Msx-2 and the regulation of organ size: Epidermal thickness and hair length

During organogenesis, the issue of size regulation is as important as shape and differentiation. We propose that the regulation of the dimensions of t he epithelium and its appendages (length, width, thickness) are based on re gulation of cell numbers in specific sites, reflecting the input and output of cells in that region. This process is in turn regulated by the flow fro m the domain of proliferating cells to the domain of postmitotic differenti ated cells. When the homeobox gene Msx-2 is over-expressed in transgenic mi ce under the control of the CMV promoter, the epidermis is thickened with h yperproliferation and hyperkeratosis. Hairs are shorter and the matrix regi on is shrunken. We suggest that Msx-2 may be one of the regulators involved in the control of organ size, and the above phenotypes are the manifestati ons of an increased cellular flow from proliferation domain to differentiat ion domain in the tissue.