Missing Concepts in De Novo Pulp Regeneration

Abstract

Regenerative endodontics has gained much attention in the past decade because it offers an alternative approach in treating endodontically involved teeth. Instead of filling the canal space with artificial materials, it attempts to fill the canal with vital tissues. The objective of regeneration is to regain the tissue and restore its function to the original state. In terms of pulp regeneration, a clinical protocol that intends to reestablish pulp/dentin tissues in the canal space has been developed--termed revitalization or revascularization. Histologic studies from animal and human teeth receiving revitalization have shown that pulp regeneration is difficult to achieve. In tissue engineering, there are 2 approaches to regeneration tissues: cell based and cell free. The former involves transplanting exogenous cells into the host, and the latter does not. Revitalization belongs to the latter approach. A number of crucial concepts have not been well discussed, noted, or understood in the field of regenerative endodontics in terms of pulp/dentin regeneration: (1) critical size defect of dentin and pulp, (2) cell lineage commitment to odontoblasts, (3) regeneration vs. repair, and (4) hurdles of cell-based pulp regeneration for clinical applications. This review article elaborates on these missing concepts and analyzes them at their cellular and molecular levels, which will in part explain why the non-cell-based revitalization procedure is difficult to establish pulp/dentin regeneration. Although the cell-based approach has been proven to regenerate pulp/dentin, such an approach will face barriers--with the key hurdle being the shortage of the current good manufacturing practice facilities, discussed herein.

Keywords: cell free; cell-based therapy; critical size defect; dentin; endodontics; stem cells.

Figure 1.

Conceptual dentin bridge formation after the entire pulp chamber roof dentin is removed. Arrows indicate the replenishment of replacement odontoblasts.

Figure 2.

Non-cell-based pulp regeneration. (A) Size of pulp defect (grayish red). (B) Hypothetic non-cell-based pulp regeneration in the pulp chamber space. Scaffold carrying growth factors (GFs) filled in the pulp chamber. White arrows indicate released GFs into the subjacent pulp in the canals. Purple arrows indicate migrating stem cells attracted by the GFs into the scaffold to regenerate pulp. (C) In a dog study, the immature tooth had pulpectomy, root canal infection, and disinfection and was filled with collagen for regeneration. Three months later, half the pulp tissue was found left behind in the canal and survived after infection and disinfection (left, indicated by P). The canal space in the right half was filled with periodontal-like tissues (PD) with cementum-like tissue ingrowth from the apical root surface cementum. Adapted from Wang et al. (2010) with permission. This figure is available in color online at http://jdr.sagepub.com.

Figure 3.

A conceptual theme of a non-cell-based revitalization/regeneration procedure. (A) Root canal space is filled with a scaffolding material carrying growth factors (GFs), which are released into the adjacent tissues to induce stem/progenitor cell migration into the canal space and to guide the differentiation of the migrated cells into odontoblast lineages. The white arrow pointing down indicates the release of GFs into the apex. Purple and blue arrows pointing up are cell migration direction into the canal. Dashed arrows indicate (1) the perivascular pericytes at the local tissues, including periapical bone and periodontal ligament, giving rise to local MSCs and progenitors and (2) cells migrating into the canal space and transdifferentiating into new odontoblasts. (B) If the apical papilla (AP) is still present, cells in this tissue (e.g., stem cells in apical papilla) may be attracted into the canal space along with more distant cells from blood. (C) If AP is no longer present, periapical (PA) cells and distant cells from blood may be attracted into the canal space. Odontogenic ameloblast-associated protein and CTGF/CCN2 (connective tissue growth factor/CCN family 2) may be coated onto the dentin wall to facilitate odontoblast differentiation. TGF-β and DMP-1 embedded in dentin may be guiding odontoblast differentiation and new dentin generation. Theme concept based in part on several reports in the literature (Kim et al., 2010; Yang et al., 2010; Ishizaka et al., 2012; Muromachi et al., 2012; Ishizaka et al., 2013; Lin et al., 2014). BDNF, brain-derived neurotrophic factor; bFGF, basic fibroblast growth factor; BMP, bone morphogenetic protein; bv, blood vessel; DMP-1, dentin matrix acidic phosphoprotein 1; HGF, hepatocyte growth factor; GDNF, glia cell line–derived neurotrophic factor; NGF, nerve growth factor; SDF-1, stromal cell–derived factor 1; TGF-β, transforming growth factor β.

Figure 4.

Histologic analysis of regenerated dentin in root canal space. The emptied root canal space of human root fragments was filled with dental stem cells, and the constructs were transplanted in the subcutaneous space of SCID (severe combined immunodeficiency) mice for 3 to 4 mo. (A-B) Dentin bridge (DB) formation underneath the mineral trioxide aggregate (MTA) cement. (C-F) Regenerated dentin on dentin walls (rDD) using stem cells in apical papilla or dental pulp stem cells. D, dentin wall; black arrows, embedded cell in DB or rDD; white open arrows, dentinal tubules; green arrows, mineralized dentinal tubules; blue open arrows, nonmineralized dentinal tubules. Images are hematoxylin and eosin stain adapted with permission from Tissue Engineering, Part A, 2009, published by Mary Ann Liebert, Inc., New Rochelle, NY (Huang et al., 2010). Scale bars: A, 100 µm; B-D and F, 50 µm; E, 20 µm.

Figure 5.

Scanning electron microscope analysis of odontoblast-like cells on dentin surface in vitro. Cells were seeded onto dentin surface (dentin discs) with open dentinal tubules. Odontoblast-like cells subsequently formed and showed cellular processes extending into the dentinal tubules. (A-C) Each odontoblast-like cell derived from human dental pulp stem cells extends 1 process (arrow) into 1 dentinal tubule. (D) One odontoblast-like cell derived from a human dental pulp stem cell extends 3 observable cellular processes, and each process extends into 1 dentinal tubule (arrows). (E) Side view of the odontoblast-like cells, derived from mouse MDPC-23 cells (Hanks et al., 1998), extending the cellular processes into the dentinal tubules (arrows). Image sources: (A, B) adapted from Huang et al. (2006b) with permission; (C, D) from Huang et al. (2006a) with permission; (E) courtesy of Dr. Carl T. Hanks.