Cell dynamics in the Growth and Differentiation of Dental Epithelium during Tooth Development: Stratum Intermedium Cells Originated From Inner Enamel Epithelium

The stratum intermedium develops as flattened cell layer on the proximal side of the ameloblast layer during the bell stage of tooth development. Stratum intermedium cells strongly express alkaline phosphatase (ALP) activity and have been considered to play a complementary role in the enamel mineralization, however the origin and/or the role of these cells have not been elucidated. In the present study, we focused on the lineage of stratum intermedium cells in continuously growing rodent incisors and analyzed it by using DiI tracers experiment and using the incisors organ culture. The results indicated that some stratum intermedium cells were originated from the inner enamel epithelium. Immunohistochemical and in situ hybridization studies showed that the stratum intermedium cells expressed the Notch-1, Notch-2, and Hes1, while the inner enamel epithelium and ameloblasts expressed their ligands Jagged-1. Furthermore, we examined th role of Notch signaling in the development of the stratum intermedium cells by use of the dental epithelial cell line, HAT-7. Recombinant Jagged1 protein enhanced the appearance of the stratum intermedium cells in HAT-7.On the other hand, anti-sense Notch1 decreased the number of stratum intermedium cells.Taken together, we propose a hypothesis that the lineage of the stratum intermedium differentiates from the ameloblasts lineage through Notch signaling.