Abstract Cementum is a specialized mineralized tissue that covers the tooth root surface and anchors the tooth to the surrounding periodontal ligament. The cervical acellular cementum (AC) is indispensable for periodontal attachment and represents a key target for periodontal regenerative therapies. However, the developmental origin and molecular identity of AC-forming cementoblasts remain poorly understood. Here, through an integrated spatial and single-cell transcriptomic analysis, we identify AC-forming cementoblasts as a distinct population of noncanonical mineralizing cells enriched for cell-matrix organization and Wnt signaling signatures, distinguishing them from cellular cementum-forming cementoblasts localized in the apical portion. Lineage tracing using a Wnt inhibitory factor 1 (Wif1-creER) demonstrates that AC-forming cementoblasts originate exclusively from peri-epithelial apical Wif1 + cells of the elongating tooth root via canonical Wnt signaling activation. Collectively, our findings uncover the unique ontogeny and molecular signature of acellular cementum, providing insights into the formation and maintenance of the periodontal attachment apparatus.

The periodontium is a dynamic composite tissue that anchors teeth to the alveolar bone and adapts continuously to mechanical loading and inflammatory challenges throughout life. Its development, homeostasis, and regeneration depend on diverse mesenchymal stem and progenitor cell populations derived from the dental follicle, apical papilla, and periodontal ligament. Recent advances in in vivo lineage-tracing studies have demonstrated that these progenitors are heterogeneous, and lineage restricted, rather than constituting a uniform stem cell pool. Distinct populations, including PTHrP⁺ dental follicle–derived progenitors and CXCL12⁺ apical papilla cells, contribute differentially to cementum, periodontal ligament, and alveolar bone formation during tooth root development and postnatal remodeling. The fate decisions of these progenitors are tightly regulated by coordinated signaling pathways, including Hedgehog–Foxf, Wnt/β-catenin, and BMP/TGF-β signaling, as well as by mechanotransducive cues. Spatially and temporally precise regulation of these pathways is essential for proper cementogenesis and alveolar bone formation, whereas their dysregulation leads to impaired periodontal regeneration and bone loss. This review summarizes current knowledge of periodontal stem cell biology with a particular emphasis on developmental origins and in vivo regulatory mechanisms and discusses emerging concepts that support endogenous stem cell–based approaches for periodontal regeneration and future therapeutic strategies.

The tooth root is a critical component of the tooth anchored to surrounding alveolar bones. Tooth root formation is driven by cells in the apical papilla (AP) that generate new dentin-forming odontoblasts at the root-forming front. Mesenchymal stem cells have been isolated from AP for regenerative use; however, how AP cells physiologically coordinate tooth root formation remains undefined. We find that CXCL12+ cells emerge in AP under hypoxic environments at the onset of tooth root formation. Using Cxcl12-creER-based cell-lineage analysis, we further find that CXCL12+ AP cells contribute not only to odontoblasts but also to cementum-forming cementoblasts of the elongating root, while showing plasticity to alveolar bone osteoblasts under regenerative conditions. Canonical Wnt inactivation inhibits odontoblast fates of CXCL12+ AP cells and induces substantial root truncation, with their aberrant fibroblast fates suppressed by TGF-β receptor inhibitor galunisertib. Therefore, CXCL12+ AP cells maintain odonto-cementogenic fates in a Wnt-dependent manner, identifying these cells as pivotal dental mesenchymal progenitor cells driving tooth root formation with substantial plasticity.

Alveolar bone supports and anchors teeth. The parathyroid hormone-related protein (PTHrP) pathway plays a key role in alveolar bone biology. Salt-inducible kinases (SIKs) are important downstream regulators of PTH/PTHrP signaling in the appendicular skeleton, where SIK inhibition increases bone formation and trabecular bone mass. However, the function of these kinases in alveolar bone remains unknown. Here, we report a critical role for SIK2/SIK3 in alveolar bone development, homeostasis, and socket healing after tooth extraction. Inducible SIK2/SIK3 (Ubq-creERt;Sik2f/f;Sik3f/f) deletion led to dramatic alveolar bone defects without changes in tooth eruption. Ablating these kinases impairs alveolar bone formation due to disrupted osteoblast maturation, a finding associated with ectopic periostin expression by fibrous cells in regions of absent alveolar bone at steady state and following molar extraction. Notably, this phenotype is the opposite of the increased trabecular bone mass observed in long bones following SIK2/SIK3 deletion. Distinct phenotypic consequences of SIK2/SIK3 deletion in appendicular versus craniofacial bones prompted us to identify a specific transcriptomic signature in alveolar versus long bone osteoblasts. Thus, SIK2/SIK3 deletion illuminates a key role for these kinases in alveolar bone biology and highlights the emerging concept that different osteoblast subsets utilize unique genetic programs.

The alveolar bone is a specialized mineralized structure supporting the lifelong functionality of the tooth in mastication. The alveolar bone develops from the dental follicle (DF) during tooth root formation due to deliberate epithelial-mesenchymal interactions. However, how DF progenitor cell fates are regulated toward alveolar bone osteoblasts remains unknown. We find that Hedgehog signaling activities are transiently activated during the onset of tooth root formation and alveolar bone formation. Parathyroid hormone-related protein (PTHrP)-expressing DF cells are highly responsive to Hedgehog signaling, yet constitutive Hedgehog activation using Pthrp-creER and Ptch1-floxed alleles potently suppresses alveolar osteoblast and ligament differentiation of PTHrP+ DF cells, resulting in striking susceptibility to alveolar bone loss. Concomitant inactivation of Hedgehog-target Foxf1 factor in Hedgehog-activated PTHrP+ DF cells partially rescued alveolar bone defects. Therefore, the Hedgehog-Foxf pathway needs to be suppressed to drive alveolar bone osteoblast fates of PTHrP+ DF cells, unraveling a unique tooth-specific mechanism of bone formation requiring deliberate on-off regulations of Hedgehog signaling.

Primary failure of eruption (PFE) is a rare disorder that is characterized by the inability of a molar tooth/teeth to erupt to the occlusal plane or to normally react to orthodontic force. This condition is related to hereditary factors and has been extensively researched over many years. However, the etiological mechanisms of pathogenesis are still not fully understood. Evidence from studies on PFE cases has shown that PFE patients may carry parathyroid hormone 1 receptor (PTH1R) gene mutations, and genetic detection can be used to diagnose PFE at an early stage. PTH1R variants can lead to altered protein structure, impaired protein function, and abnormal biological activities of the cells, which may ultimately impact the behavior of teeth, as observed in PFE. Dental follicle cells play a critical role in tooth eruption and root development and are regulated by parathyroid hormone-related peptide (PTHrP)-PTH1R signaling in their differentiation and other activities. PTHrP-PTH1R signaling also regulates the activity of osteoblasts, osteoclasts and odontoclasts during tooth development and eruption. When interference occurs in the PTHrP-PTH1R signaling pathway, the normal function of dental follicles and bone remodeling are impaired. This review provides an overview of PTH1R variants and their correlation with PFE, and highlights that a disruption of PTHrP-PTH1R signaling impairs the normal process of tooth development and eruption, thus providing insight into the underlying mechanisms related to PTH1R and its role in driving PFE.

Bone development starts with condensations of undifferentiated mesenchymal cells that set a framework for future bones within the primordium. In the endochondral pathway, mesenchymal cells inside the condensation differentiate into chondrocytes and perichondrial cells in a SOX9-dependent mechanism. However, the identity of mesenchymal cells outside the condensation and how they participate in developing bones remain undefined. Here we show that mesenchymal cells surrounding the condensation contribute to both cartilage and perichondrium, robustly generating chondrocytes, osteoblasts, and marrow stromal cells in developing bones. Single-cell RNA-seq analysis of Prrx1-cre-marked limb bud mesenchymal cells at E11.5 reveals that Notch effector Hes1 is expressed in a mutually exclusive manner with Sox9 that is expressed in pre-cartilaginous condensations. Analysis of a Notch signaling reporter CBF1:H2B-Venus reveals that peri-condensation mesenchymal cells are active for Notch signaling. In vivo lineage-tracing analysis using Hes1-creER identifies that Hes1+ early mesenchymal cells surrounding the perichondrium at E13.5, subsequently becoming growth plate chondrocytes, osteoblasts of trabecular and cortical bones, and marrow stromal cells in postnatal bones. In contrast, Hes1+ cells in the perichondrium at E12.5 or E14.5 do not generate chondrocytes within cartilage, contributing to osteoblasts and marrow stromal cells only through the perichondrial route. Therefore, Hes1+ peri-condensation mesenchymal cells give rise to cells of the skeletal lineage through cartilage-dependent and independent pathways, supporting the theory that early mesenchymal cells outside the condensation also play important roles in early bone development.

The bone marrow contains various populations of skeletal stem cells (SSCs) in the stromal compartment, which are important regulators of bone formation. It is well-described that leptin receptor (LepR)+ perivascular stromal cells provide a major source of bone-forming osteoblasts in adult and aged bone marrow. However, the identity of SSCs in young bone marrow and how they coordinate active bone formation remains unclear. Here we show that bone marrow endosteal SSCs are defined by fibroblast growth factor receptor 3 (Fgfr3) and osteoblast-chondrocyte transitional (OCT) identities with some characteristics of bone osteoblasts and chondrocytes. These Fgfr3-creER-marked endosteal stromal cells contribute to a stem cell fraction in young stages, which is later replaced by Lepr-cre-marked stromal cells in adult stages. Further, Fgfr3+ endosteal stromal cells give rise to aggressive osteosarcoma-like lesions upon loss of p53 tumor suppressor through unregulated self-renewal and aberrant osteogenic fates. Therefore, Fgfr3+ endosteal SSCs are abundant in young bone marrow and provide a robust source of osteoblasts, contributing to both normal and aberrant osteogenesis.

In endochondral bone development, bone-forming osteoblasts and bone marrow stromal cells have dual origins in the fetal cartilage and its surrounding perichondrium. However, how early perichondrial cells distinctively contribute to developing bones remain unidentified. Here we show using in vivo cell-lineage analyses that Dlx5 + fetal perichondrial cells marked by Dlx5-creER do not generate cartilage but sustainably contribute to cortical bone and marrow stromal compartments in a manner complementary to fetal chondrocyte derivatives under the regulation of Hedgehog signaling. Postnatally, Dlx5 + fetal perichondrial cell derivatives preferentially populate the diaphyseal marrow stroma with a dormant adipocyte-biased state and are refractory to parathyroid hormone-induced bone anabolism. Therefore, early perichondrial cells of the fetal cartilage are destined to become an adipogenic subset of stromal cells in postnatal diaphyseal bone marrow, supporting the theory that the adult bone marrow stromal compartments are developmentally prescribed within the two distinct cells-of-origins of the fetal bone anlage.

The periodontium is comprised of multiple units of mineralized and nonmineralized tissues including the cementum on the root surface, the alveolar bone, periodontal ligament (PDL), and the gingiva. PDL contains a variety of cell populations including mesenchymal stem/progenitor cells (MSCs) termed PDLSCs, which contribute to periodontal regeneration. Recent studies utilizing mouse genetic models shed light on the identities of these mesenchymal progenitors in their native environment, particularly regarding how they contribute to homeostasis and repair of the periodontium. The current concept is that mesenchymal progenitors in the PDL are localized to the perivascular niche. Single-cell RNA sequencing (scRNA-seq) analyses reveal heterogeneity and cell-type specific markers of cells in the periodontium, as well as their developmental relationship with precursor cells in the dental follicle. The characteristics of PDLSCs and their diversity in vivo are now beginning to be unraveled thanks to insights from mouse genetic models and scRNA-seq analyses, which aid to uncover the fundamental properties of stem cells in the human PDL. The new knowledge will be highly important for developing more effective stem cell-based regenerative therapies to repair periodontal tissues in the future.

Periodontal disease is a chronic inflammation of tooth-supporting tissues, and the destruction of these tissues results in tooth loss. Regeneration of periodontal tissues is the ultimate goal of periodontal treatment. We previously reported that transplantation of conditioned medium (CM) of periodontal ligament stem cells (PDLSCs) demonstrated the enhancement of periodontal tissue regeneration, compared to CM from fibroblasts (Fibroblast-CM). We hypothesized that the angiogenic effects of PDLSC-CM might participate in the enhanced wound healing of periodontal tissues. The aim of this study was to investigate the effect of PDLSC-CM on the functions of endothelial cells. PDLSCs were cultured from periodontal ligament tissues obtained from healthy volunteers. Human gingival epithelial cells, dermal fibroblasts, osteoblasts, and umbilical vein endothelial cells (HUVECs) were purchased from commercial sources. The functions of endothelial cells were examined using immunostaining of Ki67, observation of nuclear fragmentation and condensation (apoptosis), and network formation on Matrigel. Vascular endothelial cell growth factor (VEGF) level was measured using an ELISA kit. HUVECs demonstrated higher cell viability in PDLSC-CM when compared with those in Fibroblast-CM. HUVECs demonstrated a higher number of Ki67-positive cells and lower apoptosis cells in PDLSC-CM, compared to Fibroblast-CM. Additionally, HUVECs formed more capillary-like structures in PDLSC-CM than Fibroblast-CM. PDLSC-CM contained higher levels of angiogenic growth factor, VEGF, than Fibroblast-CM. Our results showed that PDLSC-CM increased cell viability, proliferation, and capillary formation of HUVECs compared to Fibroblast-CM, suggesting the angiogenic effects of PDLSC-CM, and the effect is a potential regenerative mechanism of periodontal tissues by PDLSC-CM.

The periodontium is essential for supporting the functionality of the tooth, composed of diversity of mineralized and non-mineralized tissues such as the cementum, the periodontal ligament (PDL) and the alveolar bone. The periodontium is developmentally derived from the dental follicle (DF), a fibrous tissue surrounding the developing tooth bud. We previously showed through in vivo lineage-tracing experiments that DF contains mesenchymal progenitor cells expressing parathyroid hormone-related protein (PTHrP), which give rise to cells forming the periodontal attachment apparatus in a manner regulated by autocrine signaling through the PTH/PTHrP receptor. However, the developmental relationships between PTHrP+ DF cells and diverse cell populations constituting the periodontium remain undefined. Here, we performed single-cell RNA-sequencing (scRNA-seq) analyses of cells in the periodontium by integrating the two datasets, i.e. PTHrP-mCherry+ DF cells at P6 and 2.3kb Col1a1 promoter-driven GFP+ periodontal cells at P25 that include descendants of PTHrP+ DF cells, cementoblasts, osteoblasts and periodontal ligament cells. This integrative scRNA-seq analysis revealed heterogeneity of cells of the periodontium and their cell type-specific markers, as well as their relationships with DF cells. Most importantly, our analysis identified a cementoblast-specific metagene that discriminate cementoblasts from alveolar bone osteoblasts, including Pthlh (encoding PTHrP) and Tubb3. RNA velocity analysis indicated that cementoblasts were directly derived from PTHrP+ DF cells in the early developmental stage and did not interconvert with other cell types. Further, CellPhoneDB cell-cell communication analysis indicated that PTHrP derived from cementoblasts acts on diversity of cells in the periodontium in an autocrine and paracrine manner. Collectively, our findings provide insights into the lineage hierarchy and intercellular interactions of cells in the periodontium at a single-cell level, aiding to understand cellular and molecular basis of periodontal tissue formation.

The perichondrium, a fibrous tissue surrounding the fetal cartilage, is an essential component of developing endochondral bones that provides a source of skeletal progenitor cells. However, perichondrial cells remain poorly characterized due to lack of knowledge on their cellular diversity and subset-specific mouse genetics tools. Single cell RNA-seq analyses reveal a contiguous nature of the fetal chondrocyte-perichondrial cell lineage that shares an overlapping set of marker genes. Subsequent cell-lineage analyses using multiple creER lines active in fetal perichondrial cells – Hes1-creER , Dlx5-creER – and chondrocytes – Fgfr3-creER – illustrate their distinctive contribution to endochondral bone development; postnatally, these cells contribute to the functionally distinct bone marrow stromal compartments. Particularly, Notch effector Hes1-creER marks an early skeletal progenitor cell population in the primordium that robustly populates multiple skeletal compartments. These findings support the concept that perichondrial cells participate in endochondral bone development through a distinct route, by providing a complementary source of skeletal progenitor cells.

The dental pulp, a non-mineralized connective tissue uniquely encased within the cavity of the tooth, provides a niche for diverse arrays of dental mesenchymal stem cells. Stem cells in the dental pulp, including dental pulp stem cells (DPSCs), stem cells from human exfoliated deciduous teeth (SHEDs) and stem cells from apical papilla (SCAPs), have been isolated from human tissues with an emphasis on their potential application to regenerative therapies. Recent studies utilizing mouse genetic models shed light on the identities of these mesenchymal progenitor cells derived from neural crest cells (NCCs) in their native conditions, particularly regarding how they contribute to homeostasis and repair of the dental tissue. The current concept is that at least two distinct niches for stem cells exist in the dental pulp, e.g., the perivascular niche and the perineural niche. The precise identities of these stem cells and their niches are now beginning to be unraveled thanks to sophisticated mouse genetic models, which lead to better understanding of the fundamental properties of stem cells in the dental pulp and the apical papilla in humans. The new knowledge will be highly instrumental for developing more effective stem cell-based regenerative therapies to repair teeth in the future.

The aim of this study is to analyze the relationship between Hepatocyte Growth Factor (HGF) levels in oral rinses using water and clinical parameters of periodontitis; and furthermore, to evaluate the potential of a prototype HGF immunochromatographic paper test strip (HGF-TS) for screening of periodontitis, in comparison with a commercially-available occult blood (hemoglobin) test strip (Hb-TS). Clinical periodontal parameters were recorded, and oral rinses were collected, from 125 subjects. Then, the presence of HGF, and hemoglobin (Hb), in each sample was detected using a prototype HGF-TS and an Hb-TS. In addition, the concentrations of HGF and Hb were also determined in each sample is necessary HGF concentrations in oral rinses showed significant correlations with clinical parameters of periodontitis. The positive rate and read value on HGF-TS showed significantly high values in cases of severe periodontitis compared to healthy subjects. Hb-TS showed generally higher positive rates than HGF-TS; however, it showed false positive results in healthy subjects. The concentration of HGF in oral rinses showed close association with the severity of periodontitis, suggesting that the prototype HGF-TS has potential for use in the diagnosis of periodontitis, although further refinement of the test strip is required to increase the sensitivity.

Bone marrow stromal cells (BMSCs) are versatile mesenchymal cell populations underpinning the major functions of the skeleton, a majority of which adjoin sinusoidal blood vessels and express C-X-C motif chemokine ligand 12 (CXCL12). However, how these cells are activated during regeneration and facilitate osteogenesis remains largely unknown. Cell-lineage analysis using Cxcl12-creER mice reveals that quiescent Cxcl12-creER + perisinusoidal BMSCs differentiate into cortical bone osteoblasts solely during regeneration. A combined single cell RNA-seq analysis demonstrate that these cells convert their identity into a skeletal stem cell-like state in response to injury, associated with upregulation of osteoblast-signature genes and activation of canonical Wnt signaling components along the single-cell trajectory. β-catenin deficiency in these cells indeed causes insufficiency in cortical bone regeneration. Therefore, quiescent Cxcl12-creER + BMSCs transform into osteoblast precursor cells in a manner mediated by canonical Wnt signaling, highlighting a unique mechanism by which dormant stromal cells are enlisted for skeletal regeneration.

A number of studies have previously shown variations of inferior alveolar, however, only a few reports focused on nearby the foramen ovale. In a formalin fixed cadaver, we identified three minor branches (anterior, middle, and posterior branches) arising from the main trunk of the mandibular nerve adjacent to the foramen ovale, passing lateral to the maxillary artery (MA), and joining the inferior alveolar nerve. The diameter of the branches was 0.68 mm, 1.43 mm, and 0.40 mm, respectively. The branches traveled inside the lateral pterygoid muscle (LPM) or between the LPM and tensor/levator veli palatini. Moreover, all of the branches were superficial to MA. Knowledge of such a variation might be helpful to dentists during, for example, anesthetic blockade and various oral surgeries.