According to 2022 World Health Organization (WHO) Global Oral Health Status Report, approximately 1/3 of entire world population suffers from dental caries. This number is particularly high in the low/middle-income countries. Carious diseases present significant economic burdens that negatively impacts the everyday life quality. Caries are usually associated with pain and discomfort, thus affecting the overall health of the individuals, the mastication process and the sleep quality. The detrimental effect of dental caries is observed in all age populations. For example, caries might affect the academic performance of children and young adults. Similarly, they could reduce work productivity in adults, therefore causing further economic difficulties. Individuals are affected economically by carious diseases, as well as the society as a whole, with global cost close to $250 billions, according to ACFF (The Alliance for a Cavity-Free Future).

 Dental caries is a complex multifactorial disease, caused by a range of mainly environmental factors. Of these the most common one is the high-sugar-intake diet, which facilitates detriment of the healthy oral environment and creates a permissive milieu for bacteria that eventually results in a progressive destruction of tooth mineralized matrices. Teeth are composed of a crown and a root, which are both covered with dentin, while the dentin of the tooth crown is covered with enamel and the dentin of the tooth root with cementum. Dental caries destruction starts at the surface enamel, or most rarely at the cementum, and progresses through the dentin. The speed and the extent of destruction are variable between patients, and dependent on bacterial cause and the age and overall health of the individual. If the carious lesion left untreated, the progressive destruction of dentin exposes the soft tissue of the tooth composing the dental pulp to detrimental effects of bacteria that can lead to pulp necrosis.

The progression of a carious lesion is marked by changes in the dental pulp tissue, which include immune responses to bacterial infection and reparative responses aimed at restoring the damage of the mineralized tissue barrier between the dental pulp and hard tissues of the tooth by re-establishing tooth physiology. Dental pulp tissue plays a pivotal role in the reparative responses to carious injuries, as it is a known source of stem cells that process regenerative capacity. Ideally, harnessing the regenerative dental pulp capacity in situ would be an ideal therapeutic approach, but this requires a deep knowledge and understanding of the changes that occur in the dental pulp tissue due to caries.

The above-mentioned group of researchers, which recently also generated the first single-cell atlas of human teeth, used state-of-the-art, single cell transcriptomics technology to unravel the effects of caries on the molecular and cellular composition of the human dental pulp. The work revealed the broad extent of a complex immune response to bacterial invasion, that reportedly varies between the patients. Because the microbial and pathogen status of the caries samples in this study is unknown the authors were unable to determine, whether the observed cytokine profile reflects the response to specific bacterial pathogens. However, the study provides remarkable resource for future research in this direction.

The most intriguing finding is the ambiguous changes in the composition of the stromal compartments of dental pulp and in the biomechanical properties of the pulp extracellular matrix (ECM). These changes are indicative of an ongoing repair in the carious pulp. Additionally, they share similarities with the fibrotic changes observed in various organs upon injury or infection. Particularly fascinating finding in this study are changes related to Tenascin-C (TNC) and Fibronectin (FN1), among other ECM constituent molecules, that have been previously linked to myofibroblast activation during fibrosis, also observed in carious dental pulps. The study reports presence of two distinct myofibroblast-like cell populations in the carious stromal compartment: 1) The CDH11 and CTHRC1 expressing population in the fibroblast subcluster, and 2) The ACTA2, MYH11, and MYL9 expressing population in the subcluster marked by mesenchymal stem cell (MSC) markers. Whether there is a developmental hierarchy between these two myofibroblast-like populations remains unclear, due to high variability between the different pulp samples that hampered further analyses and lineage modelling. The myofibroblast-like cells reside within the new carious ECM that is characterized by increased levels of relaxed Fibronectin. This feature is characteristic for ECM of a variety of organs that were impacted by infection or cancer. These changes likely influence cell adhesion, cell motility and determine the speed of healing and repair during caries.  

Another surprising finding was relatively high number of myofibroblast-like cells and TNC expressing cells in healthy pulp. Unlike other soft tissues, dental pulp is enclosed to a space delimited by dentin, which significantly diminishes the stress from physical and mechanical forces during mastication. Due to ageing, the dental pulp chamber is narrowing, thus resulting to dental pulp modifications that include the diminishment of pulp cell numbers and the concomitant expansion of collagen matrix. The results of this study show that the age-related fibrotic changes within the dental pulp are due to the encompass transformations of ECM and alterations of dental pulp cell composition. The fibrotic status of the human dental pulp is very similar to fibrotic alterations that have been observed in many other pathological organs.

Overall, the study provides a comprehensive and detailed atlas of dental pulp cells of carious human teeth at single cell level, and an outstanding resource for the community that opens new research avenues. In addition, the present findings raise a fascinating possibility that the onset of aging in the dental pulp of healthy human teeth coincides with the activation of TNC and myofibroblast phenotype, which can be used as a model for studying fibrosis events not only in teeth, but in other organs as well.