of stem cells in tissues with respect to these topics. These are the notions of micro-environments (niches) and cellular hierarchy and the significance of this cellularprosopopoeia(manifestation) to tissue homeostasis in adult organs. The simplest definition of an adult tissue stem cell might be that it is any cell with a high capacity for self-renewal that extends throughout adult life. However, even in our present understanding, this definition already seems too restrictive. In this issue, it will be clear that among the epithelia of the mammary gland, there are cells that have the capacity to selfrenew and give rise to divergent epithelial progeny but clearly do not maintain this capacity throughout an adult lifetime. Most somatic stem cell research has employed the hematopoetic stem cell model as a basis for identifying and prospectively isolating stem cells from normal tissues. The gold standard sought is the characterization and isolation at the single cell level, the property of re-creation of a particular tissue upon engraftment and the repeat of this property upon sequential transplantation. Early clonal tracking studies of individual hematopoetic stem cells (HSC) have shown that HSCs exist as both long-term and short-term repopulating capacities, indicating