Distinction of dental mesenchymal cell populations during molar tooth germ development via analysis of the spatiotemporal expression patterns of mRNAs for Shh ligand receptors Ptch1 and Hip1 using in situ hybridization.

Abstract

Background: During early tooth development, cranial neural crest cells migrating to the oral region form dental mesenchyme. To date, the molecular mechanism by which dental mesenchymal cells further migrate and differentiate into odontoblasts remains largely unknown. We hypothesize that the dental epithelium, being the only source of Shh, a chemotactic peptide, attracts future odontoblasts expressing the Shh receptor Ptch1.

Results: Changes in the spatiotemporal distribution of cells expressing Shh, Ptch1, and Hip1 and of nestin protein throughout early tooth development were evaluated via in situ hybridization and immunohistochemistry, respectively. Dental mesenchymal cells were found to be separated into three putative cell population types based on spatiotemporal gene expression patterns: presumptive Ptch1(-)/Hip1(-) non-odontoblastic cells forming the initial dental papilla cell cluster; presumptive Ptch1(+)/Hip1(-) odontoblast lineage cells; and presumptive Ptch1(+)/Hip1(+) cells possibly forming dental follicle and surrounding mesenchyme. Ptch1(+)/Hip1(-) cells seemed to invade the developing dental papilla actively at embryonic day 15.5 (E15.5), resulting in an abundant distribution of the cells by the early bell stage at E16.5. The distribution pattern of presumptive Ptch1(+)/Hip1(-) cells partially overlapped with that of nestin-expressing cells.

Conclusion: Presumptive Ptch1(+)/Hip1(-) dental mesenchymal cells may migrate to the cusp region of the dental papilla, leading to odontoblast differentiation.