Sox2 controls asymmetric patterning of ameloblast lineage commitment by regulation of FGF signaling in the mouse incisor

Mouse incisors are covered by enamel only on the labial side and the lingual side is covered by dentin without enamel. This asymmetric distribution of enamel makes it possible to be abrased on the lingual side, generating the sharp cutting edge of incisor on the labial side. The abrasion of mouse incisors is compensated by the continuous growth throughout life. Epithelium stem cells responsible for its continuous growth are reported to localize within the labial cervical loop. The transcription factor Sox2 plays important roles in the maintenance of stem cell pluripotency and organ formation. We previously found that Sox2 mainly expressed in the dental epithelium. Besides, Sox2 has been reported to be a dental epithelium stem cell marker in the incisor. However, the exact mechanism of Sox2 controlling amelogenesis is still not quite clearly elucidated. Here we report that conditional deletion of Sox2 in the dental epithelium using Shhcre caused impaired ameloblast differentiation in the labial side and induced ectopic ameloblast-like cell differentiation on the lingual side. Abnormal FGF gene expression was detected by RNAscope in situ hybridization in the mutant incisor. Collectively, we speculate that asymmetric ameloblast lineage commitment of mouse incisor might be regulated by Sox2 through FGF signaling.

Spatial and temporal expression of Sox9 during murine incisor development

Mouse incisors are capable of continuously growing due to the renewal of dental epithelium stem cells and mesenchymal stem cells residing at the proximal ends. The transcription factor Sox9 plays important roles in maintaining the stem cells of hair follicles, retinal progenitor cells and neural crest stem cells. Whether Sox9 is involved during mouse incisor development is not reported yet. In this study, we examined the expression pattern of Sox9 during mouse incisor development by in situ hybridization and immunohistochemistry. Sox9 mRNA and protein showed similar expression pattern from embryonic day (E) 13.5 to postnatal (PN) day 10. At E13.5 and E14.5, Sox9 was strongly expressed in the dental epithelium. At E16.5, Sox9 started to be detected in the mesenchymal cells within the dental pulp, especially the dental pulp cells that adjacent to the labial cervical loop. Similarly with E14.5, Sox9 was strongly detected in the labial cervical loop, including the basal epithelium, the stellate reticulum and the outer enamel epithelium from E16.5 to PN10. The mesenchyme adjacent to the labial cervical loop also showed strong signal of Sox9. The spatiotemporal expression of Sox9 suggested its possible involvement during mouse incisor development.

Stem Cells in Tooth Development, Growth, Repair, and Regeneration

Human teeth contain stem cells in all their mesenchymal-derived tissues, which include the pulp, periodontal ligament, and developing roots, in addition to the support tissues such as the alveolar bone. The precise roles of these cells remain poorly understood and most likely involve tissue repair mechanisms but their relative ease of harvesting makes teeth a valuable potential source of mesenchymal stem cells (MSCs) for therapeutic use. These dental MSC populations all appear to have the same developmental origins, being derived from cranial neural crest cells, a population of embryonic stem cells with multipotential properties. In rodents, the incisor teeth grow continuously throughout life, a feature that requires populations of continuously active mesenchymal and epithelial stem cells. The discrete locations of these stem cells in the incisor have rendered them amenable for study and much is being learnt about the general properties of these stem cells for the incisor as a model system. The incisor MSCs appear to be a heterogeneous population consisting of cells from different neural crest-derived tissues. The epithelial stem cells can be traced directly back in development to a Sox10(+) population present at the time of tooth initiation. In this review, we describe the basic biology of dental stem cells, their functions, and potential clinical uses.

Mesenchymal TGF-β signaling orchestrates dental epithelial stem cell homeostasis through Wnt signaling

In mouse, continuous growth of the postnatal incisor is coordinated by two populations of multipotent progenitor cells, the dental papilla mesenchymal cells and dental epithelial stem cells, residing at the proximal end of the incisor, yet the molecular mechanism underlying the cooperation between mesenchymal and epithelial cells is largely unknown. Here, transforming growth factor-β (TGF-β) type II receptor (Tgfbr2) was specifically deleted within the postnatal dental papilla mesenchyme. The Tgfbr2-deficient mice displayed malformed incisors with wavy mineralized structures at the labial side as a result of increased differentiation of dental epithelial stem cells. We found that mesenchymal Tgfbr2 disruption led to upregulated expression of Wnt5a and downregulated expression of Fgf3/10 in the mesenchyme, both of which synergistically enhanced Lrp5/6-β-catenin signaling in the cervical loop epithelium. In accord with these findings, mesenchyme-specific depletion of the Wnt transporter gene Wls abolished the aberrant mineralized structures caused by Tgfbr2 deletion. Thus, mesenchymal TGF-β signaling provides a unifying mechanism for the homeostasis of dental epithelial stem cells via a Wnt signaling-mediated mesenchymal-epithelial cell interaction.

An In vivo Model for Short-Term Evaluation of the Implantation Effects of Biomolecules or Stem Cells in the Dental Pulp

The continuously growing rodent incisor is a widely used model to investigate odontogenesis and mineralized tissue formation. This study focused on evaluating the mouse mandibular incisor as an experimental biological tool for analyzing in vivo the capacity of odontoblast-like progenitors or bioactive molecules to contribute to reparative dentinogenesis. We describe here a surgical procedure allowing direct access to the forming part of the incisor dental pulp Amelogenin peptide A+4 adsorbed on agarose beads, or dental pulp progenitor cells were implanted in the pulp following this procedure. After 10 days A+4 induced the formation of an osteodentin occluding almost the totality of the pulp compartment. Implantation of progenitor cells leads to formation of islets of osteodentin-like structures located centrally in the pulp. These pilot studies validate the incisor as an experimental model to test the capacity of progenitor cells or bioactive molecules to induce the formation of reparative dentin.