It is now well established that the enamel organ (EO) has two major phases of activity; matrix production and maturation. Dramatic differences in cell structure between secretory and post-secretory ameloblasts, stratum inter medium and papillary cells, are characteristic of each phase. The applicant has contributed to research in this area, describing the ultrastructure of the post-secretory enamel organ and its vasculature, as well as being the first to document the nature of complex nexustype junctions within the papillary layer of the enamel organ. The post-secretory E.O. possesses many features that are usually found in transport epithelia. Among these are increased surface area, extensive nexus-type junctions, increased pinocytosis and increased numbers of mitochondria. In this application we propose to examine the cell surfaces of the secretory and post-secretory EO by transmission electron microscopy, freeze-fracture methods, cytochemistry and autoradiography. The major objectives are to quantitate and functionally characterize nexus-type junctions, to study the distribution of membrane particles, to localize membrane Na-K-ATPase by both cytochemistry and 3H-ouabain binding and to investigate membrane cycling, including coated vesicle function. Pursuit of these objectives should lead to a better understanding of the function of the EO in enamel maturation.