The cellular basis for epithelial-mesenchymal interaction during mouse tooth development

In this study, using mouse molar as the model, we developed a dual fluorescence reporter mouse to precisely track and analyze dental epithelium and mesenchyme at single-cell resolution from early embryonic to postnatal stages. Moreover, we constructed the virtual molar explorer (VMEx) to spatially map 15,967 molar-expressed genes and identified that Msx1+ Sdc1+ marked the developing dental papilla while surrounded by Msx1+ Sdc1- molar niche. Through tooth germ reconstitution and organoid culture in vitro and kidney capsule transplantation in vivo, we provided evidence that the Msx1+ Sdc1- dental follicle cells might function as the tooth organizers that promoted epithelium survival and tooth germ organization. Furthermore, the appearance of Msx1+ Sdc1+ dental papilla cells relied on the interaction between dental epithelium and Msx1+ Sdc1- dental follicle cells. Together, our results revealed the cellular dynamics of tooth development in mice and identified that the dental follicle might be the key driver of epithelial-mesenchymal interaction and tooth morphogenesis. Tooth germs were dissected and collected at different development stages, including bud stage (E12.5), cap stage (E14.5), bell stage (E16.5), postnatal day 1 (PN1) and day 7 (PN7). Epithelial and mesenchymal cells were enriched by FACS sorting using copGFP and tdTomato fluorescence. Cells were then used for single-cell RNA sequencing using 10X Genomics platform.