Enhancing Our Understanding of Endodontic Research Advances Through Skeletal Biology and Regeneration Literature

By Laura Doherty

The main “progenitor” or “stem cell” we acknowledge in the world of endodontics is the dental pulp stem cell (DPSC) residing within the in the pulp. These progenitors function during tooth development but also in adulthood as sources of postnatal stem cells, contributing to new tissue formation following injury (as is the case with reparative dentinogenesis)¹. DPSC biology, and an understanding of their role in the future of the specialty, is a major component of the endodontist’s comprehensive toolkit. The AAE attests that “knowledge in the fields of pulp biology…can be applied to deliver biologically based regenerative endodontic treatment” and that “these developments in the regeneration of a functional pulp-dentin complex have a promising impact on efforts to retain the natural dentition, the ultimate goal of endodontic treatment”². Accordingly, these dental pulp cells are a major focus of research in endodontology-focused basic science.

In skeletal biology, there are analogous mesenchymal lineage cells that are widely studied, including bone marrow stromal cells (BMSCs) and periosteal progenitors, both of which contribute to development as well as bone regeneration (e.g. fracture healing)³. There are additionally bona fide, true skeletal stem cells which comprise a smaller, self-renewing population^4,5. We know that DPSCs differentiate into odontoblasts, whereas skeletal progenitors differentiate toward the osteoblast lineage; they comprise two discrete, different cell types (and, indeed, they do express different stemness markers and genes, give rise to distinct mineralized tissues, and exhibit different properties in terms of proliferation)⁶. However, they share similar characteristics that are tough to ignore and make them difficult to study in isolation.

DPSCs and skeletal progenitors both give rise to matrix-secreting, functional cells of mineralized tissue. The process of their differentiation and maturation is mediated largely by common signaling pathways and mechanisms, following similar transcriptional and morphological changes over the time course of terminal differentiation⁷. With a notable major exception of the odontogenic marker dentin sialophosphoprotein, which is present mainly in odontoblastic cells, odontoblasts and osteoblasts even express similar gene profiles, including the critical osteogenic genes Runx2, osteocalcin, and type I collagen⁸. Similar methodologies are used to study these cells in vitro (e.g. mineralization assays, proliferation/apoptosis analyses, and multi-lineage differentiation potential experiments commonly seen in the basic science literature). Genetically modified mouse models being used in the field of reparative dentinogenesis echo methodologies in current skeletal regeneration studies, allowing for important analyses including lineage tracing of progenitor populations and genetic functional studies using knockouts and other mutants⁹. In general, the field of basic science pulp biology appears to complement current trends in skeletal biology. One example of this is studies on the Wnt/β-catenin pathway, where canonical Wnt signaling is one of the most widely studied mechanisms of mesenchymal cell differentiation in both the bone biology and pulp biology fields.

Despite this, during a recurrent endodontic literature search, there is likely temptation to limit the selection to those articles specific to our field, and specifically to dental pulp-associated cell types and studies on odontoblastic differentiation and function. Are we doing ourselves a disservice by not expanding our search to include the advances in bone biology?

Worldwide research in the field of endodontics is robust, yet published skeletal biology studies largely overshadow those of basic science in pulp biology and dental pulp stem cells. A PubMed search limited to the past five years reveals ~1,700 journal articles that mention “dental pulp stem cells”; but compare this to “bone marrow stromal cells” at 36,000, over a 20-fold increase, and “skeletal stem cells” at ~4,200. There are 62 journal articles in the past five years that mention “reparative dentinogenesis”, compared to ~6,500 that mention “fracture healing” (a striking 100-fold difference) and ~4,000 for “skeletal regeneration”. Often, and due to the sheer amount of studies being done, bone biology studies will discover new advances in mineralized tissue biology before dental researchers.  However, the same can be true in the opposite direction, where studies on dental pulp have given skeletal biologists new and important insights into pathways of research in bone. For example, Wnt-associated stem cell marker Lgr6 was described as present in teeth in 2014¹⁰; years later, and after publication of preliminary in vitro studies, Lgr6 has been established as a putative marker of osteochondral progenitors, and is now emerging as a marker of skeletal progenitors in the context of regeneration^11,12.

While I am not advocating to treat dental pulp like bone marrow, or equate odontoblasts to osteoblasts, there is immense value in keeping current with the trends in skeletal biology and regeneration. The field of skeletal biology is enormous in comparison to that of dental pulp-related research, but these studies shed light on stem cells of mineralized tissues, how they are regulated, and how we can harness them to enhance regenerative capabilities for mineralized tissue healing and repair. It is likely that newly discovered methods of enhancing mineralization in bone will be applicable to dental pulp and teeth as well.

The field of regenerative endodontics is fairly new and still emerging, but also exciting; and a broader understanding of how other mineralized tissues in the body are repaired is of enormous value to the endodontist or other dental practitioners. A strong command of dental pulp science comes not only from being up to date with the current endodontic literature, but also from seeking to comprehend high impact studies within bone biology. Therefore, we can and should utilize current literature in bone biology to our advantage in the field of endodontics, and these efforts will undoubtedly help keep current our understanding of regenerative endodontology as the field evolves and expands rapidly.

Laura Doherty is a sixth-year D.M.D./Ph.D. Candidate, UConn School of Dental Medicine Class of 2023.

References

1. Neves, V. C. M. & Sharpe, P. T. Regulation of Reactionary Dentine Formation. J. Dent. Res. 97, 416–422 (2018).
2. Regenerative Endodontics. American Association of Endodontists Clinical Resources https://www.aae.org/specialty/clinical-resources/regenerative-endodontics/.
3. Galea, G. L., Zein, M. R., Allen, S. & Francis‐West, P. Making and shaping endochondral and intramembranous bones. Developmental Dynamics 250, 414–449 (2021).
4. Chan, C. K. F. et al. Identification and specification of the mouse skeletal stem cell. Cell 160, 285–298 (2015).
5. Chan, C. K. F. et al. Identification of the Human Skeletal Stem Cell. Cell 175, 43-56.e21 (2018).
6. Shi, S., Robey, P. G. & Gronthos, S. Comparison of human dental pulp and bone marrow stromal stem cells by cDNA microarray analysis. Bone 29, 532–539 (2001).
7. Lim, W. H. et al. Wnt signaling regulates pulp volume and dentin thickness. J. Bone Miner. Res. 29, 892–901 (2014).
8. Sagomonyants, K., Kalajzic, I., Maye, P. & Mina, M. FGF Signaling Prevents the Terminal Differentiation of Odontoblasts. J Dent Res 96, 663–670 (2017).
9. Vidovic, I. et al. αSMA-Expressing Perivascular Cells Represent Dental Pulp Progenitors In Vivo. J. Dent. Res. 96, 323–330 (2017).
10. kawasaki, M. et al. R-spondins/Lgrs expression in tooth development: R-Spondin2 in Tooth Development. Developmental Dynamics 243, 844–851 (2014).
11. Doherty, L. & Sanjay, A. LGRs in skeletal tissues: an emerging role for Wnt‐associated adult stem cell markers in bone. JBMR Plus (2020) doi:10.1002/jbm4.10380.
12. Khedgikar, V. & Lehoczky, J. A. Evidence for Lgr6 as a Novel Marker of Osteoblastic Progenitors in Mice: LGR6 AS A MARKER OF OSTEOPROGENITORS. JBMR Plus 3, e10075 (2019).