Plasticity of Dental Cell Types in Development, Regeneration, and Evolution

The impact of biological variables on cell kinetics and differentiation dynamics in the mouse incisor epithelium

The mouse incisor is a key model system for understanding the regulatory mechanisms governing dental stem cells (SCs). However, the effects of sex, age and strain on mouse incisor morphology and epithelial SC function remain unclear. We used micro-computed tomography and histology to analyze the apical region in males and females, two age groups, and two commonly used strains. Cell kinetics, enamel density, and volume were assessed to determine their impact on SC behavior and enamel properties. No differences were found in cell kinetics or enamel properties between male and female mice at 8 weeks of age. However, 3-week-old mice exhibited higher cell proliferation, lower enamel density, and reduced volume than 8-week-olds, highlighting age-dependent changes in SC activity and enamel formation. Additionally, strain-specific variations were observed, with ICR mice showing increased numbers of preameloblasts and higher enamel volume with lower density when compared to C57BL/6 mice at 8 weeks old. Our results establish a standardized framework for the examination of mouse incisor epithelial SCs. These standards will enhance research reproducibility and consistency, facilitate constructive critique by reviewers, and enable a deeper understanding of the complex factors influencing SC behavior across diverse physiological contexts.

RNAseq of peripheral blood mononucleated cells exposed to platelet-rich fibrin and enamel matrix derivatives

Platelet-rich fibrin (PRF) and Enamel Matrix Derivatives (EMD) can support the local regenerative events in periodontal defects. There is reason to suggest that PRF and EMD exert part of their activity by targeting the blood-derived cells accumulating in the early wound healing blastema. However, the impact of PRF and EMD on blood cell response remains to be discovered. To this aim, we have exposed human peripheral blood mononucleated cells (PBMCs) to PRF lysates prepared by a swing-out rotor and EMD, followed by bulk RNA sequencing. A total of 111 and 8 genes are up- and down-regulated by PRF under the premise of an at least log2 two-fold change and a minus log10 significance level of two, respectively. Representative is a characteristic IFN response indicated by various human leukocyte antigens (HLA-DPA1, HLA-DPB1, HLA-DQA1, HLA-DQA2, HLA-DRA, HLA-DRB1, HLA-DRB5), gamma Fc receptors (FCGR1A, FCGR1B, FCGR3B), chemokines (CXCL9-11), and calprotectin (S100A8/9 and S100A12), complement (C1QA/B, C2) and interferon-induced guanylate-binding proteins (GBP1, GBP5). With EMD, 67 and 29 genes are up- and down-regulated, respectively. Characteristic of the upregulated genes are tensins (TNS1 and TNS3). Among the genes downregulated by EMD were epsilon Fc receptors (FCER1A; FCER2), Fc receptor-like proteins (FCRL1, FCRL3) and CX3CR1. Genes commonly upregulated by PRF and EMD were most noticeably NXPH4 and MN1, as well as FN1, MMP14, MERTK, and AXL. Our findings suggest that PRF provokes an inflammatory response, while EMD dampens IgE signaling in peripheral mononucleated blood cells.

Bank vole genomics links determinate and indeterminate growth of teeth

BACKGROUND

Continuously growing teeth are an important innovation in mammalian evolution, yet genetic regulation of continuous growth by stem cells remains incompletely understood. Dental stem cells responsible for tooth crown growth are lost at the onset of tooth root formation. Genetic signaling that initiates this loss is difficult to study with the ever-growing incisor and rooted molars of mice, the most common mammalian dental model species, because signals for root formation overlap with signals that pattern tooth size and shape (i.e., cusp patterns). Bank and prairie voles (Cricetidae, Rodentia, Glires) have evolved rooted and unrooted molars while retaining similar size and shape, providing alternative models for studying roots.

RESULTS

We assembled a de novo genome of Myodes glareolus, a vole with high-crowned, rooted molars, and performed genomic and transcriptomic analyses in a broad phylogenetic context of Glires (rodents and lagomorphs) to assess differential selection and evolution in tooth forming genes. Bulk transcriptomics comparisons of embryonic molar development between bank voles and mice demonstrated overall conservation of gene expression levels, with species-specific differences corresponding to the accelerated and more extensive patterning of the vole molar. We leverage convergent evolution of unrooted molars across the clade to examine changes that may underlie the evolution of unrooted molars. We identified 15 dental genes with changing synteny relationships and six dental genes undergoing positive selection across Glires, two of which were undergoing positive selection in species with unrooted molars, Dspp and Aqp1. Decreased expression of both genes in prairie voles with unrooted molars compared to bank voles supports the presence of positive selection and may underlie differences in root formation.

CONCLUSIONS

Our results support ongoing evolution of dental genes across Glires and identify candidate genes for mechanistic studies of root formation. Comparative research using the bank vole as a model species can reveal the complex evolutionary background of convergent evolution for ever-growing molars.

Vole genomics links determinate and indeterminate growth of teeth

Continuously growing teeth are an important innovation in mammalian evolution, yet genetic regulation of continuous growth by stem cells remains incompletely understood. Dental stem cells responsible for tooth crown growth are lost at the onset of tooth root formation. Genetic signaling that initiates this loss is difficult to study with the ever-growing incisor and rooted molars of mice, the most common mammalian dental model species, because signals for root formation overlap with signals that pattern tooth size and shape (i.e., cusp patterns). Different species of voles (Cricetidae, Rodentia, Glires) have evolved rooted and unrooted molars that have similar size and shape, providing alternative models for studying roots. We assembled a de novo genome of Myodes glareolus, a vole with high-crowned, rooted molars, and performed genomic and transcriptomic analyses in a broad phylogenetic context of Glires (rodents and lagomorphs) to assess differential selection and evolution in tooth forming genes. We identified 15 dental genes with changing synteny relationships and six dental genes undergoing positive selection across Glires, two of which were undergoing positive selection in species with unrooted molars, Dspp and Aqp1. Decreased expression of both genes in prairie voles with unrooted molars compared to bank voles supports the presence of positive selection and may underlie differences in root formation. Bulk transcriptomics analyses of embryonic molar development in bank voles also demonstrated conserved patterns of dental gene expression compared to mice, with species-specific variation likely related to developmental timing and morphological differences between mouse and vole molars. Our results support ongoing evolution of dental genes across Glires, revealing the complex evolutionary background of convergent evolution for ever-growing molars.

Protostylid: Contributing and Aggravating Factor for Periodontitis

Clinicians should be well-versed in the anatomy, variations, and teeth anomalies. Developmental disturbances of the teeth can lead to alterations in size, shape, number, structure, and eruption of the teeth. Developmental disturbances can lead to germination, fusion, concrescence, dilaceration, talons, cusps, dens in dente, etc. Protostylid, an additional cusp on the buccal aspect of the maxillary molar, which is a rare clinical finding, can lead to plaque accumulation, making oral hygiene maintenance difficult. This leads to clinical attachment loss and bone loss. This condition may often go undiagnosed. It should be diagnosed to prevent further complications. This case has been reported to make clinicians aware of the importance of diagnosing the case at the earliest possible time so that complications can be prevented and management is easier. From the perspective of forensic dentistry, this morphological feature, though uncommon, may be useful for the classification and identification of victims in mass causalities and bite marks on bodies or inanimate objects. This is one of the rarest cases of protostylids reported to date. This may not only pose a significant problem in endodontic therapy due to morphological alterations in root canals and periodontal therapy due to grove formation leading to an inability to maintain a plaque-free area (bone loss) but also be of very significant interest from the perspective of forensic dentistry.

Mechanobiology of Dental Pulp Cells

The dental pulp is the inner part of the tooth responsible for properly functioning during its lifespan. Apart from the very big biological heterogeneity of dental cells, tooth microenvironments differ a lot in the context of mechanical properties-ranging from 5.5 kPa for dental pulp to around 100 GPa for dentin and enamel. This physical heterogeneity and complexity plays a key role in tooth physiology and in turn, is a great target for a variety of therapeutic approaches. First of all, physical mechanisms are crucial for the pain propagation process from the tooth surface to the nerves inside the dental pulp. On the other hand, the modulation of the physical environment affects the functioning of dental pulp cells and thus is important for regenerative medicine. In the present review, we describe the physiological significance of biomechanical processes in the physiology and pathology of dental pulp. Moreover, we couple those phenomena with recent advances in the fields of bioengineering and pharmacology aiming to control the functioning of dental pulp cells, reduce pain, and enhance the differentiation of dental cells into desired lineages. The reviewed literature shows great progress in the topic of bioengineering of dental pulp-although mainly in vitro. Apart from a few positions, it leaves a gap for necessary filling with studies providing the mechanisms of the mechanical control of dental pulp functioning in vivo.

PRX1-positive mesenchymal stem cells drive molar morphogenesis

Mammalian teeth, developing inseparable from epithelial-mesenchymal interaction, come in many shapes and the key factors governing tooth morphology deserve to be answered. By merging single-cell RNA sequencing analysis with lineage tracing models, we have unearthed a captivating correlation between the contrasting morphology of mouse molars and the specific presence of PRX1+ cells within M1. These PRX1+ cells assume a profound responsibility in shaping tooth morphology through a remarkable divergence in dental mesenchymal cell proliferation. Deeper into the mechanisms, we have discovered that Wnt5a, bestowed by mesenchymal PRX1+ cells, stimulates mesenchymal cell proliferation while orchestrating molar morphogenesis through WNT signaling pathway. The loss of Wnt5a exhibits a defect phenotype similar to that of siPrx1. Exogenous addition of WNT5A can successfully reverse the inhibited cell proliferation and consequent deviant appearance exhibited in Prx1-deficient tooth germs. These findings bestow compelling evidence of PRX1-positive mesenchymal cells to be potential target in regulating tooth morphology.

Using Ex Vivo Live Imaging to Investigate Cell Divisions and Movements During Mouse Dental Renewal

The continuously growing mouse incisor is emerging as a highly tractable model system to investigate the regulation of adult epithelial and mesenchymal stem cells and tooth regeneration. These progenitor populations actively divide, move, and differentiate to maintain tissue homeostasis and regenerate lost cells in a responsive manner. However, traditional analyses using fixed tissue sections could not capture the dynamic processes of cellular movements and interactions, limiting our ability to study their regulations. This paper describes a protocol to maintain whole mouse incisors in an explant culture system and live-track dental epithelial cells using multiphoton timelapse microscopy. This technique adds to our existing toolbox for dental research and allows investigators to acquire spatiotemporal information on cell behaviors and organizations in a living tissue. We anticipate that this methodology will help researchers further explore mechanisms that control the dynamic cellular processes taking place during both dental renewal and regeneration.