AbstractThe tooth is one of the ectodermal organs that develop from epithelial–mesenchymal interactions during embryonic development. An understanding of the underlying molecular mechanisms would improve our knowledge of the growth factors that regulate cell proliferation and differentiation. One of the related aspects is mitogen‐activated protein kinase (MAPK) signaling in tooth differentiation. The extracellular‐signal regulated kinase (ERK)/mitogen‐activated protein kinase kinase (MEK) cascade plays a pivotal role in many of the essential cellular processes underlying embryonic development, including responses to major developmental changes. However, the role of the ERK pathway in molar development is unclear. This study investigated epithelial patterning and tooth growth in the mouse embryo by monitoring ERK and fibroblast growth factor (FGF) signaling. ERK, MEK, and phosphatase and tensin homolog (PTEN) were activated at different levels and locations in the developing tooth at E13.5 to E16.5 and PN2. ERK was activated in the inner dental epithelium and cervical loop, while PTEN was activated in the outer dental epithelium. In addition, only ERK was activated in secretory ameloblast at PN2. To further define the pathways involving FGF and ERK, tooth germs were cultured in the presence of compounds to inhibit MAPK/ERK‐mediated signaling. Western blot analysis indicated that pERK2 was strongly activated in the tooth germ. Moreover, the activation level of pERK1 was dramatically increased by exogenous FGF10 alone and by combined treatment with FGF10 and U0126. The reported results will improve our understanding of the unique developmental processes of the dental epithelium and tooth growth, and will help to elucidate the fundamental mechanisms of ERK signaling underlying tooth development. J. Exp. Zool. (Mol. Dev. Evol.) 312B:901–911, 2009. © 2009 Wiley‐Liss, Inc.