Dental Stem Cells: The Future Of Regenerative Dentistry

INTRODUCTION

The term “stem cell” has roots as far back as 1868.¹ By definition; a stem cell is capable of self-renewal, differentiationinto at least one cell type, and functional reconstitution ofthe tissue of origin.²

When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell. Given their unique regenerative abilities, stem cells offer new potentials for treating diseases such as diabetes, and heart disease and tooth regeneration. Similar to umbilical cord blood, teeth are one of the few tissues in the body that are naturally shed (or extracted during the normal course of dental care) that contain multi-potent mesenchymal stem cells, creating an opportunity to save this tissue for when it is needed. ³

Over the last few years, dentistry has begun to explore the potential application of stem cells and tissue engineering towards the repair and regeneration of dental structures. It is becoming increasingly clearer that this conceptual approach to therapy, named “regenerative dentistry”, will have its place in the clinical practice of dentistry in the future. ⁴This review is aimed to give an overview on recent knowledge about dental stem cells for their prospective use in regenerative dentistry and medicine.

Classification of stem cells

According to Alp Can, based upon their origin or location, the stem cells are classified in two categories: Embryonic stem cells (ESCs) and Adult stem cells or Tissue-specific stem cells (TSSCs) and a functional classification based on their developmental potential as totipotent, pluripotent, multipotent, and unipotent.Embryonic stem cells are pluripotential; they can differentiate into all somatic cell types as well as germ cells when injected into a blastocyst, and form mature progeny of all three embryonic germ layers in vitro. Adult stem cells present in developmental stages beyond the embryo, They are undifferentiatedcell found among differentiated cells in a tissue or organ, can renew itself and can differentiate to yield the major specialized cell types of the tissue or organ.²

A totipotent stem cell can give rise to a new individual if provided with appropriate maternal support. They have the ability to give rise to all the cell types of the body plus all of the cell types that make up the extra embryonic tissues such as the placenta. A pluripotent stem cell can give rise to all cell types of the embryo proper, including somatic and germ cells. Pluripotent cells cannot make extra-embryonic tissues such as the amnion, chorion, and other components of the placenta. Multipotent stem cells have the potential to differentiate into different types of specialized cells constituting a specific tissue or organ e.g. dental stem cells. A unipotent stem cell can contribute only to one mature cell type e.g. myosatellite cells of muscle.²

Sources of adult stem cellsortissue specific stem cells (TSSCS)

Endo-dermal Origin: Pulmonary Epithelial SCs. Gastrointestinal Tract SCs, Pancreatic SCs, Hepatic Oval Cells, Mammary and Prostatic Gland SCs, Ovarian and Testicular Scs.

Meso-dermal Origin: Hematopoietic SCs, Mesenchymal SCs. (E.g. Dental Stem Cells), Satellite cells of muscle . Ecto-dermal Origin:Neural SCs, Skin SCs, Ocular SCs.²

DENTALSTEM CELLS

Stem cells from Human Exfoliated Deciduous teeth (SHEDs): Recent findings demonstrated the isolation of mesenchymal progenitors from the pulp of human deciduous incisors. These cells were named SHEDs (Stem cells from Human Exfoliated Deciduous teeth). They express the mesenchymal stem cell marker Stro-1, CD146, CD44 and CD73 and exhibited multi-potentiality, since they could differentiate into odontoblast, osteoblast, chondrocyte, myocyte, neurocyte, adipocyte and induced pluripotent stem cells. They have a potential contribution in bone regeneration and dentin and pulp regeneration in bio-tooth formation.⁵

Dental Pulp Stem Cells (DPSCs) have been isolated from pulp tissues in different types of teeth including exfoliated primary incisors, permanent teeth, natal teeth, and even supernumerary teeth. Dental Pulp Stem cells are thought to reside in a perivascular niche, but little is known on the exact location and molecular regulation of this niche. They express the mesenchymal stem cell marker Stro-1, CD29, CD106 CD105, CD146, CD44 and CD73 and have multi-potentiality, being capable of differentiating into odontoblast, osteoblast, chondrocyte, myocyte, neurocyte, adipocyte, corneal epithelial cell, melanoma cell and induced pluripotent stem cells.They have a potential contribution in systemic diseases for bone regeneration, central nervous degeneration, liver fibrosis, myocardial infarction, corneal recon struction and potential contributions inbio-tooth regeneration (dentin and pulp regeneration).^6-12

Periodontal ligament stem cells (PDLSCs): They express stem cell surface markers STRO-1, CD146, CD73, CD166, CD106 and CD105 and maintain certain plasticity since they can differentiate into adipogenic, osteogenic and chondrogenic phenotypes in vitro.¹⁴ It is thus obvious that PDLitself contains progenitors, which can be activated to self-renew and regenerate other tissues such as PDL, cementum and alveolar bone.¹⁵

Stem cells from the apical papilla (SCAPs): A new class of dental stem cells was isolated from the apical papilla of wisdom teeth (SCAP, stem cells from apical papilla). The dental papilla is an embryonic-like tissue that becomes dental pulp during maturation and formation of the crown. Therefore, SCAPs can only be isolated at a certain stage of tooth development. They express stem cell surface markers STRO-1, CD146, CD73, CD24, CD90 and CD29.These stem cells have multipotentiality, beingcapable of differentiating into odontoblast, osteoblast, neurocyte, adipocyte and induced pluripotent stem cells. They have a potential contribution in bone regeneration and dentin, pulp and root regeneration in tooth formation.^16,17 

Stem cells from the dental follicle (DFPCs) have been isolated from follicle of human third molars and express the stem cell markers Notch1, STRO-1 andmnestin. These cells can differentiate into cementoblasts in vitro and are able to form cementum in vivo.^18-20 

Epithelial stem cells from developing molars (EpSC). Several studies describe the use of EpSC isolated from newborn or juvenile animals, usually from third molar teeth. In these studies, epithelia were removed and cells dissociated enzymatically. Precursors were then amplified and associated with MSC (originated from the same tooth) in vitro in contact with biomaterials such as collagen sponges or synthetic polymers. These approaches are promising for tooth formation and/or regeneration. However, its clinical application is difficult, if not unrealistic, since it would require the donation of a tooth germ from children. The use of autologous stem cells is desirable but raises the question of a good and reliable source. ^21-23

Various methods have been developed to isolate stem cells from dental origin, the conventional ones being: Size-sieved Isolation, Stem Cell Colony Cultivation, Magnetic Activated Cell Sorting and Fluorescence Activated Cell Sorting.⁶

Clinical Applications of Dental Stem Cells

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.In this regard, the tooth is well-suited for the study of organogenesis, because it is easily accessible and easily monitored, and tooth failure is not life-threatening.The discovery of dental stem cells and recent advances in cellular and molecular biology have led to the development of novel therapeutic strategies that aim at the regeneration of tissues that were injured by disease or trauma.^24,25

Regenerative dentistry seek to replace lost or damaged tissues due to any reason, and this needs three major ingredients which are:^26,27

1- Morphogenic signals such as growth factors and differentiation factors; these factors play an important role in the multiplication and differentiation of stem cells into the specifically needed type of cells. BMPs (bone morphogenic proteins) and cytokines play a major role in organogenesis, and in the dental aspect specifically GDf-11 (growth/differentiation factor 11) which is a novel member of BMP/TGF B family is expressed in differentiating odontoblasts and plays a major role in differentiation of dental pulp stem cells into odontoblasts which is the corner stone in teeth tissue engineering.

2- Responding stem cells which are originally harvested from the patient and preserved under good conditions to maintain their special ability to differentiate into a wide range of cells.

3- Scaffold materials are three-dimensional tissue structures that guide the organization, growth and differentiation of cells. Scaffolds must be biocompatible and designed to meet both nutritional and biological needs for the specific cell population. Example: Scaffold of extra cellular matrix, which provide these stem cells with the environment for cell adhesion, growth, and differentiation within the construct and mold to grow into what we want them to become and function.

Dental stem cells for regenerative dentistry

Regenerative dentistry is based in part on the concept that stem cells will become increasingly useful in dentistry; strategies include using the patient's stem cells in situ to stimulate healing, or the use of laboratory-cultured stem cells with or  without tissue engineering scaffolds.^27,28 

Tooth Regeneration: Tooth loss or absence is a common and frequent situation that can result from numerous pathologies such as periodontal and carious diseases, fractures, injuries or even genetic alterations. In most cases this loss is not critical, but for aesthetical and psychological reasons, replacement of the missing teeth is important. Recent efforts made in the field of biomaterials have led to the development of dental implants composed of biocompatible materials such as titanium that can be inserted in the maxillary and/or mandibular bone to replace the missing teeth. However, implants are still not completely satisfactory and their successful use greatly depends on many factors such as osteointegration, quality and quantity of bone. To overcome these difficulties, stem cell technology and tissue engineering have emerged.Since teeth are formed from two different tissues, building a tooth logically requires the association/cooperation of odontogenic mesenchymal and epithelial cells. The recombination of dissociated dental epithelial and mesenchymal tissues leads to tooth formation both in vitro and in vivo. In a study by Nakao et al., epithelial and mesenchymal cells were sequentially seeded into a collagen gel drop and then implanted into the tooth cavity of adult mice. With this technique the presence of all dental structures such as odontoblasts, ameloblasts, dental pulp, blood vessels, crown, periodontal ligament, root and alveolar bone could be observed. Thus, the implantation of these tooth germs in the mandible allowed their development, maturation and eruption indicating that stem cells could be used in the future for the replacement of missing teeth in humans.^28-32 

Periodontium Regeneration: PDL stem cells differentiate into cells or tissues very similar to the periodontium. Recently, PDLstem cells have also been isolated from sheep and pigs. It has been shown that a functional periodontium could successfully be established using PDL stem cells. Feng, et al demonstrated clinical and experimental evidence supporting the safe and efficacious use of autologous PDL cells to treat periodontitis in 3 humans in a multiyear study.^33-34

Alveolar Bone Regeneration: dental stem cells (DPSCs, SHEDs and SCAPs) were used in humans to regenerate alveolar bone in human clinical studies. Defects of at least 1.5 cm in the alveolar ridge of human volunteers were filled with a construct of stem cells and seeded onto a collagen matrix. One year later in many cases, the gap was  filled with regenerated bone.^{35, 36}

Sinus Lift Augmentation: Bone augmentation to reconstruct atrophic jaws provides the base for sufficient functional and aesthetic implantsupported oral rehabilitation. Stem cells derived from dental pulp poured onto collagen sponge is a useful method for bone regeneration in atrophic maxilla.^{37, 38} 

Dental stem cells for regenerative medicine:The mesenchymal stem cells found in teeth may be beneficial for the treatment of neurodegenerative diseases and the repair of motor nerves following stroke or injury. This exciting research will lead to future treatment options that allow muscles to repair themselves following injury, such as the muscle damage that occurs after a heart attack, or the structural damage that occurs following a knee injury.

Liver Disease: Stem cells from third molars were differentiated into hepatocytes in cell culture, and in an animal model of liver disease, they prevented liver fibrosis and increased levels of albumin and  billirubin.³⁸  Myocardial Infarction (heart attacks): Human dental stem cells injected intramyocardially into a rat model of acute myocardial infarction showed an increase in angiogenesis, improvement in cardiac function, and a reduction in infarct size.³⁹ Diabetes : Dental stem cells have been shown to produce insulin and to modulate the immune system by suppressing Tcell response in laboratory and animal testing. Dental stem cells therefore represent an easily available source of stem cells for potentially both of  these therapeutic approaches to diabetes.^40,41 Spinal Cord Injury and Other Neurological Diseases/ Disorders: The dental pulp originates from the neural crest, a structure in the developing animal that gives rise to the autonomic nervous system including the spinal cord. Neurons have been generated from dental stem cells. Altogether, the ability of the dental pulp to mediate the inflammatory process, direct axon growth, and even provide a source of neurons for cell therapy suggest that dental stem cells could play an important role in treating human spinal cord injury, as well as neurodegenerative diseases such as  Parkinson's disease. ^42-45

The New Role of Dentist

New developments in stem cell research have created an environment in which dentists will be in the position to assume a leading role in the treatment chain of medical disease. Hence, both dentists and patients need to be aware that the option exists to preserve the stem cells from healthy extracted or exfoliating teeth as a resource for future clinical applications. ^46,47

Dental Stem Cell Banking

1. Patient education: The dentist will have a dual role to play in dental stem cell preservation: educating patients and/or their parents about the existence and use of dental stem cells, and helping them select an appropriate approach to collecting and preserving these stem cells. 2. Tooth collection: Place the tooth (extracted/exfoliated) in sterile saline solution or fresh milk along with frozen gel packs in transport kit. 3. Sample transport: the sample should be delivered to the stem cell bank at the earliest. 4. Isolation of stem cells: dental stem cells are isolated under stringent protocol. 5. Storage of stem cells: Cryopreservation: Cells are preserved by cooling them to subz e ro temperatures, at which biological activity is stopped. The cells are preserved in a liquid nitrogen vapor at a temperature of less than −150° C. Magnetic freezing: In this technique stem cells are completely chilled below the freezing point, by  using a magnetic field, without freezing.^46-49

CONCLUSION

In conclusion, recent findings clearly indicate that the control of morphogenesis and cytodifferentiation is a challenge that necessitates a thorough understanding of the cellular and molecular events involved in development, repair and regeneration of teeth. The stem cell regenerative dentistry requires complex and multidisciplinary approach. Progress will depend on the collaboration between clinicians and researchers working together toward the goal of developing biological approaches to regenerate dental tissues.