Epithelial cells have a polarized morphology, with distinct basal, lateral, and apical cell surfaces. It would be of considerable interest to know how the polarized morphology develops during embryogenesis. Both the tubular and glomerular epithelial cells of the kidney develop from mesenchymal stem cells during embryogenesis. A unique conversion of nonpolar cells to polarized epithelial cells thus occurs in the embryonic kidney. This conversion also occurs in vitro if the mesenchymal cells are properly induced. Organ cultures of mesenchymal cells from the mouse embryonic kidney have therefore been much used to study the development of epithelial cell polarity. We have used this model system to study the role of basement membrane glycoproteins in development. The results obtained suggest that laminins are particularly important for epithelial cell development. There are many different types of laminins. Developing kidney tubule cells synthesize a laminin isoform with the chain composition A-B1-B2, and it seems to promote development by interacting with specific integrin receptors on the cell surface. The mesenchymal stem cells also produce laminin B chains, but not the A chain, and they also lack the integrin receptor that interacts with A-B1-B2 laminin.