Spatiotemporal expression of CD166 and osterix during rat molar development.

OBJECTIVES: To compare the spatiotemporal localization of Activated leukocyte cell adhesion molecule (Alcam; CD166) and Osterix (OSX), encoded by Sp7, an established odontoblast differentiation marker, during rat molar development using immunohistochemical approaches. DESIGN: Mandibles from Wistar male rats were collected at embryonic days (E) 15-20 and postnatal days (P) 2-49. The expression patterns of CD166 and OSX during molar development and maturation were examined using immunohistochemistry and immunofluorescence staining to assess the differentiation of odontoblast lineage cells. RESULTS: During embryonic development (E15-E20), CD166 immunoreactivity was absent in the dental papilla but appeared in a portion of the dental follicle. At the late bell stage (P2), OSX immunoreactivity was observed in odontoblasts adjacent to newly formed primary dentin, whereas CD166 immunoreactivity was not detected in the coronal pulp at this stage. From P7, when root formation begins, and throughout root elongation, CD166 immunoreactivity was observed in short cells located near Hertwig's epithelial root sheath. More coronally positioned cells exhibited OSX-positive odontoblasts undergoing differentiation. At P28, CD166 and OSX immunoreactivity showed overlapping localization in a subset of cells in the apical region; however, by P49, both immunoreactivities had largely diminished. CONCLUSIONS: The CD166-positive cells appeared earlier than OSX expression but were restricted to the root apex during the root elongation period. These findings suggest that CD166 may serve as a potential surface marker for progenitor cells involved in odontoblast differentiation during root development.