The purpose of the proposed studies is to improve our understanding of the basic physico-chemical mechanisms of initial enamel mineralization and formation of the dentin-enamel interface. Enamel mineralization starts at the surface of a layer of mineralized dentin, which may play an important role in initial enamel formation. Dentin and enamel are two mineralized tissues with strikingly different mechanical and structural properties that normally perform jointly for tens of years, without failure. Such an outstanding mechanical endurance requires an extraordinarily strong bond between these two tissues. Studies of tooth- related genetic disorders and knockout animals demonstrate that the correct formation of the dentin-enamel interface is essential for the proper tooth function. The problem of interface stability is also very important with respect to tissue repair, where often implant failure occurs due to a weak interface between tissues and repair materials. It is likely that interactions between dentin and enamel tissues during initial mineralization play an important role in the proper formation of this critically important interface. We hypothesize that the onset of mineralization at the boundary between enamel and dentin is a highly integrated and specialized process, essential for the proper formation of this mechanically robust interface. Wealso hypothesize that the mechanism of initial enamel formation at this interface is different from other stages of amelogenesis. We have identified three major factors that distinguish initial enamel formation from later stages of amelogenesis: 1) the presence of dentin mineral at the site of initial enamel formation; 2) the presence of transient mineral phases; and 3) a unique macromolecular composition at the site of initial enamel formation. Thus, we propose a comprehensive study of the formation of the dentin-enamel interface using a variety of in vivo and in vitro approaches to gain a better understanding of basic mechanisms of initial enamel formation. Specifically, we propose in Aim1: To study the interactions between dentin and enamel crystals at the site of initial enamel mineralization, with special emphasis on the epitaxial relationship between dentin and enamel crystals; in Aim 2: To study the role of transient amorphous mineral phases in initial stages of enamel formation; and in Aim 3: To study the macromolecular composition at the initial enamel mineralization site with special emphasis on protein-mineral interactions, and to elucidate the possible functions of these macromolecules and supra-molecular assemblies in initial enamel formation using a mechanistic approach, in a series of in vitro mineralization experiments. We expect that the results of this study will improve our knowledge of the basic processes involved in the formation of the dentin-enamel interface that will hopefully lead to the development of new advanced materials and/or procedures for mineralized tissue repair.