Stem Cell Contributions to Cementoblast Differentiation

Due to the unique feature of stem cells, they are implemented in various clinical trials, and therapeutic applications such as the treatment of Periodontitis. Periodontitis, a serious bacterial gum infection loosens teeth as it damages the soft tissues and the bone that supports teeth. In a recent study, it was found that certain stem cells help in the formation of Cementoblasts, which helps curb periodontitis.

Stem cells are undifferentiated cells that can divide to produce offspring cells that continue as stem cells or cells that are destined to differentiate and become specialized. They are the ongoing sources that make up cells and tissues of different organisms. With this feature they are implemented in various clinical trials, and therapeutic applications.

Periodontitis, a serious bacterial gum infection, loosens teeth as it damages the soft tissues and the bone that supports teeth. The cells responsible for producing cementum that connects teeth to the soft tissue are known as cementoblasts. The two different stem cells Axin2+ and CD90+ form cementoblasts during the postnatal phase, whereas Axin2+ form cementoblasts in the adult stage. 

Introduction

Periodontal disease, the main cause of tooth loss in adults, is caused by an initial loss of attachment between the gingival and the tooth, creating space for bacteria to populate and proliferate.

Materials and methods

Three consecutive intraperitoneal injections of tamoxifen were given at a dose of 2 mg/30 gbw (Sigma, T5648) over 3 days to the mice. They were sacrificed for tissue dissection and tissue processing.

After tissue collection, the samples were placed into fresh 4% polymeric formaldehyde (PFA) at 4°C overnight. Samples were snap‐frozen in a sagittal orientation in optimal cutting temperature matrix in dry ice and 100% ethanol bath.

For mice, older than 8 weeks in mild periodontitis group, the sutures were placed around upper second molars and removed immediately; samples were collected on day 7.

For mice tested for mechanical force, the additional mechanical force was applied by deliberately placing the tweezers between upper molars and exerting force.

Tamoxifen was injected for 3 days to induce Axin2. Samples were collected 1 month after the one month of EdU injection and processed.

Images are obtained after sections were then subject to permeabilization and stained by multiple antibodies.

The single celled RNAs are sequenced and analysed. Scanned by a SCANCO, samples were analyzed by MicroView software.

Results

1. Wnt‐responsive Axin2 expressing cells form cementoblasts during homeostasis

During early postnatal development Axin2⁺ cells contributed to cementoblast formation. Both the perivascular cells and cementoblasts express Axin2.The cells that give rise to cementoblasts, cells from the PDL were collected for single‐cell RNA sequencing. Sequenced PDL cells are of 3 types, mesenchymal cells, immune cells, and endothelial cells.

2.  Thy1/CD90 expressing cells transiently form cementoblasts during PDL development

During postnatal development CD90 is expressed in stem cells of tooth pulp in a small percentage in the incisor.  CD90 being pericytes (contractile cells of mesenchymal origin), are located close to blood vessels. Unlike Axin2+ cells, CD90 was only expressed in progenitor cells but not in their progeny.

3. CD90 and Axin2 coexpression

Coexpression with PDGFR‐β suggested that CD90 and Axin2 are distinct cell types. 

4.  Thy1/CD90 expressing cells form cementoblasts during disease

CD90+ cells were observed to form cementoblasts in large numbers in mild periodontitis, but this decreased significantly in severe periodontitis. The contribution of CD90+ cells to cementoblasts was increased following additional mechanical force and considerably reduced when LPS was applied.

Discussions

The complex tissue structure of the periodontal ligament is peculiar because of its attachment to the mineralized tissues, tooth and bone. With increase in the spread of periodontal disease treatments aim at preventing its progression instead of restoring the extensive tissue loss. The function of cementoblasts is to form cementum (cementogenesis) and provide attachment to the collagen fibers in the PDL.

Conclusion

Two stem cell populations, Axin2+ and CD90+, have been identified that contribute to cementoblasts in the periodontal ligament. Valuable insights into the behavior of these resident stem cells help in developing clinical treatments.