Fig. 7. The role of Mi-2ß in skin development: a model for the development of the skin and its appendages and the role of Mi-2ß in this process. Successive stages in the development of wild-type skin are depicted on the upper time line. We propose that ectodermal cells (yellow) are first committed to an epidermal TA cell (blue) that can make epidermis but has limited proliferative potential and developmental plasticity. This cell type is then converted to an epidermal stem cell (green) with extensive proliferative capacity and plasticity to adopt alternative fates. Starting at E14.5, some of these cells are induced to become follicular progenitor cells (pink), and sometime thereafter other epidermal stem cells give rise to TA cells of the epidermis with more restricted proliferative and developmental potential. Follicular progenitors proliferate to make the hair peg, while epidermal stem and TA cells generate the stratified epidermis. Finally, a subset of follicular progenitors at the base of the follicle are specified as the matrix stem cells (orange) that give rise to the hair shaft and inner root sheath over the anagen phase of the hair cycle (right). These matrix stem cells are distinct from the follicular bulge stem cells (purple) that regenerate the lower follicle in the adult. The three phenotypes resulting from deletion of Mi-2ß at different stages of development are shown (1-3, indicated by a red cross) and interpretations of these phenotypes in the context of the model are shown beneath.