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Logo of woundMary Ann Liebert, Inc.Mary Ann Liebert, Inc.JournalsSearchAlerts
Advances in Wound Care
 
Adv Wound Care (New Rochelle). 2013 July; 2(6): 296–305.
PMCID: PMC3842883

Application of Stem Cell Technology in Dental Regenerative Medicine

Abstract

Significance

In this review, we summarize the current literature regarding the isolation and characterization of dental tissue-derived stem cells and address the potential of these cell types for use in regenerative cell transplantation therapy.

Recent Advances

Looking forward, platforms for the delivery of stem cells via scaffolds and the use of growth factors and cytokines for enhancing dental stem cell self-renewal and differentiation are discussed.

Critical Issues

We aim to understand the developmental origins of dental tissues in an effort to elucidate the molecular pathways governing the genesis of somatic dental stem cells. The advantages and disadvantages of several dental stem cells are discussed, including the developmental stage and specific locations from which these cells can be purified. In particular, stem cells from human exfoliated deciduous teeth may act as a very practical and easily accessibly reservoir for autologous stem cells and hold the most value in stem cell therapy. Dental pulp stem cells and periodontal ligament stem cells should also be considered for their triple lineage differentiation ability and relative ease of isolation. Further, we address the potentials and limitations of induced pluripotent stem cells as a cell source in dental regenerative.

Future Directions

From an economical and a practical standpoint, dental stem cell therapy would be most easily applied in the prevention of periodontal ligament detachment and bone atrophy, as well as in the regeneration of dentin-pulp complex. In contrast, cell-based tooth replacement due to decay or other oral pathology seems, at the current time, an untenable approach.

figure fig-3
Chistopher Lengner, PhD

Scope And Significance

Diseases that destroy the cellular composition and structure of teeth and surrounding tissue, such as periodontitis and pulpitis compromise patients' standard of living. Once tissue injury occurs in the oral cavity, structures are either lost permanently or heal with little scar formation. Stem cells have the ability to regenerate various differentiated cell types and thus, may be applied to promote the regeneration of functional tissue. This article compares and contrasts somatic dental stem cells and pluripotent stem cells and discusses their regenerative potential and practicality. Homing of these stem cells is essential for their regenerative potential to take effect, so the methods of delivery, proliferation, and differentiation of the stem cells are also discussed.

Translational Relevance

Gaining a strong fundamental understanding of the molecular mechanisms that govern dental tissue ontogeny during development is paramount for effect stem cell-based regenerative medicine. Successful manipulation of self-renewal, differentiation, mechanotransductive, and homing mechanisms will be critical for moving the field of dental regeneration medicine forward.

Clinical Relevance

The current treatment plans for dental related diseases, such as periodontitis that have shown some promise in tissue regeneration are bone grafting and guided tissue regeneration (GTR). However, they are performed infrequently and are less reliable than other, more traditional periodontitis treatments. Currently, thegovernment database (ClinicalTrials.gov) describes four registered clinical trials in different stages aimed at the advancement of periodontal ligament stem cells (PDLSCs) in regenerative therapy. Success of the clinical trials indicates that PDLSCs, which are discussed in this review, hold a great potential in treatment of periodontitis.

Background

Developmental origins of dental tissues

Craniofacial development is a complex process involving the combined efforts of a cohort of stem cells with varying developmental origins. The teeth alone have at least two embryonic origins. Ectoderm-derived oral epithelium gives rise to dental enamel, while the neural crest give rise to the remaining dental structures, including pulp, dentin and cementum.1 However, other craniofacial bones, including the flat bones of the skull, are derived from mixtures of progenitor cells, primarily mesodermal cells and neural crest cells.2 Stem cell therapy has garnered much attention in the dental community because of the spectrum of opportunity for autologous cell-based therapies. Limitations of current procedures have led researchers to explore the possible use of stem cells for the regeneration of lost dental structures.

Any effort to advance stem cell therapy into the therapeutic arena will require advances in directed differentiation protocols that can effectively recapitulate the embryological developmental processes of dental tissue. Thus, advancing such efforts necessitates an in-depth understanding of the normal development of dental structures. Odontogenesis starts around the 5th week of embryonic development and continues until all the permanent teeth have replaced primary teeth. After ~5 weeks of gestation, the primary epithelial bands form and thicken at the upper and lower jaws of the future dental arches. Invagination of the oral epithelium around the epithelial bands on both arches result in vestibular lamina and dental lamina. Odontogenesis initiates under the dental lamina ushering in the three stages of the dental development: bud, cap, and bell stages. During the bud stage, the epithelial cells move into the underlying ectomesenchyme, and ectomesenchymal cells pack closer together around the bud (Fig. 1A). In the next, cap stage, the bud splits into a cap-like structure due to proliferation. The epithelium forms the enamel organ, and the ectomesenchymal cells aggregate beneath the enamel organ to form dental papilla (DP) (Fig. 1B). The dental follicle forms around the papilla like a sac. During bell stage, the undersurface of the enamel organ deepens, and cells starts to differentiate (Fig. 1C). Cells of inner dental epithelium (IDE) and DP communicate with each other though the basal membrane (BM). IDE cells and DP cells stop proliferating, and their nuclei move in opposition to the BM and the cells become elongated. Cells of IDE become preameloblasts and they send signals for cells of DP to become odontoblasts. Odontoblasts subsequently secrete predentin, an organic matrix, around the area underneath the IDE, and form odotoblast processes as they move toward the center of the tooth. The secretion of hydroxyapatite (HA) and mineralization convert predentin to dentin. The presence of dentin signals the preameloblasts to make enamel and become ameloblasts.

Figure 1.
Odontogenesis initiates under the dental lamina ushering in the three stages of the dental development: bud (A), cap (B), and bell stages (C). (A) Bud stage: epithelial cells move into the underlying ectomesenchyme, and ectomesenchymal cells pack closer ...

Dental roots develop separately from crowns. The internal and external dental epithelial cell layers grow downward from the cervical loop to form Hertwig's epithelial root sheath (HERS). HERS cells induce the DP to differentiate into odontoblasts producing root dentin. The three cell types that support and stabilize teeth in their sockets are cementoblasts, periodontal ligament (PDL) fibroblasts and alveolar bone osteoblasts. These cells are derived from the dental follicle and together they are called periodontium. It is from this periodontium that stem and progenitor cells with various potential for self-renewal and differentiation have been isolated. In this review, we compare the advantages and limitations of several of the well-characterized periodontium-derived stem cells.

Discussion of Findings and Relevant Literature

Cell sources for dental regenerative medicine

When considering cell sources for regenerative medicine, it is widely appreciated that autologous transplantation will be required to effectively circumvent immune-rejection issues. For the purposes of cell replacement in the dental arena, there exist several potentially viable sources of stem/progenitor cells. First, dental follicle cells, which originate from neural crest-derived mesenchyme, give rise to cells that form PDL. Periodontium is vulnerable to attack by bacterial, chemical and mechanical trauma,3 and the results of such damage are different forms of periodontitis. Periodontitis manifests in redness of the gingival tissue, deep periodontal pockets, and movement of teeth resulting from atrophy of alveolar bone and detachment of PDL from the teeth (Fig. 2). Patients can be genetically predisposed to periodontitis; however, environmental factors, including smoking and diabetes are major risk factors for individuals contracting periodontitis. If left untreated, these conditions may lead to loss of the affected teeth.4 PDLSCs can be isolated with ease from sources, such as naturally lost or surgically removed teeth.3,5 It has also been suggested that stem cells from an inflamed PDL still retain self-renewal and differentiation capacities.6 PDL cells were found to differentiate into cementoblast-like cells, osteoblast-like cells, adipocytes, and collagen-forming cells in vitro, and transformed into cementum/PDL-like structures when they were transplanted with a calcium HA scaffold into immunocompromised rodents.7 Collagen fibers from transplanted PDLSCs were able to link with the cementoblast-like structures which resemble the physiological attachment of Shapey's fibers.7 Currently, the clinical trial.gov database describes four registered clinical trials in different stages aimed at the advancement of PDLSCs in regenerative therapy. The search engine of the International Standard Randomized Controlled Trial Number currently has zero registered trials on PDLSCs, and there are currently no published data for assessment of efficacy of PDLSCs in humans.

Figure 2.
Periodontal disease. Accumulation of periodontal bacteria trigger neutrophils to migrate into the gingival crevice. Cytokines released by the neutrophils and the bacteria attract macrophages that induce fibroblasts to degrade collagens and the underlying ...

The current treatment plans for periodontal diseases include scaling and root planning, prescribing antibiotic medications, open flap surgery, along with bone and tissue grafting.8 One disadvantage of open flap surgery, scaling, and root planning is the disruption of fibrin linkage by a long junctional epithelium, resulting in periodontal repair instead of regeneration.3,9 However, periodontal repair doesn't fully restore the function and architecture of the original tissue.9 Two techniques, bone grafting and GTR, have shown much improved regeneration of the attachment apparatus in comparison to traditional open flap surgery.9 Bone grafts are the most widely used treatment procedures for the replacement of atrophied alveolar bone. Sources of bone can include autografts, allografts, xenografts, and alloplasts. GTR allows for periodontal regeneration by excluding gingival connective tissue cells and preventing downgrowth of epithelium into the wound in the belief that they interfere with regeneration.10 However, there are many more contraindications for GTR than there are indications, making GTR reliable in only select cases of periodontitis.4,9 The indications for GTR are narrow 2- or 3- wall infrabony defects, circumferential defects, class II molar furcations, and recession defects.9

The inherent multipotency of PDL cells has lead researchers to study different delivery vehicles for growth factors (GFs) and cytokines to attract endogenous PDL stem cells and initiate periodontal regeneration.11 A preclinical approach to GF-induced periodontal regeneration was recently attempted in dogs. Mesenchymal stem cells (MSCs) transfected with fibroblast growth factor (FGF)-2 were implanted in sites of defect, resulting in faster regeneration than the delivery of control MSCs.12 A clinical trial of implantation of MSCs with GFs was completed in Japan on a 54-year-old female patient. Yamada et al. delivered a fibrin gel mixture of bone marrow stromal stem cells and platelet-rich plasma (PRP) to the root surface,13 and the result indicated improvements in probing depths, clinical attachment level, bleeding on probing and bone fill. Flores et al. demonstrated the ability of transplantable human PDL cell sheets to develop into a cementum-PDL complex in vivo14 (Fig. 2). Thus, these studies on delivery of stem cells combined with GFs show promise in early clinical stages; however, larger scale human studies are necessary to unequivocally determine their efficacy.

In addition to PDL cells, other progenitor cell sources exist within the craniofacial tissues, including dental follicle progenitor cells (DFPCs), apical papilla progenitor cells (SCAP), and dental pulp cells. These cells all share a number of features with the well-studied bone marrow-derived mesenchymal stem cells, including their ability to differentiate, albeit with varying efficacy, into cells of the major mesenchymal lineages: osteoblasts, chondrocytes, and adipocytes. However, these cells seem to differ in their propensity to differentiate into functional odontoblasts.15 Further, some of these cells, such as the SCAP, are isolated from specific developmental stages (in this case the apex of a developing tooth), and thus, do not serve as practical sources of cells for autologous cell-based transplantation.

Dental pulp stem cells (DPSCs), stem cells from human exfoliated deciduous teeth (SHED), SCAP, and PDLSCs have all been successfully isolated and characterized both in vitro and in vivo. Because of the nature of the deciduous teeth, guiding the development of permanent teeth and eventual replacement by the permanent teeth, deciduous teeth may act as a very practical and easily accessibly reservoir for autologous stem cells and hold the most value in stem cell therapy.16 SHED are obtained by enzymatic digestion of pulp from deciduous teeth and are more proliferative than the four other common dental stem cell types.15,16 SHED exhibit the capacity to undergo osteogenic and adipogenic differentiation. In vitro, SHED express different neural cell markers.17 Similar to DSPCs and SHEDs, SCAP has the potential to undergo odotogenic differentiation in vitro. SCAP is found apical to the epithelial diaphragm, and there is a cell-rich zone between the apical papilla and the pulp.18 PDLSCs can also be easily obtained and isolated from surgical waste as previously described. DPSCs are not as readily accessible as SHED or PDLSCs, because they are conventionally obtained from freshly extracted third molars (wisdom teeth) of healthy adults.19 This is problematic when patients have congenitally missing third molars, late eruption of third molars or surgical splitting of impacted teeth which renders them unfit for DPSCs isolation.20,21 Successful isolation of DPSCs was demonstrated from pulps of diseased dentitions (excluding pulpitis teeth); however, whether DPSCs from diseased tissues have the same proliferative and differentiation capacity as those isolated from healthy tissue remains to be clarified.22 One important feature of DPSCs is their odontoblastic differentiation potential. In vitro, DPSCs exhibited osteogenic, chondrogenic and myogenic differentiation potential.23 DFPCs were previously isolated from human dental follicles of impacted third molars, and thus, suffer from the same lack of availability for some individuals as DPSCs. DFPCs have the potential to differentiate into periodontium, such as cementum, PDL and alveolar bone. Comparisons of osteo/odontogenic, neurogenic, and chondrogenic biomarkers among the five groups of dental stem cells reveal that DPSCs may most readily differentiate into cells that support dentinogenesis. Second to DPSCs are the SCAP, and then SHED, with PDLSCs and DFPCs expressing just a few biomarkers.15

Whether the more broadly studied bone marrow-derived or perivascular mesenchymal stem cells are capable of functional differentiation into odontoblasts, or whether the aforementioned dental-derived progenitor cells inherently differ from their bone marrow/perivascular-derived cousins in their being preprogrammed for odontoblastic fate specification remains a matter of some controversy. While Huang et al.15 propose that the differentiation programs that allow dental-derived progenitors to form odontoblasts are analogous to those that allow MSCs to generate osteoblasts based on differential transcriptome profiling of these populations,24 other studies have observed pericyte (perivascular) MSCs directly differentiating into odontoblasts in vivo,25 suggesting that these particular MSCs might be capable of adopting both an osteogenic and odontogenic cell fate. Curiously, human dental pulp cells are the only mesenchymal cells that, to date, have been shown to harbor the ability to differentiate into osteoblasts, odontoblasts, and adipocytes in vitro.26 Thus, these dental pulp cells might offer an invaluable experimental platform for elucidating the signals that induce odontogenesis versus osteogenesis.

Gaining a comprehensive understanding of the molecular regulatory programs that specify odontoblast versus osteoblast cell fate and whether dual differentiation potential can be held by an individual progenitor cell in vivo is of critical importance for advancing one of the most promising stem cell types, the induced pluripotent stem (iPS) cell into the dental regenerative arena.

iPS cells: promises and limitations in dental regenerative medicine

While somatic stem cells, including MSCs or dental progenitors discussed above offer promise as sources for cell-replacement therapies, they have inherent limitations. In particular, somatic stem cells cannot always be derived from the patient for use in autologous transplantation (Table 1). Another significant concern is the limited replicative potential of these cells that may preclude their clinical use if sufficient numbers for transplantation cannot be generated in vitro. To overcome these issues, increasing attention has been paid to the potential use of pluripotent cells for cell-based regenerative medicine.

Table 1.
Sources of somatic dental stem cells

Pluripotent cells are characterized by their ability to differentiate into any cell type of an organism, unlike somatic stem cells that are regarded as multipotent due to their ability to differentiate only into cells of the tissue from which they were isolated. The pluripotent state is characterized by the activity of the pluripotency gene regulatory network, which has the transcription factors Oct4, Sox2, and Nanog at its core. Numerous reports have been published suggesting the presence of pluripotent cells in the adult stroma; however, none have been verified and strong evidence suggests that the pluripotent state, and therefore, the pluripotency regulatory network, does not exist in somatic cells in vivo.27,28 Human embryonic stem (ES) cells represent the gold standard source of pluripotent cells. ES cells can self-renew indefinitely in culture without losing their capacity to differentiate, in stark contrast to their somatic counterparts where long periods of in vitro culture frequently come at the cost of decreased differentiation capacity and replication-induced senescence. The obvious drawback of applying ES cells in regenerative medicine is the inability to access these cells in patients: ES cells must be isolated from preimplantation embryos, usually necessitating the destruction of that embryo. Thus, intense research has been directed at the generation of pluripotent cells from adult (somatic) tissues to fulfill the promise of an autologous, pluripotent cell source for regenerative medicine.

A breakthrough in generating pluripotent cells from somatic tissue came in the 1990s with the advent of cloning, or somatic cell nuclear transfer, in which a somatic genome was transplanted into an enucleated oocyte.29 Unknown factors in the ooplasm could act upon the somatic genome and “reprogram” it to the embryonic, pluripotent state. Unfortunately, this procedure was technically challenging and inefficient in mice, and required the use of donor oocytes that are not readily available in the human population. Thus, successful somatic cell nuclear transfer was never achieved in humans.

When the prospects for generating autologous pluripotent cells looked bleak, a major breakthrough came from the laboratory of Shinya Yamanaka, who went on to win the 2012 Nobel Prize in Physiology or Medicine for his pioneering work. In 2006, Yamanaka and Takahashi reported the generation of pluripotent mouse cells (called induced pluripotent stem, or iPS cells) from a somatic cell source through the simple overexpression of four transcription factors: Oct4, Sox2, Klf4, and cMyc, delivered by retroviral transduction.30 The robust nature of this methodology enabled rapid verification in a number of laboratories, and soon the procedure was successful in humans.31,32 The ease of this reprogramming strategy allowed the technology to be applied widely, and soon it became clear that all nucleated somatic cells are susceptible to iPS cell generation. Further advances allowed for the generation of virus-free iPS cell lines that are functionally indistinguishable from true ES cells.33

Despite the great promise held by iPS cell technology, there are several limitations that must be addressed before the application of these cells in regenerative medicine. First, the reprogramming process appears to be stochastic and incomplete reprogramming can result in iPS lines with some “epigenetic memory” of their tissue of origin. While this may be a hindrance in many circumstances, it can also potentially be advantageous. For example, generation of iPS cells from a dental pulp progenitor cell may yield cultures that are more prone to odontogenic differentiation than similar iPS cells derived from leukocytes or keratinocytes. Recently, there has also been concern that the rapid cell division that occurs during the reprogramming process may render iPS cells more susceptible to replication-induced DNA mutation, although the significance and scale of this phenomenon is not well understood.34 Such studies emphasize the need for stringent screening of iPS cells (or any cells for that matter) before using it in cell replacement therapy.

Possibly the largest barrier for the use of iPS cells in regenerative medicine is the paucity and inefficiency of existing directed differentiation protocols. Robust directed differentiation of pluripotent cells into the desired functional effector cell type is prerequisite for successful cell-based therapy, and while tremendous progress is being made in differentiating iPS cells into various terminally differentiated cells, this area remains largely unexplored in dental medicine.

In the case of tooth development, where there is a complex interplay of neural crest-derived mesenchyme and oral epithelium, lineage specification protocols must first be developed to (1) specify ectoderm, (2) specify neural crest versus odontogenic epithelium, (3) initiate coculture of neural crest and epithelial derivatives on a tissue-engineered scaffold. While such stepwise, directed differentiation protocols have not yet been refined for dental tissue, similar approaches have been very successful in generating a variety of neural subpopulations and endodermal derivatives.35,36 Given the robust literature on oral ectoderm and odontogenic mesenchyme specification, a similar approach may be viable for derivation of dental tissues from pluripotent stem cells.

A number of classic tissue transplantation experiments have identified stages of induced competence and specification of dental tissue through interactions between craniofacial epithelium and mesenchyme. Epithelial cells of the first branchial arch are capable of instructing tooth formation when cultured with neural crest-derived mesenchyme of the second branchial arch; however, the molecular basis for this competence is not well understood.37 It appears, however, that such competence may arise from the interplay of a number of signaling pathways between the two tissue types, particularly the sequential and reciprocal signaling of bone morphogenetic protein (BMP), FGF, Wnt, sonic hedgehog (SHH), and activin between the mesenchyme and epithelium.3840 These studies and others (reviewed by Thesleff and Jussila41) provide a blueprint for moving forward in establishing directed differentiation protocols, possibly involving coculture of mesenchymal and epithelial components on a bioscaffold, for the derivation of mature dental tissue from pluripotent stem cells sources, such as iPS cells.

Delivery of stem/progenitor cells for regenerative dentistry

Much excitement has recently surrounded the identification and application of stem cells with therapeutic value, but equally important is the development of delivery methods for transplantation of these cells and the promotion of their efficient engraftment. Traditionally, stem cells are introduced into the body via injection into the site of interest or into the circulation. However, the efficacy of this methodology is questionable, since studies have shown poor cell survival, engraftment, and unpredictable differentiation in vivo.42 The potential efficacy of stem cell delivery and differentiation may be improved with the adoption of tissue-engineered scaffolds for cell delivery and structural support. Delivery of dental stem cells can potentially be supported by scaffolds that provide both mechanical and molecular (GF, integrin receptor engagement) cues for differentiation. HA is a naturally occurring mineral form of calcium apatite; it can be coated onto implants and prosthesis to promote osteointegration. In vitro and in vivo studies of dental stem cells mixed with HA both showed increases in osteogenic markers, including alkaline phosphatase (ALP), Runt-related transcription factor 2 and bone sialoprotein, suggesting that HA or its derivatives may promote osteogenic differentiation of dental stem cells and may serve as a scaffold in delivering stem cells to defect sites.7,43 Other scaffold structures that were studied include calcium-silicate derivatives,44 and peptide hydrogels.45 All of these scaffolds share the common properties of being biodegradable and promoting osteogenic differentiation. Properties of scaffolds include providing cell adhesion, enabling cell proliferation and differentiation, and mimicking microenvironment observed in natural tissues and organs. Unlike injection-based delivery, scaffolds allow for superior control for stem cell delivery and allow for impregnation with time-release GFs, modulation of stiffness, pore size, and cell-substrate interaction. Some evidence suggests that in addition to directly participating in tissue regeneration, transplanted stem cells can also regulate tissue regeneration by secreting trophic factors.46 These two properties of stem cells suggest that the proper local distribution of stem cells is critical, making bioscaffold-based delivery one of the most promising strategies in cell-based regenerative dentistry.

Incorporation of GFs and/or cytokines has been shown to increase the efficacy of cell-based regeneration, both in the context of exogenous cell delivery, and recruitment of patients' endogenous stem cells from the local environment in vivo. Previously, FGF and PRP were described to enhance regeneration of PDL in vivo.12,13 GFs enhance dental stem cell activities by promoting the migration of endogenous cells and the subsequent proliferation, differentiation, angiogenesis and neuronal growth. GFs and cytokines are either autocrine or paracrine in nature and modulate cellular behavior by mediating intracellular communication. Kim et al. recently discussed the effects of some GFs in pulp-dentin regeneration.47 These GFs include platelet-derived growth factors (PDGF), transforming growth factor beta (TGFβ) family, vascular endothelial growth factor, FGF and etc. When delivered alone or with stem cells to sites of interest, they share common properties of promoting dentinogenesis, cellular proliferation and odontoblastic differentiation. PDGF, for example, normally released by platelets, induces chemotaxis and proliferation of stem cells by promoting angiogenesis and cell proliferation. PDGF has four isoform homodimers and one heterodimer: AA, BB, CC, DD, and AB. When PDGF dimers bind to their receptors, PDGFRα and PDGFRβ, the two receptors dimerize and bind to different isoforms of PDGF. PDGFRα and PDGFRβ are both present on odontoblasts.48

Findings suggest that PDGF influence osteoblastic differentation via these two receptors by stimulating the expression of dentin sialoprotein, while inhibiting ALP activity in dental pulp cells in vitro. BMPs, a subgroup of the TGFβ superfamily, promote cellular proliferation, differentiation, and apoptosis, and are well known to have osteoinductive and chondrogenic effects.49,50 BMPs were found to induce elevated mRNA expression of dentin sialophosphoproteins (DSPPs) and ALP, and stimulated differentiation of dental pulp cells into odontoblasts.51 BMPs regulate DSPP expression and odontoblastic differentiation by inducing nuclear transcription factor Y activities.52 Choi et al. demonstrated in vitro the chondrogenesis of PDLSCs when they were treated with TGFβ3 and BMP-6.53 They found upregulation of chondrogenic markers SOX9, aggrecan, and collagen type II.53 The two GFs exhibited synergistic effect when given together.53 Rizk and Rabie demonstrated chondrogenesis of DPSCs transplanted into nude mice on poly-L-lactic acid/polyethylene glycol electrospun fiber scaffolds. These DPSC transplants were more efficient at chondrogenesis when transduced with TGFβ3 virus, further supporting the notion that codelivery of GFs can stimulate basal levels of stem cell differentiation upon transplantation in vivo.54

Take-Home Messages

  • Several somatic stem cell types are capable of generating dental tissues, including DPSCs, SHED, SCAP and PDLSCs.
  • SHED may act as a very practical and easily accessibly reservoir for autologous stem cells and hold the most value in stem cell therapy.
  • DPSCs and PDLSCs should also be considered for their triple lineage differentiation ability and relative ease of isolation.
  • iPS cells are able to differentiate into any cell types of an organism; however, efficient directed differentiation protocols for generation of dental tissues remain largely unexplored.
  • Cell-based dental regeneration will likely rely heavily on biomimetic scaffolds, such as HA or others that are biocompatibable and provide niches for stem cell growth.
  • Incorporation of GFs and/or cytokines has been shown to increase the efficacy of cell-based regeneration

Conclusions

Stem cells are rapidly becoming a focus for the restoration of function and aesthetics in dentistry. Isolation of stem cells from the oral cavity has led researchers to identify five different classes of tissue-resident stem cells. Advancing cell-based therapy in dentistry will require a strong fundamental understanding of the molecular cues leading to the normal development of dental tissues. Harnessing this knowledge will enable advances in directed differentiation of stem cells and the delivery of therapeutic cell types will likely be much improved by implantation with biomimetic scaffolds and GFs. Ultimately, depending on the site of injury, clinicians need to evaluate the pros and cons of different cell sources when deciding the types of stem cells that should be employed. Stem cells should be useful in preventing detachment of PDL and bone atrophy, and regenerating dentin-pulp complex; however, the de novo generation of functional teeth will likely remain elusive in the foreseeable future due to the complex interactions between numerous cell types.

Abbreviations and Acronyms

ALP
alkaline phosphatase
BM
basal membrane
BMPs
bone morphogenetic proteins
DFPCs
dental follicle progenitor cells
DP
dental papilla
DPSCs
dental pulp stem cells
DSPP
dentin sialophosphoproteins
ES
embryonic stem cells
FGF
fibroblast growth factor
GFs
growth factors
GTR
guided tissue regeneration
HA
hydroxyapatite
HERS
Hertwig's epithelial root sheath
IDE
inner dental epithelium
iPS
induced pluripotent stem cells
MSC
mesenchymal stem cell
PDGF
platelet-derived growth factors
PDL
periodontal ligament
PDLSCs
periodontal ligament stem cells
PRP
platelet-rich plasma
SCAP
apical papilla progenitor cells
SHED
stem cells from human exfoliated deciduous teeth
TGFβ
transforming growth factor beta family

Author Disclosure and Ghostwriting

No competing financial interests exist. The content of this article was expressly written by the authors listed. No ghostwriters were used to write this article.

About the Authors

Ruoxue Feng holds a Bachelor of Science degree from the Pennsylvania State University. She is currently in her 2nd year of the Doctor of Dental Medicine (DMD) Degree Program at the University of Pennsylvania (UPenn) School of Dental Medicine. She worked with Dr. Chris Lengner's laboratory with support from the Student Research Program at the School of Dental Medicine. Her research focus is on the regulation of self-renewal and differentiation of mesenchymal stem cells by the Msi family of RNA binding proteins. Before matriculating at UPenn, she worked at Dr. Sunday Akintoye's Laboratory at the Oral Medicine Department at the UPenn School of Dental Medicine as a research technician, where she coauthored studies on how biomimetic calcium-silicate cements support differentiation of human orofacial mesenchymal stem cells. Chris Lengner, PhD, is an assistant professor in the Department of Animal Biology at the UPenn School of Veterinary Medicine, and the Department of Cell and Developmental Biology at the UPenn Perelman School of Medicine. He is also a member of the UPenn Institute for Regenerative Medicine. Dr. Lengner has authored numerous publications in the field of stem cell biology, including studies on the determinants of mesenchymal stem cell fate commitment, the molecular mechanisms underlying somatic stem cell multipotency, and the generation of iPS cells from somatic cells. The Lengner laboratory is focused on understanding the fundamental molecular pathways governing stem cell self-renewal, how these pathways can be harnessed for regenerative medicine, and how their deregulation contributes to oncogenic transformation.

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