11
Dentin-Pulp Complex Regeneration
Introduction
In this chapter we will focus on presenting new strategies for local regeneration therapy of the dentin-pulp complex; such a process is mediated by odontoblast and relates to preservation of pulp vitality, and clinical management of deep caries, necrotic pulp with apical periodontitis necrotic and immature permanent teeth; also, to understand factors involved in repairing of the damaged pulp and to review the current knowledge of the potential beneficial effects derived from the interaction of dental materials with the dentin-pulp complex as well as potential future developments.
Recent studies in the area of prevention, diagnosis and treatment of pulpal and periradicular disease has led to an increasing interest into the role of the dentin-pulp complex and its ability to repair itself and regenerate tissues. Regenerative endodontics should be considered as two concerns: local dentin-pulp complex regeneration and regenerative endodontics; the first, also called dentin/ odontoblast complex regeneration, relates to preservation of pulp vitality and pulp capping; the latter relates to regeneration of vital tissue within an empty root canal space.
Historically, pulp capping and dentin bridge formation induction were reported since the 30s, with the first experimental studies performed by Zander in 1939 (Zander 1939).
Attempts to regenerate pulp tissue were carried out in the 60s and 70s without success. Studies focusing on the formation of fibrous connective tissue inside the root canal space have been reported by Ostby and Nygaard-Ostby and Hjortdal. They determined that filling the root canal space with a blood clot could lead to regeneration of pulp tissue. Generation of a disorganized soft connective tissue was observed. Histological examination of extracted teeth revealed that fibrous connective tissue and cellular cementum were formed in the apical portion of the root canal space when the teeth previously contained vital pulp tissue (Ostby 1961; Nygaard-Ostby and Hjortdal 1971).
Publications related to regenerative endodontics have increased significantly in the last decade. In an electronic search in PubMed, with appropriate MeSh terms including ‘regenerative endodontics’, 259 studies of potential relevance were identified (18 April 2019). The first case reports of ‘revascularization’ were reported in 2001 and 2004. Successful clinical outcomes in teeth with pulp necrosis were reported without the conventional obturation of the root canal with gutta-percha or bioceramic materials. These studies defined the direction of the investigation in this topic. From these statistics, it becomes immediately clear that these two conditions remain a significant public health problem and require better strategies for disease prevention and clinical management (Iwaya et al. 2001; Banchs and Trope 2004).
Clinical Setting
Clinically, there are two scenarios that clinicians must handle correctly and that are the most common diseases of the pulp. First, when the dental pulp is still vital and potentially inflamed; in these cases, the main objective is to maintain pulp vitality. The treatment strategy will be focused on locally regenerate dentin and promote reorganization of the underlying connective tissue. In the second clinical scenario, there is a complete loss of pulp tissue, due to cell and tissue death in response to infection and subsequent bacterial invasion and uncontrolled inflammation In this condition, the strategy aims to generate new vital connective tissue, imitating the original dental pulp.
Caries is the most common disease worldwide. The Global Burden of Disease Study 2016 estimated that oral diseases affected at least 3.58 billion people worldwide, and 2.4 billion people suffer from caries of permanent teeth and 486 million children suffer from caries of primary teeth. Data in USA estimate that 92% of adults between 20 to 64 years have had dental caries in their permanent teeth and 26% have untreated decay with an average of 3.28% decayed or missing permanent teeth (National Institute of Dental and Craniofacial Research, 18 April 2019). Epidemiological studies estimate the annual incidence of dental trauma at about 4.5%. Approximately one-fifth of adolescents and adults (permanent teeth) sustained a traumatic dental injury (commonly involved the maxillary central incisors) (Lam 2016). The American Association of Endodontists has estimated that 22.3 million endodontic procedures were performed annually by different causes (American Association of Endodontists, 18 April 2019).
This condition has a clinical impact since an early loss of a permanent tooth in young patients has consequences ranging from aesthetic problems, alterations of the function and bone development of the jaws, problems with phonetics, respiration and mastication, to severe effects of the psychosocial development of the patients. Significant advances have been made in the field of caries management, leading to a better understanding of the mineralization process of the teeth and the biological behavior of the dentin-pulp complex. It is evident that the dentin-pulp complex is able to adapt to a variety of stimuli that generate defense responses to maintain its vitality, and the main role of the dentin-pulp complex is to form defense dentin. It is a new paradigm advocating the complete replacement of compromised tissue, based on tissue engineering rather than traditional restoration.
Basic Concepts
Dentin and pulp tissues are specialized connective tissues derived from ectomesenchymal cells, formed from the dental papilla of the tooth bud.
Dental Pulp
Dental pulp is a highly innervated and vascularized connective tissue of mesenchymal origin, confined within dentin and enamel; it is located in the center of a tooth and is mainly made of living pulp cells, odontoblasts, immune system cells, neurons, endothelial cells and extracellular matrix. Dental pulp has a blood vessel system to deliver nutrients and clear waste products, and a special neural system to offer protection against harmful stimuli. The immune system made by dendritic cells, macrophages and T-lymphocytes are responsible to protect teeth from microorganisms and other foreign antigens (Pashley 2002).
Dentin
Dentin is a mineralized tissue that forms the bulk of the crown and root of the tooth, giving the root its form; it surrounds coronal and radicular pulp, forming the walls of the pulp chamber and root canals; its composition is approximately 67% inorganic, 20% organic and 13% water (Glossary of Endodontic Terms 2016).
Dentin can be classified as primary, secondary or tertiary, depending on when it was formed. Primary dentin is the regular tubular dentin formed before tooth eruption, including mantle dentin. Secondary dentin is the regular circumferential dentin formed after tooth eruption, whose tubules remain continuous with that of the primary dentin. Tertiary dentin is the irregular dentin that is formed in response to abnormal stimuli, such as excess tooth wear, cavity preparation, restorative materials and caries (Cox et al. 2002).
Dentin-pulp Complex
The main internal part of the tooth under the enamel layer in the crown is the dentin-pulp complex, involving the whole tooth root covered by a thin layer of cement. The pulp is a unique tissue, which is a soft tissue of mesenchymal origin with specialized cells, the odontoblasts, arranged peripherally in direct contact with dentin matrix. The close relationship between odontoblasts and dentin, is referred to as the dentin-pulp complex. The structural integrity and isolative characteristics of the tooth are kept by highly mineralized dentin that encloses the pulp chamber and root canals; thus dentin and pulp should be considered as a functional entity made up of histologically distinct constituents (Goldberg and Lasfargues 1995).
Different stimuli and aggressions to dental tissues represent a special situation, since the pulp tissue is surrounded by non-flexible, mineralized walls. Also, the tubular structure of dentine allows permeability, which grants diffusion of bacterial metabolites, and the degradation products of the matrix produced by carious processes, which causes a response of the pulp cells, originating molecular events in response to this damage.
Dentin-pulp Complex Damage Response
When the dentin-pulp complex is injured by caries, trauma, chemical stimuli or other aggressors, the dentin-pulp complex responds dynamically as a functional unit to protect the pulp tissue, either through the formation of sclerotic dentin, calcification of the dentin tubules and/or promoting the formation of reparative dentine by the pulpal odontoblasts, which rapidly deposit dentin (Pashley 2002).
Dentin has a strong relationship with pulp tissue through the odontoblastic process. Dental caries and trauma generate cellular and molecular responses in the pulp that can cause inflammatory and/ or regenerative events at tissue and cellular levels (Goldberg et al. 2011).
The biological response of the dentin-pulp complex to different harmful stimuli is a complex interrelation between the aggression, defense mechanisms and the regeneration process. These factors are addressed independently, and the relation between factors and their relative balance is essential to determine tooth vitality. The balance of these events could directly impact the nature and capacity of any regeneration process mediated by the dentin-pulp complex (Smith 2003).
Pulp cells and odontoblasts play dynamic roles in the regeneration of damaged dentin, as a protective physical defense in the removal of exogenous stimuli by depositing tertiary dentin on the pulp chamber surface. When dentine is invaded by pathogens and their products, the first pulp cells to act are odontoblasts. These cells, located at the dentin-pulp interface with their long cellular process embedded in dentin tubules, represent the first line of defense (Durand et al. 2006).
Odontoblasts may be involved in combating bacterial invasion and activating innate and adaptive aspects of dental pulp immunity. This recognition happens through the detection of molecular structures shared by pathogens and are essential for microorganism survival (Veerayutthwilai et al. 2007).
Dentin-pulp Complex Response to Dental Caries
Dental caries is a chronic infectious disease mediated by a complex and dynamic bacterial biomass that affects the mineralized tissues of the tooth. Bacterial invasion of dentinal tubules has been described as the main cause of the inflammatory response of dental pulp (Cooper et al. 2010). The proliferation and metabolic activity of these microorganisms lead to the release of bacterial components into dentinal tubules and their diffusion towards the peripheral pulp (Cooper et al. 2011). Recognition of bacterial components by host cells at the dentin-pulp interface generates host defensive events including antibacterial, immune and inflammatory reactions and may eliminate early stages bacterial infection and block the route of its progression when accompanied by dentin formation at the dentinpulp interface.
Dental caries may end up in pulpal necrosis and potential tooth loss if not treated. Three basic reactions protect the pulp against caries: (i) a decrease in dentin permeability, (ii) tertiary dentin formation, and (iii) inflammatory and immune reactions. These responses occur concomitantly, and their robustness is highly dependent on the aggressive nature of the advancing lesion (Smith 2002). There are different factors about the dental pulp response that distinguish it from other tissues in the body. Clinically and histologically, the pulpal response to aggressions has been well studied.
Different factors limit the possibility of pulp tissue regeneration. The dental pulp has the least collateral blood supply because of the anatomical features of the pulp chamber, and this leads to a disruption in the function of the immune system for infection control. Odontoblasts, also as post mitotic cells, have a restricted ability to proliferate. Stimulating odontoblast cells in promoting their secretory activity causes losses due to superficial caries, and this leads to dental restorative competence. Only pulp tissue can lead to the regeneration of dentin; however, the regeneration of pulp tissue is difficult, because the tissue is recapped in dentin with no collateral blood supply except from the root apical foramen. Overall, the dentin-pulp complex is responsible for dental health (Farges et al. 2015).
Deep Caries Management
Minimally invasive caries excavation techniques are not new. Different excavation methods to avoid pulp exposure have been previously proposed (Marending et al. 2016). The traditional ‘invasive’ approach is to fully excavate caries. When the pulp is not exposed during complete caries excavation, there is a high probability of success, and for the pulp to remain vital (Fitzgerald and Heys 1991).
Different studies describe minimally invasive techniques. The main clinical objectives are stopping caries progression and maintaining pulp vitality. Three different options of treatments have been recognized:
1) Caries-sealing method: caries is only removed from the enamel, leaving caries in the dentin.
2) Partial caries removal: a portion of caries close to the pulp is left, where two different techniques are described:
– stepwise caries excavation, where the remaining caries is chemically treated and after a period of temporization (few weeks), it is excavated and completely removed;
– indirect capping with an immediate definitive restoration, where the cavity is filled with a permanent restorative material.
3) Complete caries removal: the softened dentin is completely removed. In case of pulp exposure, there are three methods to maintain full or partial pulp vitality: direct capping; partial pulpotomy and complete pulpotomy. For all clinical procedures, the cavity is treated with a permanent restoration, which ensures peripheral sealing.
There are no objective clinical parameters to determine how much carious dentin should be removed; the question arises as to whether to cap the exposed pulp or perform a root canal treatment directly. A systematic review reported success in different longitudinal studies when a complete root canal treatment is performed (Ng 2007). Tooth with pulp exposure subsequent to caries excavation, the cost-benefit relation between a capping procedure, and root canal treatment could still be balanced or even favor pulpectomy (Schwendicke and Stolpe 2014). Completed root formation is a prerequisite for pulpectomy after pulp exposure. As an alternative to pulpectomy, pulpotomy offered a viable alternative to root canal treatment for teeth with vital pulps in short terms (Simon et al. 2013).
Some authors report that the maintenance of pulp vitality and the promotion of biologically based management strategies are the core of deep caries management. Pulp exposure can be avoided in radiographically deep caries and asymptomatic or mildly symptomatic teeth by selective removal of caries and restoration in one or two visits (Bjorndal et al. 2019; Duncan et al. 2019).
For all cases, disinfection of the dental tissue is mandatory for the health of the tooth, the subsequent interaction between dental tissue defense and repair is complex and the fine-tuning of the regulation of these processes is important for ensuring which response predominates when vital pulp tissue can be clinically retained or regenerated.
Local Response
After primary dentinogenesis and tooth formation, the odontoblasts are responsible for regeneration or healing of injuries in the form of tertiary dentinogenesis, which may continue after injuries and, provides the basis for the development of dentine bridges in pulp exposure sites. The formation of the dentinal bridge occurs after the death of the odontoblast near the site of the exposure, and the following differentiation of cells called odontoblast-like cells derived from pulpal stem cells or progenitor cells. This event requires a sequence of cellular actions, which include the recruitment of stem cells, cytodifferentiation and the activation and up-regulation of secretory activity of cells (Choung et al. 2016).
Different factors are involved in the initiation of tertiary dentinogenesis and could be related to harmful agents such as acids and bacterial metabolic products, or by leakage from the restorative material used to fill a cavity. Tertiary dentinogenesis has been described in relation to the nature of the injury. This has led to adoption of terms like ‘reactionary’ and ‘reparative’ to subdivide tertiary dentinogenesis into the responses seen after survival and death of the primary odontoblast population, respectively. The reactionary dentinogenesis represents the focal up-regulation of a group of primary odontoblasts surviving injury to the tooth, while reparative dentinogenesis represents the response of tertiary dentin secretion by a new generation of odontoblast-like cells after death of the primary odontoblast cells (Smith et al. 2001).
Local regeneration of the dentin-pulp complex from residual dental pulp has been developed by researchers who are involved in clinical practice. Induction of appropriate pulp wound healing and formation of new dentin in tooth defects are mandatory for local repair of the dentin-pulp complex and to maintain vital pulp.
Tissue Engineering in Endodontics
A challenging problem for endodontists and pediatric dentists is the clinical management of immature permanent teeth with necrotic pulp resulting from infection or trauma (Albuquerque et al. 2014).
Here we will review the current knowledge of regenerative dentistry as an emerging concept that challenges modern dentistry to step up basic dental research and translate scientific knowledge into the future for clinical scenarios. This methodology is based on the knowledge of the essential mechanisms of tooth development and the biological processes of healing, repair or regenerating (engineering) the damaged tissue or organ (Angelova Volponi et al. 2018).
Since the beginning of the 20s, Davis was the first to recognize the importance of the integrity of the apical/periapical tissues in endodontic therapy (Gutmann 2016). As above mentioned (Ostby 1961; Nygaard-Ostby and Hjortdal 1971) produced the experimental evidence for Davis’ clinically based observation and called attention to the need for biocompatible endodontic materials.
The traditional endodontic treatment for necrotic immature permanent teeth is calcium hydroxide apexification, which has antibacterial properties and can stimulate enzyme pyrophosphatase, facilitating repair mechanisms. However, this therapy requires multiple visits over an extended period; which results in a delay of root canal obturation and placement of permanent restoration and may predispose the tooth to increased susceptibility of root fracture (Andreasen et al. 2002; Giuliani et al. 2002).
Another type of apexification named ‘apical MTA plug’ was described using Mineral Trioxide Aggregate (MTA). MTA is a repair material made of fine hydrophilic particles of tri/dicalcium silicate, tricalcium aluminate, tricalcium oxide and silicate oxide (Parirokh and Torabinejad 2010). MTA is placed into the root canal space and acts as a mechanical barrier to prevent coronal leakage and penetration of microorganisms. Some disadvantages of this material are difficulty to manipulate, the possibility of tooth discoloration and difficulty to remove from the root canal However, neither calcium hydroxide nor MTA barrier technique allow further root growth in length, maturation of the apex or root wall thickening. New calcium silicate-based materials have recently been developed with the purpose of improving clinical use and overcoming MTA limits BiodentineTM is a bioceramic made of tricalcium silicate, dicalcium silicate, zirconium oxide, calcium carbonate, calcium oxide and iron oxide. It is mixed with a hydrosoluble polymer and calcium chloride to decrease the setting time (Rajasekharan et al. 2014). This biomaterial has shown reduced setting time with interesting physical and biological properties as a dentine restorative material (Koubi et al. 2013; Topçuoglu and Topçuoğlu 2016).
It has described a procedure that allows complete root development of immature permanent teeth with necrotic pulp/apical periodontitis. This procedure suggests the use of a combination of an antimicrobial paste and irrigants, no canal walls instrumentation, induced apical bleeding to form a blood clot, and a tight seal into the root canal to promote healing, offering results in terms of penetration through dentine and antibacterial efficacy of the drug combination when the drugs are placed in root canals (Takushige et al. 2004; Hwang et al. 2018; Arruda et al. 2018; Zancan et al. 2019; Zargar et al. 2019; Fundaoğlu Küçükekenci et al. 2019; McIntyre et al. 2019; Sadek et al. 2019). The application of antibacterial drugs may represent a way to eradicate bacteria during root canal treatment; however, this local application has no effect on tissue regeneration by itself.
Regenerative Endodontics
The main goal of regenerative endodontics is the use of biologic-based procedures to arrest the disease process, preventing its recurrence while favoring the repair or replacement of damaged structures of the dentin-pulp complex (Diogenes and Ruparel 2017). The term regenerative endodontics was introduced in clinical endodontics, which also included revascularization and revitalization to describe the treatment of immature permanent teeth with necrotic pulp.
There is considerable discussion on the use of the term ‘regeneration’ because there is convincing evidence from histologic studies that the newly formed tissue following current forms of regenerative endodontic procedures does not resemble the lost dentin-pulp complex. Repaired tissue that promotes resolution of the disease and re-establishment of some or all the original tissue functions should be an acceptable goal (Simon et al. 2014; Diogenes and Ruparel 2017; Song et al. 2017).
Regenerative endodontics comprises both vital and non-vital pulp treatments. Specifically, nonvital treatments include procedures to promote new vital tissue formation after necrosis following infection. The regenerative endodontic treatment is an alternative to a conventional endodontic treatment (Murray et al. 2007).
Some terms have been used to identify clinical procedures in this field:
Regenerative endodontics. Biologically based procedures designed to physiologically replace damaged tooth structures, including dentin and root structures, as well as cells of the dentin-pulp complex.
Revascularization. The restoration of blood supply (Glossary of Endodontic Terms 2016).
Revitalization. An ingrowth of tissue that may not be the same as the original lost tissue (Wang et al. 2010).
The clinical procedures and results of regenerative endodontics are very different from conventional endodontic therapy; that has created an interest in the field of endodontics in recent years. Immature permanent teeth with necrotic pulp/apical periodontitis are traditionally treated with apexification treatment using calcium hydroxide or apical MTA plugs to induce formation of an apical hard tissue barrier before root canal filling. The calcium hydroxide apexification procedure usually takes multiple visits over an extended period, which could increase the risk of root fracture. However, an apexification procedure has no potential to restore the vitality of damaged tissue in the canal space and promote root maturation of immature permanent teeth with necrotic pulp. In the year 2001, Iwaya et al. reported a clinical case in a necrotic immature mandibular second premolar with periapical involvement in a 13-year-old patient; as an alternative to conventional root canal treatment protocol and apexification, antimicrobial agents were used in the root canal Radiographic examination showed the start of apical closure 5 months after treatment and a thickening of the canal wall and complete apical closure was confirmed 30 months after the procedure, suggesting a possible revascularization potential into the root canal space (Iwaya et al. 2001). Other reports on this new technique showed induced root maturation in infected immature teeth and described the use of a blood clot into the root canal as ‘revascularization’ (Branchs and Trope 2004). On the other hand, the term ‘revitalization’ has been suggested as it describes non-specific vital tissue rather than just blood vessels as implied by the term ‘revascularization’ (Lenzi and Trope 2012).
Regenerative Endodontic Procedures (REPs)
Regeneration is defined as reconstitution of damaged tissues by a tissue similar to the original tissue and restoration of biological functions. Repair is the replacement of the damaged tissue by tissue different from the original tissue and consequently, the loss of the original biological functions. The dental pulp has a limited potential of regeneration (Shi et al. 2005).
REPs are defined as “biologically based procedures designed to replace damaged structures, including dentin and root structures, as well as cells of the dentin-pulp complex” (Murray et al. 2007). This term has been widely accepted and refers to all procedures that aim to achieve organized repair of the dental pulp and include future therapies yet to evolve in the field of regenerative endodontics (Diogenes et al. 2017).
REPs are bioengineering treatments that aim to restore the normal physiological functions of the dental pulp, including innate pulp immunity, pulp repair through mineralization (tertiary dentin) and pulp sensibility (sensation of occlusal pressure and pain). These therapeutic techniques include a triad of elements that is integrated by stem cells, growth factors and biomaterials or scaffolds. The successful endodontic regeneration requires synchronized effects of infection control, biomaterials and stem cells (Cao et al. 2015). In regenerative endodontic procedures, growth factors embedded in the dentin matrix are released into the canal space after a cleaning protocol. However, the mesenchymal stem cells introduced into the canal space during REPs do not appear to be able to differentiate into odontoblast-like cells and produce the dentin-pulp complex in animal and human studies (Lovelace et al. 2011). Regenerative endodontics implies that further root maturation results in reestablishment of the dentine-pulp complex. REPs, suggesting repair rather than regeneration (Wang et al. 2010; Becerra et al. 2014).
Figure 11.1. Local regeneration of the dentin-pulp complex by tissue engineering strategies: Stem cells, signal biomolecules and scaffolds.
In recent years, regenerative therapies in endodontics have gained attention as a treatment for teeth without a complete root development affected by caries or trauma. The success of these clinical procedures has offered new treatment alternatives to traditional endodontic therapy. With advances in tissue engineering, the use of stem cells, and the progress made in biomaterials, it is possible to consider including regeneration procedures in daily endodontic clinical practice (Kim et al. 2016) (Fig. 11.1).
Tissue Engineering Strategies
In general, tissue engineering strategies include the evaluation of an appropriate scaffold for the regulation of cell differentiation, selection of growth factors that can promote stem cell differentiation, and an appropriate source of stem cell/progenitor cells (Hargreaves et al. 2013; Albuquerque et al. 2014) (Table 11.1). The success of tissue engineering in combination with tissue regeneration depends on the behavior and cellular activity in the biological processes developed within a structure that functions as a support, better known as scaffolds or directly at the site of the injury. The cell-cell and cell-biomaterial interaction are key factors for the induction of a specific cell behavior, together with bioactive factors that allow the formation of the desired tissue (Ortiz et al. 2019) (Fig. 11.2).
Stem Cell/Progenitor Cells
Stem cells are defined as clonogenic cells capable of both self-renewal and multilineage differentiation since they are thought to be undifferentiated cells with varying degrees of potency and plasticity (Gronthos et al. 2002).
All tissues originate from stem cells, which play an indispensable role in embryonic development and tissue regeneration. These cells are capable of self-renewal, proliferation, and differentiation into multiple mature cell types. Stem cell potency describes the potential of the cell to divide and express different cell phenotypes. Totipotent stem cells are able to divide and produce all the cells in an individual. Pluripotent stem cells have not completely divided and can become many cells, but not all lineages. They are able to differentiate into any of the three germ layers: endoderm, mesoderm or ectoderm, where the progeny has multiple distinct phenotypes, whilst multipotent stem cells can differentiate into cells from multiple, but a limited number of lineages (Robey 2000).
Table 11.1. Current techniques in regenerative endodontic.
| Technique | Objective |
| Root canal revascularization | Open up the tooth apex to allow bleeding into root canals |
| Stem cells therapy | Stem cells are delivered to teeth via injectable matrix |
| Injectable scaffold | Pulp cells are seeded into a 3D scaffold made of polymers and injectable implanted |
| 3D cell printing scaffold | Ink-jet like device dispenses layers of cells in a hydrogel which is surgically implanted |
| Gene therapy | Mineralizing genes are transfected into the vital pulp cells of necrotic and symptomatic teeth |
Figure 11.2. Key components in tissue engineering: Scaffold, stem cells, and signal biomolecules.
There are two types of stem cells: embryonic and postnatal. Embryonic stem cells are pluripotent cells capable of differentiating into any cell type as well as maintaining an undifferentiated state. These cells are plastic and have the capacity to develop into various specialized cell types with an enormous potential for tissue regeneration. Postnatal stem cells have been isolated from various tissues including bone marrow, neural tissue, dental pulp and periodontal ligament. These are multipotent stem cells capable of differentiating into more than one cell type, but not all cell types (Antoniou 2001).
Studies of stem cells of dental pulp has led to a significant change in our understanding of the mechanisms involved in the preservation of dental pulp homeostasis in health and in the pulp response to damage. These cells are related to the physiology of the dental pulp tissue. Also, it has been suggested that stem cells are involved in the regulation of pulp angiogenesis in response to caries. Recently, the potential use of stem cells in dentin-pulp complex engineering has increased the interest in regenerative endodontics.
The first cellular lines established of Mesenchymal Stem Cells (MSC) obtained from different structures of teeth were Dental Pulp Stem Cells (DPSC) and Stem cells from Human Exfoliated Deciduous teeth (SHED) reported by Gronthos et al., at the National Institutes of Dental and Craniofacial Research, both with high potential for differentiation into other cell lineages (Gronthos et al. 2000; Gronthos et al. 2002; Miura et al. 2003). To date, four types of human dental stem cells have been isolated; DPSC, SHED, Stem Cells from Apical Papillae (SCAP), and Periodontal Ligament Stem Cells (PDLSC). DPSC, SHED, SCAP are derived from neural crest mesenchyme (Fig. 11.3).
Figure 11.3. Types of human dental stem cells and sites of isolated.
DPSC show a higher proliferation capacity compared to osteogenic cells and have the ability to differentiate into odontoblast-like cells which express the early odontoblast cell marker, dentine sialophosphoprotein and can form a dentine—pulp complex when transplanted in vivo. DPSC are capable of generating new stem cells or multilineage differentiation into odontoblasts, adipocytes and neural-like cells, suggesting a hierarchy of progenitors within the pulp, including a small population of stem cells amongst a larger population of more committed cells (Gronthos et al. 2002).
The fraction of DPSC in the dental pulp is small, approximately 1% of the total cells (Smith et al. 2005), and the effect of aging reduces the cell pool available to participate in regeneration which reflects the better healing outcomes seen in younger patients (Huang et al. 2008).
SHED, mesenchymal stem cells isolated from the dental pulp of exfoliated deciduous teeth, were recognized to have a high proliferative rate and capability of differentiating into various cell types, including neural cells, adipocytes and odontoblasts. These cells are distinct from DPSC regarding their higher proliferation rate, increased cell population doublings, viability, osteoinductive capacities, failure to reconstitute a dentin-pulp complex and markers (Miura et al. 2003; Koyama et al. 2009; Nakamura et al. 2009) (Table 11.2).
The transplantation of MSC obtained from dental tissues for therapeutic purposes in the dental area is a method that has been performed experimentally in animal models, demonstrating high predictability of success; however, there is still little evidence to bring these experimental protocols to controlled clinical trials in humans Although results show that dental pulp cells have the capacity to repair damaged dental tissue, when it comes to in vivo tooth repair, the dental reparative capacity is limited (Gronthos et al. 2000). It has been shown that dental pulp stem cells can regenerate a pulp-like tissue in root canals in vivo. The tissue formed had functional odontoblast-like cells able to deposit a mineralized matrix on the root canal walls (Huang et al. 2010). The advances on dental pulp tissue engineering are geared towards the generation of a viable and healthy pulp throughout the entire root canal length. SHED cells are able to attach to the dentin walls and proliferate inside root canals in vitro (Gotlieb et al. 2008).
The use of stem cells for the regeneration of pulpal tissues is promising, their use in a clinical situation to induce apexification is incipient and unpredictable; however, advances in the study and understanding of pulp regeneration processes could make this clinical procedure a viable and predictable treatment in a few years.
Table 11.2. Postnatal dental stem cells studied.
| Type | Isolation | Marker | Possible applications |
| Dental pulp stem cells (DPSC) | Dental pulp | STRO-1 CD44 CD46 | Osteogenesis and dentinogenesis inductors; without ability to produce dentin-pulp complex regeneration |
| Stem cells from human exfoliated deciduous teeth (SHED) | Exfoliated deciduous teeth and coronal pulp | CD 146 STRO-1 CD44 | Osteoinductive capacities; but failure to reconstitute a dentin-pulp complex regeneration |
| Periodontal ligament stem cells (PDLSC) | Root of extracted teeth | CD90 CD73 CD105 CD29 | Capacity for tissue regeneration and periodontal repair |
| Stem cells from the apical papilla (SCAP) | Impacted third molars | CD146 STRO-1 CD29 | Higher proliferative rate and effective for tooth formation |
| Stem cells from the dental folicle (DFSC) | Follicles of impacted third molars | CD13 CD29 CD59 CD90 CD105 | Capacity to differentiate into osteoblasts and cementoblasts, adipocytes and neurons |
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