|Year : 2014 | Volume
| Issue : 2 | Page : 112-117
Regenerative dentistry: Current and future perspectives to rejuvenate and reclaim dental tissues
Sourabh Jagannath Torvi, Kala Munniswamy
Department of Conservative Dentistry and Endodontics, Government Dental College and Research Institute, Bengaluru, Karnataka, India
|Date of Web Publication||28-Oct-2014|
Sourabh Jagannath Torvi
No. 122, 3B Cross, 6A Main Road, 7th Block, 2nd Stage, Nagarbhavi, Bengaluru - 560 072, Karnataka
Source of Support: None, Conflict of Interest: None
| Abstract|| |
While widespread advances have occurred in all details of science over the past decade, regenerative dentistry has also seen its part of breakthrough innovations. Tooth regeneration offers new and innovative approaches to common problems encountered in oral and dental sciences. In cases where a tooth is lost, it may be replaced with an implant, bridge, or a denture capable of mastication. However, in many developing countries, it is often simpler (and far more cost-effective) to remove the tooth. Strategies based upon regenerative medicine that facilitates the repair or replacement of damaged teeth may hold particular promise as a means to reduce the cost of dental care. Dental maladies aside, the tooth is also a compelling candidate as a template for organogenesis which could have far-reaching implications for the field of regenerative medicine.  A systematic review of the literature was performed using various internet-based search engines (PubMed, Medline Plus, Cochrane, Medknow, Ebsco, Science Direct, Hinari, WebMD, IndMed, and Embase) using keywords such as "dental pulp stem cells," "regeneration," "medical applications," and "tissue engineering." This review explores existing and visionary approaches in the revolutionary field of regenerative dentistry, as an extension to the familiar concepts of regenerative medicine.
Keywords: Gene therapy, growth factors, regenerative, scaffold, stem cells, tissue engineering
|How to cite this article:|
Torvi SJ, Munniswamy K. Regenerative dentistry: Current and future perspectives to rejuvenate and reclaim dental tissues
. J Int Clin Dent Res Organ 2014;6:112-7
|How to cite this URL:|
Torvi SJ, Munniswamy K. Regenerative dentistry: Current and future perspectives to rejuvenate and reclaim dental tissues
. J Int Clin Dent Res Organ [serial online] 2014 [cited 2019 May 20];6:112-7. Available from: http://www.jicdro.org/text.asp?2014/6/2/112/143496
| Introduction|| |
A tooth is a complex biological organ which consists of multiple tissues including the enamel, dentin, cementum, and pulp, encased in a biological socket comprising the periodontium, which includes the periodontal ligament (PDL), bone and gingiva which continues as the oral mucous membrane. Dental caries is one of the most common disorders in humans, second only to common cold  and tooth loss is the most common organ failure, affecting a vast majority of the population. Conventional dental treatment modalities range from simple restorations of decayed teeth, tooth devitalization and disinfection (root canal therapy), extractions and replacement by an implant supported prosthesis. Dental implants, despite being the treatment in vogue for a lost tooth might not perform as well as a natural tooth would in relation to its surrounding tissues and its function.
A systematic review of the literature was performed using various internet based search engines (PubMed, Medline Plus, Cochrane, Medknow, Ebsco, Science Direct, Hinari, WebMD, IndMed, and Embase) using keywords such as "dental pulp stem cells," "regeneration," "medical applications," "tissue engineering." The current concepts of regenerative dentistry are to create biological alternatives to root canal treatments, regeneration of dental hard tissues, revascularization, and regrowth of lost periodontal tissues. In a recently conducted survey among clinicians, 96% of participants thought that more regenerative therapies should be incorporated into treatments.  The magnitude of clinical success of the above-mentioned concepts is debatable due to the fact that the clinical studies conducted are either in a controlled environment or due to the lack of long-term observations. It might also be difficult to estimate the general outcome and usefulness of the procedure due to its inherent scarcity of the number of cases and its practicality. In spite of these limitations, regenerative dentistry has seen its fair share of successful innovative technologies through a combined multidisciplinary effort offering a wealth of opportunities, paving the way for superior treatment techniques which are closer to the natural.
| Available technologies in regenerative dentistry|| |
Due to the advancements in the field of medicine and biotechnology, a plethora of technologies have been made available. Development has been made possible only due to the extension from the most basic and commonly followed treatment modalities. Science is constantly updated, what is considered as the treatment of the present age becomes obsolete in a few years. Regenerative technology was considered a thing of the distant future, but we can already see numerous regenerative procedures being performed on a day to day basis. Some of them are as follows.
| Conventional techniques for regeneration of dental tissues|| |
Vital pulp therapy
It is now accepted that a highly proliferative and clonogenic population of progenitor/stem cells resides within the postnatal dental pulp and can differentiate into hard tissue-forming cells. Hence, damaged odontoblasts can be replaced by newly regenerated population of odontoblast-like cells derived from those progenitor/stem cells that remain in the healthy portion of the pulp. ,,,, This principle is put into practice by procedures like indirect and direct pulp capping. However, pulp capping in carious teeth has been considered unpredictable, and therefore, must be limited to a few well-selected cases.  The use of various cement-based compounds such as calcium hydroxide [Ca(OH) 2 ] and mineral trioxide aggregate, are believed to promote the activity of dentinogenesis.  Advances in the understanding of the molecular and cellular mechanisms that regulate dentinogenesis will open directions to design new regenerative methods of dental treatment.
Introduction of bleeding, formation of the blood clot
The induction of pulp tissue regeneration in the pulp space has been a long-standing quest, and earlier efforts were mainly focused on the introduction of bleeding and the formation of a blood clot in the canal space of mature teeth, with the hope of guiding the tissue repair in the canal. The resultant tissues were observed to be the granulation or fibrous tissues instead of pulp, and in some cases the ingrowth of cementum and bone occurred.  Similar efforts were conducted by Myers and Fountain (1974) in a primate study using the blood clot as a scaffold, with the aim of regenerating lost pulp tissue.
Localized delivery of growth factors
The use of recombinant growth factors for dentinal and pulpal regeneration has been widely investigated, ,, however, the data available are still often controversial. The localized delivery of growth factors for regenerative therapy today is hampered by the need for a safe and efficient delivery vehicle that can provide sustained therapeutic action without cytotoxicity or unwanted side effects. ,, Periodontal therapy using a formation of platelet-rich plasma (PRP) has been suggested as a modality to enhance the outcome of regenerative surgery, which points to a great interest in the use of PRP for potential pulp regeneration.  In vitro studies suggest that growth factors released by platelets recruit reparative cells and may augment soft-tissue repair. Unfortunately, there is no substantial evidence that the use of endogenous growth factors represents a superior alternative to potential pulp repair. While a sound biological rationale and a multitude of basic science research support the use of PRP to promote pulp tissue healing.
Conservative treatment approaches to pulp regeneration require well-selected cases in distinct clinical situations to ensure their clinical outcomes as further extensive studies need to be conducted in this area.
| Tissue engineering and regenerative medicine|| |
Tissue engineering is the field of the functional restoration of the tissue structure and the physiology for impaired or damaged tissues due to cancer, disease, and trauma.  Tissue engineering promises a wide array of solutions to challenging clinical problems in dentistry that have not been adequately addressed due to the use of conventional techniques.
Tissue engineering is generally considered to consist of three key elements [Figure 1].
Stem cells/progenitor cells
Stem cell biology has become an important field for the understanding of tissue regeneration, although much knowledge in this area has been from the in vitro studies. A stem cell is commonly defined as a cell that has the ability to continuously divide and produce progeny cells that differentiate (develop) into various other types of cells or tissues.  In general, stem cells are defined by having two major properties, cells which are capable of self-renewal and when cells divide, some daughter cells give rise to cells that eventually become differentiated cells. Depending on the type of stem cells and their ability and potency to become different tissues, the following categories of stem cells have been established:
- Totipotent stem cells: Each cell is capable of developing into an entire organism
- Pluripotent stem cells: Cells from embryos (embryonic stem cells) that when grown in the right environment in vivo are capable of forming all types of tissues; and
- Multipotent stem cells: Postnatal stem cells or commonly called adult stem cells that are capable of giving rise to multiple lineages of cells. Dental stem cells belong to this category.
Some of the suggested candidates for stem cell therapy are: 
- Dental pulp stem cells (DPSCs).
- Stem cells from exfoliated deciduous teeth (SHED).
- Stem cells from apical papilla.
- Side-population cells (from the pulp).
- Bone marrow mesenchymal stem cells.
Scaffolds or extracellular matrix
To create a more practical tissue engineering therapy, stem cells must be organized into a three-dimensional structure that can support cell organization and vascularization. This can be accomplished using a scaffold seeded with stem cells. A scaffold should contain growth factors to aid stem cell proliferation and differentiation, leading to improved and faster tissue Development. In addition, the scaffold may exert essential mechanical and biological functions needed by replacement tissue to achieve the goal of pulp tissue reconstruction. Therefore, scaffolds must meet some specific requirements which are discussed later in this section. The types of scaffold materials available are natural or synthetic, biodegradable, or permanent.
- Synthetic materials.
- Polylactic acid.
- Polyglycilic acid.
- Tricalcium phosphate.
- Synthetic hydroxyapatite.
- Injectable hydrogels - Poly(ethylene glycol).
- Natural materials.
- Chitosan (poly-N-acetyl glycosaminoglycans).
Biological requirements of a scaffold: 
- A scaffold should contain growth factors to aid stem cell proliferation and differentiation, leading to improved and faster tissue development.
- A scaffold should be effective for transport of nutrients, oxygen, and waste.
- A high porosity and an adequate pore size are necessary to facilitate cell seeding and diffusion throughout the whole structure of both cells and nutrients.
- The scaffold should be able to provide structural integrity within the body, and it should eventually break down, leaving the newly formed tissue that will take over the mechanical load.
- The rate at which degradation occurs has to coincide as much as possible with the rate of tissue formation.
Morphogens (signaling molecules)/growth factors
Growth factors are biological modulators that are able to promote cell proliferation and differentiation.  Growth factors are extracellular matrix molecules which are involved in signaling and regulating dentogenic events during tooth development, application of these exogenous signaling factors has been recommended for regenerative therapies although a number of challenges in the methods of delivery should be addressed before they are to be used in regular clinical practice. 
Rutherford et al. pioneered the use of growth factors (human cloned bioactive osteogenic protein-1) with a carrier matrix of purified bovine type-1 collagen powder, moistened with sterile saline, for inducing reparative dentine formation in monkeys. 
Some of the naturally occurring and commercially available osteogenically active growth factors are:
- Bone morphogenic/morphogenetic protein (BMP).
- Platelet-derived growth factor.
- Insulin-like growth factor.
- Fibroblast growth factor.
- Transforming growth factor-β.
- Dentine sialoprotein.
- Dentine phosphoprotein.
- Enamel Matrix Derivative (Emdogain® , Biora Inc., Chaska, MN, USA).
| Clinical applications of tissue engineering|| |
There are several areas of research for which dental stem cells are presently considered to offer potential for tissue regeneration. These include the obvious uses of cells to repair damaged tooth tissues such as dentin, PDL, and dental pulp.  Dental stem cells are also being put into use in the field of regenerative medicine to facilitate repair of tissues such as bone and nerves.  Tissue engineering in regenerative endodontics represents a new treatment modality that focuses on reestablishment of pulp vitality and continued root development.
Tissue engineering can be utilized in dentistry for in situ partial pulp regeneration, de novo pulp replacement, regeneration or replacement of mineralized tissues such as dentin, enamel, bone, and cementum, and regeneration of the periodontium. The pulp tissue repair/regeneration recapitulates tooth development and points to the brighter possibilities in the future.
In situ partial pulp regeneration
Current evidence suggests that the remaining healthy portion of the pulp could be recoverable before the final stages and may have the potential to regenerate the lost portion under certain conditions. To enhance this regeneration, inductive medical devices or engineered pulp constructs based on DPSCs may be inserted into the pulp space to facilitate the total recovery of pulp tissue and the generation of new dentin. 
De novo synthesis of a pulp replacement
Before an entire pulp tissue can be created, the major goal of a pulp tissue-engineering effort is to harness the pulp's own capacity to regenerate functionally active pulp tissue that physiologically responds to metabolic cues. When the entire pulp tissue is lost, de novo synthesis of a pulp must take place to regenerate the tissue. Generation of well-vascularized pulp-like tissue using a tooth slice model has been reported recently by several groups who are using SHED or DPSCs. 
| Nanoscale and microscale technologies for dental tissue engineering and regeneration|| |
While widespread advances in tissue engineering have occurred over the past decade, many challenges remain in the context of tissue engineering and regeneration of the tooth. In this regard, microscale approaches that spatially pattern and support the development of different cell types in close proximity can be used to regulate the cellular microenvironment and, as such, are promising approaches for tooth development. Microscale and nanotechnologies also present alternatives to conventional tissue engineering approaches in terms of scaffolds and the ability to direct stem cells. Furthermore, these techniques can be used to miniaturize many in vitro techniques and to facilitate high-throughput experimentation.
Techniques commonly used in the microelectronics and semiconductor industries to fabricate miniaturized structures are being increasingly utilized to study cellular events and interactions, as well as to generate scaffolds and cell environments with micron and nanoscale resolution.  The development of microengineered scaffolds with patterns of progenitor cells of dental-specific tissue types, growth factors, and cues to direct cell behavior, supported by a controlled microvasculature may also offer more rapid and robust methods for the generation of teeth in vitro.
The application of microscale and nanoscale technologies will likely help to advance the technology and knowledge associated with dental tissue regeneration. These technologies are likely to advance scaffold development and increase stem cell sources for dental tissue regeneration. Microscale scaffolds with controlled properties and architecture may facilitate the generation of complex, cell-laden, load-bearing vascularized scaffolds for hard tissue regeneration and the neo-vascularization essential for the in vitro development of a tooth.
| Gene therapy|| |
The year 2003 marked a major milestone in the realm of genetics and molecular biology. That year marked the 50th anniversary of the discovery of the double-helical structure of DNA by Watson and Crick. On April 14, 2003, 20 sequencing centers in five different countries declared the human genome project complete. This milestone will make possible new medical treatments involving gene therapy. All human cells contain a 1-m strand of DNA containing 3 billion base pairs, with the sole exception of nonnucleated cells like red blood cells. The DNA contains genetic sequences (genes) that control cell activity and function; one of the most well-known genes is p53.  New techniques involving viral or nonviral vectors can deliver genes for growth factors, morphogens, transcription factors, and extracellular matrix molecules into target cell populations. Delivery is brought about by viral or nonviral vectors:
- Viral vectors: Retroviruses, adenovirus, adeno-associated virus, herpes simplex virus, and lentivirus.
- Nonviral vectors: Plasmids, peptides, gene guns, DNA-ligand complexes, electroporation, sonoporation, and cationic liposomes.
One use of gene delivery in endodontics would be to deliver mineralizing genes into pulp tissue to promote tissue mineralization. However, a literature search indicates there has been little or no research in this field, except for the work of Rutherford.  He transfected ferret pulps with cDNA-transfected mouse BMP-7 that failed to produce a reparative response, suggesting that further research is needed to optimize the potential of pulp gene therapy. Gene therapy is a relatively new field, and evidence lacks to demonstrate that this therapy has the potential to rescue necrotic pulp. At this point of time, the potential benefits and disadvantages are largely theoretical.
| Regenerative Dentistry: Barriers and Future Directions|| |
Tooth regeneration by cell transplantation is a meritorious approach. However, there are hurdles in the translation of cell-delivery-based tooth regeneration into therapeutics. The most important one of these difficulties is inaccessibility of autologous embryonic tooth germ cells for human applications. Xenogenic embryonic tooth germ cells (from nonhuman species) may elicit immune rejection and tooth dysmorphogenesis. Autologous postnatal tooth germ cells (e.g., third molars) or autologous DPSCs are of limited availability and remain uncertain as a cell source to regenerate an entire tooth. Regardless of cell source, cell-delivery approaches for tooth regeneration, similar to cell-based therapies for other tissues, encounter translational barriers. The costs of the commercialization process and difficulties in regulatory approval in association with ex vivo cell manipulation have precluded any significant clinical translation effort to date in tooth regeneration. As in tissue engineering of other biological structures, regeneration of an entire tooth or various tooth and associated structures, including the enamel, dentin, cementum, dental pulp, and the periodontium, by cell transplantation encounters a number of scientific, translational, and regulatory difficulties.
The near future
Restoring lost tooth tissue is a technology that will be studied for many years to come. Although tooth transplantation and dental implants have existed for many years, they have never been totally satisfactory. With a renewed focus, regenerative endodontics is going to have a significant effect on the clinical practices of dentistry and regenerative medicine.
Technologies which may be practical for clinical application in the near future:
- Establishment of stem cell banks which are readily accessible can make stem cell therapy a clinical probability.
- Procedures involving biological pulp implants and the respective implant storage banks.
- Mainstream tissue engineering specifically related to tissue regeneration in dentistry. This may involve cell printing and assembly on biological templates.
- Advanced and practical scaffold and growth factor delivery techniques.
- Amenable advances in biological microscale and nanoscale technologies.
The distant future
The next generation of regenerative treatment techniques may involve the synthesis and assembly of bio proteins by the nanorobots. Where, these entities are simply injected to the desired location, and they weave up the collagen framework onto which the proteins are assembled, also are the possibilities where dental tissues are grown to specific requirements and transplanted on a regular basis. The possibilities are infinite and coveting. These ground breaking strategies may provide an innovative and novel biology-based new generation of clinical treatments for dental disease.
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