Regenerative Endodontics in BRIEF • Reviews Advances in Regenerative RESEARCH Endodontics

Regenerative endodontics IN BRIEF • Reviews advances in regenerative RESEARCH endodontics. S. Simon*1–3 and A. J. Smith4 • Suggests regenerative endodontics as it is currently performed represents more of a reparative than regenerative therapeutic strategy.

Background Significant advances in our understanding of the biological processes involved in tooth development and repair at the cellular and molecular levels have underpinned the newly emerging area of regenerative endodontics. Development of treatment protocols based on exploiting the natural wound healing properties of the dental pulp and applying tissue engineering principles has allowed reporting of case series showing preservation of tissue vitality and apexogenesis. Aim To review current case series reporting regenerative endodontics. Results Current treatment approaches tend to stimulate more reparative than regenerative responses in respect of the new tissue generated, which often does not closely resemble the physiological structure of dentine-pulp. However, despite these biological limitations, such techniques appear to offer significant promise for improved treatment outcomes.Conclusions Improved biological outcomes will likely emerge from the many experimental studies being reported and will further contribute to improvements in clinical treatment protocols.

INTRODUCTION and regeneration following pulpal disease extensively investigated to provide a Dentistry has long been a pioneer of and preserve tooth vitality. The concept blueprint to understand the molecular and regenerative medicine with the use of of regenerative endodontics5 is broad and cellular events involved in pulp repair and calcium hydroxide to stimulate tissue repair encompasses a wide variety of strategies regeneration. Many parallels appear to after pulp exposure1,2 and more recently to clinically translate our biological exist in these processes, albeit with altered with newer agents including mineral understanding of pulp regeneration into regulatory control.6 trioxide aggregate (MTA)3 and tri-calcium improved patient management approaches. In considering the responses of the silicate (Biodentine).4 Lack of clarity on Thus, these strategies range from focus on dentine-pulp complex to injury, whether of the mechanisms of action of these various clinical protocols to harness the natural traumatic or carious origin, it is important agents has led to their application being wound healing capacity of the pulp, through to distinguish initially the extent of this somewhat empirical, however, they have interventions to promote revascularisation injury since it has significant implications for contributed significantly to the preservation of an empty root canal and optimise the subsequent cellular events. When the injury is of pulp vitality in diseased teeth. Significant actions of canal irrigants to use of stem in its earlier stages, or relatively low grade in research advances in the broader fields of cells for tissue engineering. Clearly some of nature, the pulp responds by upregulation of biology and tissue engineering over the these approaches show promise for shorter the secretory activity of the odontoblasts at last two decades have been paralleled by term translation while others are longer term the site of injury resulting in focal deposition developments in pulp biology, which have clinical goals. Nevertheless, the over-riding of a ‘reactionary’ variant of tertiary dentine at underpinned our mechanistic understanding emphasis on preservation of tooth vitality will the pulp-dentine interface.7 This response may of the responses of the dental tissues be beneficial to the practice of endodontics. be regarded as both defensive by increasing to injury, wound healing and clinical This article seeks to outline some of the key the barrier to ongoing carious bacterial attack intervention. This has provided a platform biological advances underpinning these new and regenerative in restoring the structural from which novel therapeutic strategies have strategies in regenerative endodontics and integrity of the tissue. Since the reactionary emerged, which aim to promote tissue repair to highlight the opportunities for clinical dentinogenesis results from the activity of translation. existing primary odontoblasts, the new dentine 1Department of Oral Biology, School of Dentistry, deposited has a regular tubular structure Université de Paris Diderot, Paris, France; 2Hôpital de PULP BIOLOGY AND ITS showing tubular continuity with the existing la Pitié Saléptrière, Paris, France; 3Research Centre UNDERPINNING OF primary/secondary dentine matrices. As a des Cordeliers, 15 rue école de médecine, 75006 Paris, REGENERATIVE ENDODONTICS France; 4School of Dentistry, University of Birmingham, consequence, the new tissue will be expected St Chad’s Queensway, Birmingham, B4 6NN, UK By definition, wound healing is a to behave identically to the existing tissue. *Correspondence to: Dr Stéphane Simon Email: [email protected] pathological process and tissue events may These responses are typical of an early carious not be under the same regulatory control lesion or perhaps significant wear to the tooth Online article number E13 as the physiological processes of tissue surface. With further progression of disease Refereed Paper - accepted 17 December 2013 DOI: 10.1038/sj.bdj.2014.243 development and homeostasis. Nevertheless, and the stimulus from the injury becoming ©British Dental Journal 2014; 216: E13 these physiological processes have been more intense, cellular events in the pulp will

likely change significantly. Local death of have the potential to give rise to odontoblast- by enamel organ-derived molecular signals some of the primary odontoblasts at the site like cells including the undifferentiated (including growth factors of the TGF‑b of injury will ensue and the inflammatory mesenchymal cells in the cell rich layer of the superfamily). The absence of enamel responses within the pulp will intensify. If mature pulp derived from the developmental epithelium in the mature tooth requires an the tissue conditions are conducive to repair stages of odontogenesis8 and the various alternative signalling source for odontoblast- and regeneration, that is, the inflammation pulp stem cell populations more recently like cell differentiation during dentine-pulp has not become uncontrolled and the pulp is reported. These pulp stem cells include dental regeneration. While dentine has traditionally not grossly infected, a typical wound healing pulp stem cells (DPSCs),9 stem cells from the been regarded as a relatively inert tissue response will be initiated in which the pulp apical part of the papilla (SCAPs),10 stem comprising largely of the structural protein will strive to repair and regenerate itself by cells from human exfoliated deciduous teeth collagen and mineral, it is now clear that it recruiting and differentiating new cells. The (SHEDs)11 and side-population pulp stem contains a variety of bioactive molecules with odontoblast, however, is a unique cell in the cells.12 Despite characteristic cell surface potent cell signalling functions (reviewed body in many respects. It is a post-mitotic cell marker profiles, all of these cells appear to in Smith et al.).16 Among these bioactive meaning that it is unable to divide and give be mesenchymal stem cell (MSC) derived molecules are a variety of growth factors rise to progeny. Furthermore, this absence of and have varied localisations ranging from and cytokines, many of which have been turnover of odontoblasts leads to its continued perivascular niches to the apical papilla implicated in signalling developmental events presence for the life of the tooth in the absence of the tooth. There has been considerable including odontoblast differentiation. Carious of injury. Clearly, the odontoblast is a very interest in the identification of these cells not dissolution of dentine matrix will release versatile cell with its alternating phases of only for their potential application in dental many of these bioactive molecules and their activity and quiescence throughout life. The regeneration but also much broader clinical diffusion to the pulp can lead to signalling consequence of the odontoblast’s post-mitotic applications in regenerative medicine and of odontoblast-like cell differentiation. nature is that following cell death, any new tissue engineering. Currently, MSCs are The presence of these bioactive signalling generation of odontoblast-like cells must arise most commonly isolated from bone marrow molecules in dentine explains the ability of by recruitment of stem/progenitor cells and for regenerative medicine, however, pulp dentine chips to stimulate dentine bridge signalling of their differentiation. When this offers the opportunity to isolate such cells formation17 and the demonstration of their occurs, the new generation of odontoblast- less invasively, especially in the case of stimulation of reparative dentinogenesis like cells focally secretes a ‘reparative’ SHED cells from exfoliated teeth. Indeed, (reviewed in Smith et al.)16 offers exciting variant of tertiary dentine at the injury site.7 commercial stem cell banks have now opportunities for clinical application. Reparative dentinogenesis constitutes the emerged for storage of these cells. Although Sequestration of these bioactive molecules process for dentine bridge formation at sites resident pulp stem cells represent a possible in a fossilised state in dentine provides an of pulp exposure after pulp capping. source of odontoblast-like cell progenitors, exquisite self-repair mechanism for the tooth. Reparative dentine can show considerable MSC recruitment from outside the tooth It is likely that clinical exploitation of these heterogeneity in its structure, the appearance (possibly bone marrow) to sites of injury in molecules will target protocols to release of which can range from a normal regular the pulp occurs.13 Once recruited, these cells endogenous stores within the dentine (see tubular structure to a very disorganised are exposed to the niche environment of the below) rather than rely upon their application poorly tubular or atubular structure. pulp, which may give rise to their specific within new materials. This reflects the pathological nature of phenotypic characteristics. Cell recruitment reparative dentinogenesis, which lacks the from outside the pulp may be advantageous Odontoblast-like cell regulatory control seen during physiological to rapidly providing the significant stem cell secretory activity dentinogenesis. As a consequence, the stem/ numbers required post-injury. Achieving successful dentine-pulp progenitor cells recruited for odontoblast- Despite many reports proposing stem cell regeneration clinically relies upon regulation like cell differentiation may have varied transplantation for pulp regeneration, there of cell secretory activity if pulp canal derivations and thus the formative cells is still considerable debate as to whether obliteration and loss of tooth vitality is to for reparative dentine may be somewhat such cell-based approaches are necessary be avoided.18 Much of the research focus on heterogeneous in nature. Reparative when cell-free approaches may be feasible tooth development and pulp regeneration dentinogenesis is clearly a more complex and avoid the significant challenges of has been concerned with the triggers to process than reactionary dentinogenesis and introducing cell transplantation protocols switch-on odontoblast differentiation, but is characterised by several key events: into general dental practice. Some of the subsequent regulation of secretory activity • Recruitment and proliferation of stem/ constituents of dentine matrix promote cell and the signals downregulating odontoblast progenitor cells to the site of injury recruitment in the pulp14 and their release activity after primary dentinogenesis have • Signalling of differentiation of during carious dissolution of the tooth may received little attention. Transcriptome odontoblast-like cells from the recruited contribute to MSC recruitment to the pulp analyses of young and mature odontoblasts stem/progenitor cells through the vasculature during natural have revealed modulation in expression of a • Upregulation of secretory activity in the wound healing by promoting such processes number of genes associated with changes in new generation of odontoblast-like cells. through cell homing.15 cellular activity, including downregulation of the p38-MAPK pathway.19 P38-MAPK Stem/progenitor cell recruitment Odontoblast-like cell signalling may be key to the regulation of Despite considerable debate, there is still lack differentiation odontoblast activity since it is stimulated of consensus as to which cell population(s) During tooth development, reciprocal again on odontoblast20 and pulp cell21 are the progenitors of the new generation epithelial-mesenchymal interactions in the activation during exposure to bacterial of odontoblast-like cells for reparative tooth germ are responsible for signalling products and bioactive components of dentinogenesis and dentine regeneration. A cellular events, including odontoblast dentine matrix during tertiary dentinogenesis number of locally resident cell populations differentiation, the latter of which is triggered and regenerative events in the pulp. This

pathway provides a key clinical target for control in endodontics, which are exacerbated Clearly, there is a strong relationship regulation of dentinogenic activity and the by the lack of specificity of most disinfection between canal preparation procedures availability of inhibitory agents may allow agents to bacterial rather than host cells. A and treatment outcomes for regenerative pharmaceutical control of tissue events. wide variety of disinfection and irrigation endodontics. Careful attention to infection protocols are commonly used with little control (for example, rubber dam, disinfection REGENERATIVE ENDODONTICS AS consensus on their use. Sodium hypochlorite protocols etc), the extent of surgical excision A CLINICAL TREATMENT STRATEGY and ethylenediaminetetraacetic acid (EDTA) of inflamed tissue and case selection in the A number of single case and case series are widely used in canal preparation and context of preservation of tissue vitality are reports of regenerative endodontics are now their order of application has marked effects central features of regenerative endodontics. emerging in the literature,22–25 which highlight on tissue damage. Application of sodium the potential for such treatment strategies. hypochlorite followed by EDTA allows Natural pulp wound healing versus These reports focus on use of treatment preservation of dentine structure, while tissue engineering approaches protocols, which build on our knowledge the converse order of application leads to The scope of regenerative endodontics of the biology of the dentine-pulp complex significant dentine damage due to reduction ranges from procedures aimed at promoting in health and disease and aim to preserve in the protective effects of the mineral against natural wound healing events in the pulp to tissue vitality. The approaches range from the oxidising effects of hypochlorite.29 application of tissue engineering principles adoption of protocols that strive to create Hypochorite is a powerful oxidising agent to treatment protocols. At this stage, there is a conducive tissue environment for natural and the broad range of concentrations that insufficient evidence that either is superior, wound healing processes to ensue in the it is used at can have significant implications but advances in pulp biology will likely lead pulp, to those which seek to exploit the basic for further pulpal injury. Generally, lower to clinical introduction of tissue engineering principles of tissue engineering to promote concentrations are preferable if used in the techniques, either for the pulp or the whole pulp regeneration. A common theme for all context of regenerative endodontics. tooth in the future. There is already proof- of these approaches, however, is focus on The rich reservoir of growth factors and of-principle at the laboratory level that those aspects widely recognised to be primary other bioactive molecules sequestered in a such techniques are feasible39 and such determinants of treatment outcome. Thus, fossilised state within the dentine matrix studies offer exciting future possibilities for control of infection and inflammation is a key offers exciting opportunities for exploitation novel treatment modalities in endodontics. consideration in all of these approaches, as in to drive regenerative events in pulp.16 While Currently, there are attempts to apply tissue any endodontic management plan. In contrast carious demineralisation of dentine will engineering principles within regenerative to more traditional root canal treatment initiate release of some of these molecules, endodontics.22 In tissue engineering, cells approaches, there is greater emphasis on various irrigants and etchants can also locally (often stem or progenitor cells) are combined more gentle infection and inflammation release and expose these molecules30–33 and with scaffolds and appropriate signalling control measures to minimise tissue damage. stimulate dentinogenic events.34 molecules to construct tissues resembling These control measures may pose a slightly Release of bioactive molecules may also be their physiological counterparts. Although higher risk in terms of their efficacy by stimulated by pulp-capping agents, including application of cells for regenerative virtue of their need to preserve host tissue calcium hydroxide and mineral trioxide endodontic procedures has been restricted vitality and thus, case selection for adoption aggregate (MTA).35,36 Such effects may also to experimental studies, there have been of regenerative endodontics is of significant favour stem cell recruitment through cell attempts to introduce scaffolds through blood importance. Regenerative endodontics will homing15 as a consequence of local release clot formation – termed ‘revascularisation’ likely be the treatment of choice for those of dentine matrix components at sites of procedures.22 This emulates the role of cases where infection and inflammation are injury allowing chemotactic attraction the blood clot as a scaffold for tissue clinically judged to be not too extensive. of pulp stem cells.14 Careful attention to regeneration in natural soft-tissue wound This highlights the critical challenge facing canal irrigation and disinfection protocols healing. While encouraging results have endodontics of the constraints imposed by will be paramount to successful treatment been observed with this approach, there is current diagnostic techniques.26,27 The lack of outcomes in regenerative endodontics and insufficient evidence to determine whether it robust diagnostic markers hampers treatment considerable scope remains for optimisation is better than other regenerative endodontic planning and can lead to treatment protocols of these protocols. approaches. This perhaps highlights the being more empirical and not exploiting The extent of surgical intervention during challenges of haemostasis during endodontic our biological understanding of the pulp canal preparation may also be a determinant procedures and the problems of controlling optimally. The majority of the clinical reports of treatment outcome in terms of preservation tissue events in a carefully defined manner of regenerative endodontics have focused of tissue vitality. Pulpotomy is commonly within revascularisation procedures. on treatment of immature teeth where used in the management of the infected stimulation of completion of root growth is pulp, although difficulties in the evaluation Regeneration or repair – semantics a particular benefit to the treatment approach. of the depth of penetration of infection and or clinical reality? Currently, the evidence base for use of inflammation can result in more radical Most case reports/case series in regenerative endodontics for permanent teeth surgery than necessary. The observation regenerative endodontics show examples of is very limited and conventional treatment that pulp is frequently only inflamed to a revascularisation of the pulp space where approaches may be preferable for such teeth depth of two mm37 led to the proposal of there is a pre-existing endodontic lesion. at this stage. limiting surgical intervention to a more A number of these reports demonstrate superficial partial pulpotomy.38 Adoption of radiographically completion of apexogenesis Infection and inflammation a more conservative approach to pulpotomy with increased root dentinal wall thickness control measures likely contributed to the successful outcomes and reduction in volume of the root canal The complexity of the pulpal microbiome28 reported recently for use of regenerative space implying treatment success. Where poses significant challenges to infection endodontics in a case-series.24 histological analysis of teeth has been possible

after treatment by a simple revascularisation advances being reported at the experimental 21. Botero T M, Son J S, Vodopyanov D, Hasegawa M, 40 Shelburne C E, Nör JE. MAPK signalling is required procedure or by application of platelet-rich level. Such approaches will likely target for LPS-induced VEGF in pulp stem cells. J Dent Res plasma,41 deposition of a mineralised layer recruitment of specific stem/progenitor 2010; 89: 264–269. on the radicular walls was observed, which cell populations and exploit endogenous 22. Trope J. Regenerative potential of dental pulp. J Endod 2008; 34: S13–17. appeared to be of periodontal rather than signalling molecules sequestered in dentine 23. Shimizu E, Ricucci D, Albert J et al. Clinical, pulpal origin. This suggests that the new to regenerate dentine-pulp tissue with radiographic, and histological observation of a tissue was not of dentinogenic origin and physiological characteristics. human immature permanent tooth with chronic apical abscess after revitalization treatment. J Endod emphasises the limitations of radiography 1. Hermann B. Ein weiterer Beitrag zur Frage der 2013; 39: 1078–1083. for characterising new mineralised tissue Pulpenbehandlung. Zahnärztl Rundsch 1928; 24. Simon S, Perard M, Charpentier E et al. Should formation in endodontics. Nevertheless, 37: 1327–1376. pulp chamber pulpotomy be seen as a permanent treatment? Preliminary findings of a pilot study.Int effective apical closure is a key goal of 2. Zander H A. Reaction of the pulp to calcium hydroxide. J Dent Res 1939; 18: 373–379. Endod J 2012; 46: 79–87. apexification techniques42 and whether 3. Nair P N, Duncan H F, Pitt Ford T R, Luder H U. 25. Diogenes A, Henry M A, Teixeira F B, Hargreaves K M. this is achieved with a dentinogenic or a Histological, ultrastructural and quantitative An update on clinical regenerative endodontics. investigations on the response of healthy human Endod Topics 2013; 28: 2–23. periodontal structure might be of lesser pulps to experimental capping with Mineral Trioxide 26. Mejàre I A, Axelsson S, Davidson T et al. Diagnosis importance Aggregate: a randomized controlled trial. Int Endod J of the condition of the dental pulp: a systematic review. 2012; 597–613. It is important, however, to return to the 2009; 42: 422–444. Int Endod J 45: 4. Zanini M, Sautier J M, Berdal A, Simon S. Biodentine 27. Petersson A, Axelsson S, Davidson T et al. objectives when trying to assess treatment induces immortalized murine pulp cell differentiation Radiological diagnosis of periapical bone tissue success. If the objective is to induce healing into odontoblast-like cells and stimulates lesions in endodontics: a systematic review. Int Endod J 2012; 45: 783–801. of the periapical tissues, stimulate bone biomineralization. J Endod 2012; 38: 1220–1226. 5. Hargreaves K M, Diogenes A, Teixeira F B. Treatment 28. Hsiao W W, Li K L, Liu Z, Jones C, Fraser-Liggett C M, regeneration, and render the patient free options: biological basis of regenerative endodontic Fouad A F. Microbial transformation from normal from any signs or symptoms, then current procedures. Paediatr Dent 2012; 35: 129–140. oral microbiota to acute endodontic infections. BMC Genomics 2012; 13: 345. regenerative treatments can be regarded 6. Smith A J, Lesot H. Induction and regulation of crown dentinogenesis – embryonic events as a 29. Haapasalo M, Shen Y, Qian W et al. Irrigation in a clinical success (although perhaps a template for dental tissue repair. Crit Rev Oral Biol endodontics. Dent Clin North Am 2010; 54: 291–312. biological failure). Filling the root canal Med 2001; 12: 425–437. 30. Smith A J, Smith G. Solubilisation of TGF‑B1 by 7. Smith A J, Cassidy N, Perry H, Begue-Kirn C, Ruch dentine conditioning agents. J Dent Res 1998; space with a vital biological tissue has J V, Lesot H. Reactionary dentinogenesis. Int J Dev 77: 1034. the significant advantage of providing an Biol 1995; 39: 273–280. 31. Zhao S, Sloan A J, Murray P E et al. Ultrastructural immuno-competent root canal filling with 8. Fitzgerald M, Chiego D J, Heys D R. Autoradiographic localisation of TGF‑b exposure in dentine analysis of odontoblast replacement following by chemical treatment. Histochem J 2000; defensive capabilities for any future bacterial pulp exposure in primate teeth. Arch Oral Biol 1990; 32: 489–494. exposure like normal pulp tissue However, 35: 707–715. 32. Galler K M, Hartgerink J D, Cavender A C, Schmalz G, if the objective is restitutio ad integrum, 9. Gronthos S, Brahim J, Li W et al. Stem cell properties D’Souza R N. A customized self-assembling peptide of human dental pulp stem cells. J Dent Res 2002; hydrogel for dental pulp tissue engineering. Tissue that is, to regenerate a physiological-like 81: 531–535. Eng Part A 2012; 18: 176–184. pulp tissue, then these current treatments 10. Sonoyama W, Liu Y, Yamaza T et al. Characterization 33. Ferracane J L, Cooper P R, Smith A J. Dentin Matrix Component Solubilization by Solutions of pH should be regarded as failures. Nevertheless, of the apical papilla and its residing stem cells from human immature permanent teeth: a pilot study. Relevant to Self-etching Dental Adhesives. J Adhes reports of some of the experimental studies J Endod 2008; 34: 166–171. Dent 2013; 15: 407–412. involving recruitment of stem/progenitor 11. Miura M, Gronthos S, Zhao M et al. SHED: stem cells 34. Murray P E, Smith A J, Garcia-Godoy F, Lumley P J. Comparison of operative procedure variables cells with dentinogenic potentiality to sites from human exfoliated deciduous teeth. Proc Natl Acad Sci USA 2003; 100: 5807–5812. on pulpal viability in an ex vivo model. Int Endod J of pulp injury suggest that true pulp-dentine 12. Iohara K, Zheng L, Wake H et al. A novel stem cell 2008; 41: 389–400. regeneration may be a clinical reality in the source for vasculogenesis in ischemia: subfraction 35. Graham L, Cooper P R, Cassidy N, Nor J E, Sloan of side population cells from dental pulp. Stem Cells A J, Smith A J. The effect of calcium hydroxide future. 2008; 26: 2408–2418. on solubilisation of bi-active dentin matrix 13. Feng J, Mantesso A, De Bari C, Nishiyama A, components. Biomater 2006; 27: 2865–2873. CONCLUSIONS Sharpe P T. Dual origin of mesenchymal stem cells 36. Tomson P L, Grover L M, Lumley P J, Sloan A J, Smith contributing to organ growth and repair. Proc Natl A J, Cooper P R. Dissolution of bio-active dentine Regenerative endodontics offers a number Acad Sci USA 2011; 108: 6503–6508. matrix components by mineral trioxide aggregate. of exciting opportunities for preservation of 14. Smith J G, Smith A J, Shelton R M, Cooper P R. J Dent 2007; 35: 636–642. pulp vitality following episodes of trauma Recruitment of dental pulp cells by dentine and 37. Mjor I A, Tronstad L. Experimentally induced pulpitis. pulp extracellular matrix components. Exper Cell Res Oral Surg Oral Med Oral Path Oral Radiol Endod and disease and the many biological advances 2012; 318: 2397–2406. 1972; 34: 102–108. have helped to underpin the development of 15. Kim J Y, Xin X, Moioli E K et al. Regeneration of 38. Cvek M. A clinical report on partial pulpotomy and capping with calcium hydroxide in permanent this approach. From a semantics viewpoint, dental‑pulp‑like tissue by chemotaxis-induced cell homing. Tissue Eng Part A 2010; 16: 3023–3031. incisors with complicated crown fracture. J Endod regenerative endodontics as it is currently 16. Smith A J, Scheven B A, Takahashi Y, Ferracane J, 1978; 4: 232–237. performed represents more of a reparative Shelton R M, Cooper P R. Dentine as a bioactive 39. Cordeiro M M, Dong Z, Kaneko T et al. Dental pulp extracellular matrix. Arch Oral Biol 2012; 57: tissue engineering with stem cells from exfoliated than regenerative therapeutic strategy. 109–121. deciduous teeth. J Endod 2008; 34: 962–969. Treatment outcomes from techniques, such 17. Anneroth G, Bang G. The effect of allogeneic 40. Shimizu E, Jong G, Partridge N, Rosenberg P A, Lin as revascularisation procedures, generally demineralized dentin as a pulp capping agent in L M. Histologic observation of a human immature Java monkeys. Odontol Revy 1972; 23: 315–328. permanent tooth with irreversible pulpitis after give rise to a vital biological tissue, albeit 18. McCabe P S, Dummer P M. Pulp canal obliteration: revascularization/regeneration procedure. J Endod not necessarily representative of the an endodontic diagnosis and treatment challenge. 2012; 38: 1293–1297. physiological structure of pulp. Despite these Int Endod J 2012; 45: 177–197. 41. Martin G, Ricucci D, Gibbs J L, Lin L M. Histological 19. Simon S, Smith A J, Lumley P J et al. Molecular findings of revascularized/revitalized immature current limitations, such treatment strategies characterization of young and mature odontoblasts. permanent molar with apical periodontitis using still offer significant clinical benefits, Bone 2009; 45: 693–703. platelet-rich plasma. J Endod 2013; 39: 138–144. especially for immature teeth. True pulp 20. Simon S, Smith A J, Berdal A, Lumley P J, Cooper 42. Simon S, Rilliard F, Berdal A, Machtou P. The use of P R. The MAPK pathway is involved in odontoblast mineral trioxide aggregate in one-visit apexification regeneration will emerge as a viable clinical stimulation via p38 phosphorylation. J Endod 2010; treatment: a prospective study. Int Endod J 2007; treatment strategy from the many recent 36: 256–259. 40: 186–197.