1. ^*Department of Pathology, School of Medicine, University of Southern California, Los Angeles, California, U.S.A.
2. ^†Department of Molecular Biology, Tzu Chi College of Medicine and Humanity, Hualien, Taiwan, R.O.C.

Correspondence: Dr Cheng-Ming Chuong, Department of Pathology, School of Medicine, University of Southern California, HMR 315 B, 2011 Zonal Ave., Los Angeles, CA 90033. Email: chuong@pathfinder.hsc.usc.edu

Received 26 January 1999; Revised 22 July 1999.

Abstract

During organogenesis, the issue of size regulation is as important as shape and differentiation. We propose that the regulation of the dimensions of the epithelium and its appendages (length, width, thickness) are based on regulation 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 from the domain of proliferating cells to the domain of postmitotic differentiated cells. When the homeobox gene Msx-2 is over-expressed in transgenic mice under the control of the CMV promoter, the epidermis is thickened with hyperproliferation and hyperkeratosis. Hairs are shorter and the matrix region 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 manifestations of an increased cellular flow from proliferation domain to differentiation domain in the tissue.