In tooth development, the oral ectoderm and mesenchyme coordinately and reciprocally interact through the basement membrane for their growth and differentiation to form the proper shape and size of the tooth. Laminin alpha 5 subunit-containing laminin-10/11 (LM-511/521) is the major laminin in the tooth germ basement membrane. Here, we have examined the role of laminin alpha 5 (Lama5) in tooth development using laminin alpha 5-null mouse primary dental epithelium and tooth germ organ cultures. Lama5-null mice develop a small tooth germ with defective cusp formation and have reduced proliferation of dental epithelium. Also, cell polarity and formation of the monolayer of the inner dental epithelium are disturbed. The enamel knot, a signaling center for tooth germ development, is defective, and there is a significant reduction of Shh and Fgf4 expression in the dental epithelium. In the absence of laminin alpha 5, the basement membrane in the inner dental epithelium becomes discontinuous. In normal mice, integrin alpha 6 beta 4, a receptor for laminin alpha 5, is strongly localized at the basal layer of the epithelium, whereas in mutant mice, integrin alpha 6 beta 4 is expressed around the cell surface. In primary dental epithelium culture, laminin-10/11 promotes cell growth, spreading, and filopodia-like microspike formation. This promotion is inhibited by anti-integrin alpha 6 and beta 4 antibodies and by phosphatidylinositol 3-kinase inhibitors and dominant negative Rho-GTPase family proteins Cdc42 and Rac. In organ culture, anti-integrin alpha 6 antibody and wortmannin reduce tooth germ size and shape. Our studies demonstrate that laminin alpha 5 is required for the proliferation and polarity of basal epithelial cells and suggest that the interaction between laminin-10/11-integrin alpha 6 beta 4 and the phosphatidylinositol 3-kinase-Cdc42/Rac pathways play an important role in determining the size and shape of tooth germ.