The Basal population complements the puStm population to form the basal compartment and appears higher in the hierarchy than the puPgt due to its bi-potent characteristic. However, it exhibited lower sphere formation than the puStm and retained only an intermediate propagation rate. This population might represent a mixture of bipotent progenitors and more differentiated myoepithelial cells. Very little is known about the mechanism regulating myoepithelial cell formation and their differentiation pathway, hindering our and others�� attempts to dissect this lineage. Nonetheless, during the preparation of this study for GSK2118436 Raf inhibitor publication, an elegant study in human BEZ235 side effects mammary organoids was published demonstrating the involvement EGF in early expansion of myoepithelial cells via the ERK 1/2-RSK pathway, and dissecting the effect of HER1 ligands in determining the myoepithelial lineage . Notably, mouse mammary organoids are much less responsive to HER1��s effect than their human counterparts. This striking difference designates the bovine mammary gland as an attractive candidate for further studies of mammalian diversity in EGF-MAP kinase��s regulatory role in delineating the basal/myoepithelial mammary lineage. The puPgt population is uni-potent, as it forms only CK18- stained colonies, and does not preserve the ability to form organized structures from any tested re-sorted cell fraction. Yet its highest level of Stat5a expression particularly marks luminal progenitors and its high level of sphere formation depicts its relatively high position in the cell hierarchy. The Lum population complements the puPgt in comprising the luminal compartment and represents the lower boundary of the luminal lineage. It encompasses differentiated luminal cells expressing high levels of the luminal genes CK18, GATA3, and Cx32, as well as ERa and PR, indicating little in-vivo stem cell activity in mice and humans . These cells also exhibited low sphere formation and development, and negative propagation potential in culture. Apparently, the Lum population exhibits the characteristics of the milk-producing cells in the lactating cow that show almost no proliferation. Their gradual apoptosis during the lactation period is the main cause for decreased milk production . Of note is the propagation-rate analysis that was applied here to evaluate the distance of an epithelial cell from its fully differentiated state. Once stem cells are removed from their invivo environment and seeded under adherent culture conditions, they lose their quiescent state and initiate proliferation followed by differentiation . It is this very property that warrants the development of various methods to maintain stem cells undifferentiated in culture .