Developmental changes in the properties of mouse mammary stem cells

Recent findings have suggested that the normal breast is a hierarchically organized tissue in which differentiated cells incapable of further division are continuously produced from a common, self-sustaining stem cell population. However, little is known about the origin, timing or properties of mammary stem cells during development. In some tissues, cells with tissue-specific stem cell properties are known to develop after the first recognizable tissue elements are in place, and these early stem cells also have qualitatively different properties than their adult counterparts. I hypothesized that this would also be true of the mammary gland. I therefore designed a series of experiments to identify, isolate and characterize mammary stem cells when they first appear during development. I also established robust in vitro and in vivo systems to enable their growth and regenerative potential to be compared with that of similarly assessed adult mammary stem cells. I found that a reproducibly detectable mammary stem cell population with transplantable regenerative activity is first detected late in gestation after the early mammary bud is formed. This population then expands in parallel with a more prevalent population detectable as cells with clonogenic activity in vitro. I also discovered that single EpCAM⁺ fetal mammary epithelial cells in semi-solid Matrigel cultures supplemented with added irradiated fibroblasts have a direct and inducible but highly variable growth and regenerative potential that is, nevertheless greater on average than that of their adult counterparts. Analysis of the 4-week regenerative activity of fetal as compared to adult mammary epithelial cells in vivo also indicated an increased output of total cells and clonogenic progenitors from the fetal cells, again with marked variation in this assay. Analysis of published gene expression profiles of fetal and adult mammary enriched stem cell subsets have suggested a number of changes in the external cues that govern their responses as well as the intrinsic molecular network they use to execute these responses. Further elucidation of developmentally-determined changes in the properties of mammary stem cells are likely to be important in understanding perturbations of adult mammary cells that endow them with malignant properties.Medicine, Faculty ofMedical Genetics, Department ofGraduat