Myoepithelial cells in the control of mammary development and tumorigenesis: data from genetically modified mice.

Until recently, myoepithelial cells-the second major cell population in the mammary epithelium-were not considered to play an important role in the morphogenetic events during gland development. Mouse mutants with changes in the gene expression pattern characteristic of the basal myoepithelial cell layer have been generated and used to show that these cells influence the proliferation, survival and differentiation of luminal cells, modulate stromal-epithelial interactions and actively participate in mammary morphogenesis. Various cellular and molecular mechanisms may underlie the observed phenotypes. These include an unbalanced expression of matrix degrading metalloproteinases (MMPs) and their inhibitors, leading to changes in the composition and organization of the (extracellular matrix) ECM, the production of soluble growth factors affecting stromal and epithelial cell growth and differentiation and direct signaling through cell-cell contacts between the myoepithelial and luminal cell layers.