Progenitor Cells of the Mandibular Condylar Cartilage

A PTHrP Gradient Drives Mandibular Condylar Chondrogenesis via Runx2

The mandibular condylar cartilage (MCC) is an essential component of the temporomandibular joint, which orchestrates the vertical growth of the mandibular ramus through endochondral ossification with distinctive modes of cell differentiation. Parathyroid hormone-related protein (PTHrP) is a master regulator of chondrogenesis; in the long bone epiphyseal growth plate, PTHrP expressed by resting zone chondrocytes promotes chondrocyte proliferation in the adjacent layer. However, how PTHrP regulates chondrogenesis in the MCC remains largely unclear. In this study, we used a Pthrp-mCherry knock-in reporter strain to map the localization of PTHrP+ cells in the MCC and define the function of PTHrP in the growing mandibular condyle. In the postnatal MCC of PthrpmCherry/+ mice, PTHrP-mCherry was specifically expressed by cells in the superficial layer immediately adjacent to RUNX2-expressing cells in the polymorphic layer. PTHrP ligands diffused across the polymorphic and chondrocyte layers where its cognate receptor PTH1R was abundantly expressed. We further analyzed the mandibular condyle of PthrpmCherry/mCherry mice lacking functional PTHrP protein (PTHrP-KO). At embryonic day (E) 18.5, the condylar process and MCC were significantly truncated in the PTHrP-KO mandible, which was associated with a significant reduction in cell proliferation across the polymorphic layer and a loss of SOX9+ cells in the chondrocyte layers. The PTHrP-KO MCC showed a transient increase in the number of Col10a1+ hypertrophic chondrocytes at E15.5, followed by a significant loss of these cells at E18.5, indicating that superficial layer-derived PTHrP prevents premature chondrocyte exhaustion in the MCC. The expression of Runx2, but not Sp7, was significantly reduced in the polymorphic layer of the PTHrP-KO MCC. Therefore, PTHrP released from cells in the superficial layer directly acts on cells in the polymorphic layer to promote proliferation of chondrocyte precursor cells and prevent their premature differentiation by maintaining Runx2 expression, revealing a unique PTHrP gradient-directed mechanism that regulates MCC chondrogenesis.

Development of the human primary and secondary jaw joints

This study investigates the development of the primary and secondary jaw joints in humans, focusing on their concomitance and subsequent disconnection. Development begins with the primary temporomandibular joint as a connection between Meckel's cartilage and the incus, while the secondary temporomandibular joint develops anteriorly as an articulation between the mandibular condyle and the mandibular fossa. Previous research in mice has provided insights into the morphogenesis of these joints, but their specific development of the 3D morphogenesis in humans remains unclear. To address this gap, histological serial sections of embryos and fetuses ranging from 19 to 230 mm crown-rump length were analyzed. The 3D morphogenesis of the middle ear and the temporomandibular joint was examined, paying attention to the morphological characteristics, timing, and potential mechanisms of movement and disconnection. The primary jaw joint is initially formed at 25 mm (8th week), followed by the appearance of the secondary jaw joint arising at 87 mm (12th week). Both joints persist present simultaneously, until a separation occurs between 150 and 230 mm (18th-24th week). It is remarkable that both joints remain concomitant and function somehow for a period exceeding 6 weeks, with the mechanism of their separation still unclear. Understanding the precise timing and functional movements involved with these temporarily connected joints is crucial for comprehending the overall development of the temporomandibular joint. Further research is needed to explore the molecular and cellular processes underlying these developmental changes.

NG2/CSPG4 attenuates motility in mandibular fibrochondrocytes under serum starvation conditions

The migration of mandibular fibrochondrocytes is important for the development of the mandible, the homeostasis of the mandibular cartilage, and for the capacity of the tissue to respond to injury. Mandibular fibrochondrocytes have to overcome formidable obstacles during migration including a dense and heterogeneous three-dimensional matrix. Guiding the direction of cell migration and commitment to a migratory phenotype in this microenvironment necessitates a multivalent response to chemotactic and extracellular matrix-mediated stimuli. One of the key matrix components in the cartilage of the temporomandibular joint is type VI collagen. Neuron/glial antigen 2 (NG2/CSPG4) is a transmembrane proteoglycan that binds with collagen VI and has been implicated in a wide range of cell behaviors including cell migration, motility, adhesion, and proliferation. While NG2/CSPG4 has been shown to be a key regulator of mandibular cartilage homeostasis, its role in the migration of mandibular fibrochondrocytes during normal and cell stress conditions has yet to be resolved. Here, we address this gap in knowledge by characterizing NG2/CSPG4-dependent migration in mandibular fibrochondrocytes using primary mandibular fibrochondrocytes isolated from control and full length NG2/CSPG4 knockout mice, in primary mandibular fibrochondrocytes isolated from NG2|DsRed reporter mice and in an immortalized mandibular fibrochondrocyte cell line with a mutated NG2/CSPG4 ectodomain. All three cells demonstrate similar results, with loss of the full length or truncated NG2/CSPG4 increasing the rate of cell migration in serum starvation/cell stress conditions. These findings clearly implicate NG2/CSPG4 as a key molecule in the regulation of cell migration in mandibular fibrochondrocytes in normal and cell stress conditions, underscoring the role of NG2/CSPG4 as a mechanosensitive signaling hub in the mandibular cartilage.

Temporomandibular Joint Fibrocartilage Contains CD105 Positive Mouse Mesenchymal Stem/Progenitor Cells with Increased Chondrogenic Potential

Objective

A specific type of mesenchymal stem/progenitor cells (MSPCs), CD105+ is reported to aid in cartilage regeneration through TGF-β/Smad2-signalling. The purpose of this study was to identify and characterize CD105+ MSPCs in temporomandibular joint (TMJ) cartilage.

Materials and Methods

MSPCs were isolated from mouse TMJ condyle explants and evaluated for their clonogenicity and pluripotential abilities. MSPC were examined for CD105 antigen using immunohistochemistry and flow cytometry.

Results

Immunohistochemistry revealed presence of CD105+ MSPCs in the proliferative zone of condyle's cartilage. Only 0.2% of isolated MSPCs exhibited CD105, along with the stem cell surface markers CD44 and Sca-1. In CD105+ MSPCs, intracellular immunostaining revealed significantly higher (p < 0.05) protein levels of collagen type 1, 2, proteoglycan 4. Ability for chondrogenic differentiation was found to be significantly higher (p < 0.05) after 4 weeks compared to CD105- cells, using alcian blue staining. CD105+ cells were found to resemble an early MSPC subgroup with significantly higher gene expression of biglycan, proteoglycan 4, collagen type 2, Gli2, Sox5 (p < 0.001) and Sox9 (p < 0.05). In contrast, significantly lower levels of Runx2 (p < 0.05), Osterix, Trps1, Col10a1 (p < 0.01), Ihh (p < 0.001) related to chondrocyte senescence and commitment to osteogenic lineage, were observed compared to CD105- cells.

Conclusion

The study showed the existence of a CD105+ MSPC subgroup within TMJ fibrocartilage that may be activated to aid in fibrocartilage repair.

The adaptive response of the mandibular condyle to increased load is disrupted by ADAMTS5 deficiency

OBJECTIVE

To determine the impact of increased load on the temporomandibular joint (TMJ) from mice deficient in the extracellular matrix protease ADAMTS5.

MATERIALS AND METHODS

Wire springs exerting 0.5 N for 1 h/day for 5 days (Adamts5+/+ -n = 18; Adamts5-/- n = 19) or 0.8 N for 1 h/day for 10 days (Adamts5+/+-n = 18; Adamts5-/- n = 17) were used to increase murine TMJ load. Safranin O-staining was used to determine mandibular condylar cartilage (MCC) morphology. Chondrogenic factors Sox9 and aggrecan were immunolocalized. Microcomputed topography was employed to evaluate mineralized tissues, and Tartrate-Resistant Acid Phosphatase staining was used to quantify osteoclasts.

RESULTS

Increased load on the mandibular condyle of Adamts5-/- mice resulted in an increase in the hypertrophic zone of mandibular condylar cartilage (MCC) compared to normal load (NL) (P < 0.01). In the trabecular bone of the mandibular condyle, the total volume (TV), bone volume (BV), trabecular thickness (TbTh), and trabecular separation (TbSp) of the mandibular condyles in Adamts5-/- mice (n = 27) did not change significantly with increased load, compared to Adamts5+/+ (n = 38) that exhibited significant responses (TV-P < 0.05; BV-P < 0.001; TbTh-P < 0.01; TbSp-P < 0.01). The bone volume fraction (BV/TV) was significantly reduced in response to increased load in both Adamts5-/- (P < 0.05) and Adamts5+/+ mandibular condyles (P < 0.001) compared to NL. Increased load in Adamts5-/- mandibular condyles also resulted in a dramatic increase in osteoclasts compared to Adamts5-/- NL (P < 0.001) and to Adamts5+/+ with increased load (P < 01).

CONCLUSION

The trabeculated bone of the Adamts5-/- mandibular condyle was significantly less responsive to the increased load compared to Adamts5+/+. ADAMTS5 may be required for mechanotransduction in the trabeculated bone of the mandibular condyle.

Influence of COL2A1-G1405S polymorphism on mandibular skeletal malocclusions: A genetic association study and in silico analysis

OBJECTIVE

The current study aimed to assess the association between collagen type II alpha 1 chain (COL2A1) single nucleotide polymorphism (SNP: rs2070739; C>T; G1405S) and mandibular skeletal malocclusions in the population of Mazandaran (North Iran).

DESIGN

During 13 months, 102 control samples, 81 samples with skeletal Class III malocclusion contributed by mandibular prognathism and 82 samples with skeletal Class II malocclusion contributed by mandibular retrognathism were screened. Cephalometric analysis was performed to determine the type of abnormalities. COL2A1-G1405S genotyping was performed using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. The HOPE tool was used to investigate the effect of COL2A1-G1405S on the three-dimensional structure of protein.

RESULTS

Results showed that there is no significant correlation between genotypes and alleles related to COL2A1-G1405S and mandibular prognathism (CT genotype: p-value= 0.210; T allele: p-value= 0.222). On the other hand, an association was observed between COL2A1-G1405S and mandibular retrognathism (CT genotype: p-value= 0.008; T allele: p-value= 0.011). The outputs of the HOPE tool also showed that COL2A1-G1405S can disrupt the NC1 domain of the protein.

CONCLUSIONS

Here, we provide evidence that COL2A1-G1405S polymorphism may have positive correlation with the risk of skeletal Class II malocclusion contributed by mandibular retrognathism in the population of Mazandaran. Given that the COL2A1-G1405S occurs in NC1 domain, it is possible that this domain plays an important role in signaling pathways related to ossification. So, we suggest that the study of COL2A1 SNPs can help researchers understand the significant role of this collagen in mandibular skeletal malocclusions.

Cell depleted areas do not repopulate after diphtheria toxin-induced killing of mandibular cartilage chondrocytes

OBJECTIVE

Growth of mandibular condylar cartilage (MCC) is associated with cell proliferation within the polymorphic cell layer and subsequent differentiation into chondrocytes that reside along the condylar surface and along the cartilage/subchondral bone interface. We examined whether cells in the polymorphic layer would proliferate and repopulate toxin-induced cell-depleted areas in MCCs of adult mice.

METHOD

We induced diphtheria toxin (DTA) expression (ROSA26l-s-lDTA) to cell-autonomously kill large fractions of MCC chondrocytes throughout the cartilage or along the articular cartilage surface with Aggrecan-CreERt2 (AcanCreERt2) or Lubricin-CreERt2 (Prg4CreERt2) Cre-recombinase-inducible mice, respectively. We examined MCCs from these mice shortly after cell killing or several months later with histology and confocal microscopy for evidence of chondrocyte proliferation and repopulation.

RESULTS

AcanCreERt2-induced DTA expression killed an average of 53% MCC chondrocytes in adult mice after 1 week (39-66%, 95% confidence interval (CI)). Twelve weeks later, surviving chondrocytes had proliferated but not migrated to cell depleted areas. Prg4CreERt2-induced DTA expression killed an average of 24% surface chondrocytes in mice after 5 weeks (14-34% CI). After thirteen weeks there was 34% fewer surface chondrocytes (4-63% CI) in Prg4CreERt2 DTA-induced mice compared to controls.

CONCLUSION

In adult mice, after diphtheria toxin-mediated chondrocyte killing, cell depleted areas within MCC cartilage are not repopulated by new cells.

Two-photon fluorescence and second harmonic generation characterization of extracellular matrix remodeling in post-injury murine temporomandibular joint osteoarthritis

End stage temporomandibular joint osteoarthritis (TMJ-OA) is characterized by fibrillations, fissures, clefts, and erosion of the mandibular condylar cartilage. The goal of this study was to define changes in pericellular and interterritorial delineations of the extracellular matrix (ECM) that occur preceding and concurrent with the development of this end stage degeneration in a murine surgical instability model. Two-photon fluorescence (TPF) and second harmonic generation (SHG) microscopy was used to evaluate TMJ-OA mediated changes in the ECM. We illustrate that TPF/SHG microscopy reconstructs the three-dimensional network of key fibrillar and micro-fibrillar collagens altered during the progression of TMJ-OA. This method not only generates spatially distinct pericellular and interterritorial delineations of the ECM but distinguishes early and end stage TMJ-OA by signal organization, orientation, and composition. Early stage TMJ-OA at 4- and 8-weeks post-injury is characterized by two structurally distinct regions containing dense, large fiber collagens and superficial, small fiber collagens rich in types I, III, and VI collagen oriented along the mesiodistal axis of the condyle. At 8-weeks post-injury, type VI collagen is locally diminished on the central and medial condyle, but the type I/III rich superficial layer is still present. Twelve- and 16-weeks post-injury mandibular cartilage is characteristic of end-stage disease, with hypocellularity and fibrillations, fissures, and clefts in the articular layer that propagate along the mediolateral axis of the MCC. We hypothesize that the localized depletion of interterritorial and pericellular type VI collagen may signify an early marker for the transition from early to end stage TMJ-OA, influence the injury response of the tissue, and underlie patterns of degeneration that follow attritional modes of failure.

Diverse contribution of Col2a1-expressing cells to the craniofacial skeletal cell lineages

OBJECTIVES

Craniofacial skeletal development requires deliberate coordination of two distinct mechanisms of endochondral and intramembranous ossification. Col2a1-expressing cells encompass growth-associated skeletal progenitors in endochondral bones of the limb. The objective of this study was to determine the contribution of Col2a1-expressing cells to the craniofacial skeletal cell lineages. We hypothesize that Col2a1-expressing progenitors significantly contribute to various modes of ossification associated with the craniofacial development.

METHODS

Cellular fates of Col2a1-expressing cells were studied based on a cre-loxP system using a Col2a1-cre transgene and an R26R-tdTomato reporter allele. We analysed three distinct locations of the craniofacial skeletal complex representing unique ossification mechanisms: the cranial base, the calvaria and the mandibular condyle.

RESULTS

Col2a1-cre consistently marked a majority of skeletal cells in the cranial base. Interestingly, Col2a1-cre also marked a large number of osteoblasts and suture mesenchymal cells in the calvaria, in addition to chondrocytes in the underlying transient cartilage. In the mandibular condyle, Col2a1-cre marked chondrocytes and osteoblasts only during the growth phase.

CONCLUSIONS

Col2a1 is expressed by progenitors of the skeletal lineage in canonical endochondral bone formation occurring in the cranial base. In contrast, other ossification mechanisms of the craniofacial complex utilize Col2a1-expressing cells in a different manner, whereby Col2a1 may be expressed in more differentiated or transient cell types of the skeletal lineage.

Developmental characteristics of secondary cartilage in the mandibular condyle and sphenoid bone in mice

OBJECTIVE

Secondary cartilage develops from osteochondral progenitor cells. Hypertrophic chondrocytes in secondary cartilage increase within a very short time and then ossify rapidly. In the present study, we investigated the sequential development process of osteochondral progenitor cells, and the morphology and size of hypertrophic chondrocytes in secondary cartilage.

DESIGN

ICR mice at embryonic days (E) 14.5-17.5 were used. The mandibular condyle and the medial pterygoid process of the sphenoid bone were observed as secondary cartilage, and the cranial base and the lateral pterygoid process of the sphenoid bone, which is primary cartilage, were observed as a control. Thin sections were subjected to immunostaining and alkaline phosphatase (ALP) staining. Using a confocal laser microscope, 3D stereoscopic reconstruction of hypertrophic cells was performed. To evaluate the size of hypertrophic chondrocytes objectively, the cell size was measured in each cartilage.

RESULTS

Hypertrophic chondrocytes of secondary cartilage first expressed type X collagen (Col X) at E15.5. SRY-box 9 (Sox 9) and ALP were co-expressed in the fibroblastic/polymorphic tissue layer of secondary cartilage. This layer was very thick at E15.5, and then rapidly became thin. Hypertrophic cells in secondary cartilage were markedly smaller than those in primary cartilage.

CONCLUSIONS

The small hypertrophic cells present in secondary cartilage may have been a characteristic acquired in order for the cartilage to smoothly promote a marked increase in hypertrophic cells and rapid calcification.