Abstract
Since the identification of mesenchymal stem cells, stem cell biology is a greatly researched field of regenerative medicine and tissue engineering therapies and has become an essential part of dentistry. Mesenchymal stem cells are multipotent stem cells that can differentiate into many cell types. Dental mesenchymal stem cell populations have been identified in dental pulp, human exfoliated deciduous teeth, periodontal ligament, dental follicle of third molars, tooth germ of third molars, gingiva of periodontium, alveolar bone, and apical papilla. Dental stem cells are the most natural, noninvasive source of stem cells that have been identified, and they have gained recent attention due to their accessibility and the associated relatively low cost of integration into regenerative therapy. Long-term preservation of dental stem cells is becoming a popular consideration and mirrors the ideology of banking umbilical cord blood. This review outlines the recent progress in the mesenchymal stem cells used in dentistry as well as some advancements that are being made in preserving dental stem cells for future personalized medicine. The aim of this study was to completely and concisely review the current use of adult dental stem cells specifically oral sources of stem cells, banking of dental stem cells, and applications or uses of dental stem cells specifically in oral regions and in a clinical setting.
Keywords
- dental mesenchymal stem cells
- stem cell banking
- regenerative dentistry
1. Introduction
There exists an association between oral health and overall health throughout the lifetime of an individual. Because of this association, advancements are being made in dental practice to maintain teeth and orofacial bone as a way to combat general decline in oral health. Current common practice includes the use of artificial materials in dental implant procedures and other prosthodontic therapies. While often effective, these methods frequently require costly and invasive procedures and may result in failure or rejection. Recent research has given much attention to bettering therapeutic treatment and regenerative medicine associated with maintaining oral health. At the forefront of this research is investigating the use of stem cells in tissue regeneration, specifically in dental practices [1]. The use of stem cells in dentistry focuses on making the transition from using artificial materials in treatments to using individualized biological materials resulting in greater personalized medicine. The oral cavity has many locations from which to obtain stem cells, and stem cells can be harvested throughout many stages in an individual’s life. Stem cell preservation through banking must be considered as a preventative way to obtain he biological materials necessary to make personalized medicine a common practice.
Adult stem cells, specifically mesenchymal stem cells (MSCs), can be loosely defined as immature multipotent cells that mature and multiply, and turn into many and more specific types of cells and tissues through the process of differentiation. The MSCs can be first derived from neural crest cells and pluripotent progenitor cells both of which develop during embryonic and child development [2]. A cell must be able to self-replicate and differentiate into two or more cell types to be considered MSCs [3]. Stem cells are the greatest source for means of regenerative medicine because they are autologous to the native tissue with low tumorigenesis risk and unlikely immune rejection [4]. Tooth derived MSCs, from both primary and permanent teeth, are of great interest regarding regenerative medicine because of how accessible they are due to the minimally invasive means through which these stem cells are obtained.
Stem cell use, specifically in dentistry, is on the forefront of research and clinical application because there are many sources of stem cells in the oral cavity. Treatment of a number of dental, oral, and craniofacial diseases using stem cells is currently being explored. Tooth-derived MSCs have great capability to proliferate and differentiate into many mesoderm lineages, and their performance aligns with MSCs from non-oral origins [5].
Discoveries are continuing to be made regarding which stem cells have the most advantageous characteristics for success in allogenic treatment in dental regenerative medicine. Dental stem cells come from a number of sources that can be obtained at different points throughout an individual’s life. These sources include dental pulp [6], human exfoliated deciduous teeth (SHED) [7], periodontal ligament of periodontium [8], dental follicle of third molars [9], tooth germ progenitor cells of third molars [10], gingiva of periodontium [11], alveolar bone [12], and apical papilla [13, 14, 15]. These sources each have varying levels of proliferation and a range of differentiation capabilities.
Two types of stem cells are necessary for dental hard tissue regeneration: epithelial stem cells, responsible for the formation of enamel, and mesenchymal stem cells, responsible for the formation of dentin [16]. While research and clinical applications exist regarding the use of MSCs in dentistry, no information is currently available regarding dental epithelial stem cells [1, 16].
The success of stem cell therapy is exemplified through a process known as stem cell banking which includes harvesting, storing, and cryopreserving the cells. Stem cell banking highlights the ideology of individually personalized medicine being the best preparation for future treatment and is quickly increasing in research and clinical attention [17]. Advantages to dental stem cell banking include: (a) safety in harvesting, (b) low ethical concerns present compared to that of applications of embryonic stem cells [3], (c) noninvasive means of harvesting [7], and (d) autologous source of stem cells.
2. Background
MSCs are the group of stem cells that are the most heavily researched and commonly used in clinical application. Being multipotent, they possess the capability to mature and differentiate into multiple mesenchymal tissue lineages: mesodermal, ectodermal, and endodermal lineages [18]. MSCs are also characterized by their ability to self-renew and proliferate to maintain the source of undifferentiated stem cells. Additionally MSCs have been shown to possess immunomodulatory effects including angiogenesis, anti-inflammation, and antiapoptosis [1] showing their great potential for use in regenerative medicine. Furthermore, MSCs are associated with minimal ethical controversy and can be obtained from many adult tissues in the body [19]. The study of human MSCs began with their being identified and researched in bone marrow. Through experimentation, bone marrow derived stem cells (BM-MSCs) have been found to regenerate certain skeletal tissues including bone, cartilage, and adipose and fibrous tissues [20]. While research has shown effective use of BM-MSCs in tissue regeneration, the rather invasive measures through which these cells must be obtained make BM-MSCs an unlikely candidate for expansion and normalization of personalized medical treatments. Scientists since have investigated means to obtain and isolate MSCs from other various tissues in the body. Some of these include adipose tissue [21], endometrial tissue [22], salivary gland [1], umbilical cord [23], synovial membrane and fluid [24], blood [25], and oral sources [1]. MSCs from oral sources are among the most easily accessible stem cells and often may be obtained through minimally invasive measures. They have also been found to possess similar immunomodulatory characteristics as BM-MSCs [26]. Because of their accessibility, researchers continue to investigate their clinical applications in regenerative medicine. There are eight identified populations of MSCs found in teeth and their supporting structures of the oral cavity.
3. Dental pulp stem cells
In 2000 Gronthos
Gronthos
Gronthos
4. Stem cells from human exfoliated deciduous teeth
In 2003 Miura
Miura
The same MSCs markers as BM-MSCs and DPSC have displayed were observed on SHED suggesting similar differentiation capabilities among the various sources of stem cells. However, SHED were found to differentiate into odontoblasts
Miura
5. Stem cells from periodontal ligament of periodontium
The periodontal ligament (PDL) was found by Seo
Seo
Seo
6. Stem cells from dental follicle of third molars
Stem cells obtained from the dental follicle of third molars have also been determined to be a source of oral derived MSCs valuable in contributing to regenerative medicine [9]. The dental follicle was investigated based on its supposed capacity to differentiate into cementoblasts, osteoblasts, and periodontal ligament cells as well as its ease in accessibility from extracted teeth. Dental follicle cells possess the progenitors for osteoblasts that are responsible for aiding the development of many tooth related structures during growth and eruption of teeth and development of alveolar bone [9]. Findings from Gronthos
Morsczeck
A similar study was conducted by Park
7. Tooth germ progenitor cells from third molars
In 2007 Ikeda
The cell populations were tested both
Ikeda
8. Gingiva-derived mesenchymal stem cells
In 2009, Zhang
The study was conducted using both
The GMSC were transplanted using a HA-TCP carrier to mice infected with experimentally induced colitis.
More recent research has shown GMSC ability to differentiate into osteogenic cells
Continued research on GMSC examine the advantages the cells offer to cutaneous wound healing [37], tendon regeneration [38], as well as regeneration of peripheral nerve defects
9. Alveolar bone-derived mesenchymal stem cells
In 2005 Matsubara
Matsubara
Matsubara
Another
10. Stem cells from apical papilla
In 2006 Sonoyama
SCAP were collected and isolated from human apical papilla from normal extractions of third molars and allowed to proliferate to form cell populations for
An additional
Sonoyama
11. Current clinical applications
The current advancements and the recognition of the use of stem cells in regenerative medicine has resulted in human clinical trials. Various human clinical trials applying stem cells from different sources in treatment of oral conditions are presented below. Additionally, there continue to be preclinical studies aiming to perfect the transition from autologous source to implementation of stem cells prior to the application of research in human clinical trials. These preclinical studies often aim to determine which scaffolding methods and materials are the most promising to ensure effectiveness of stem cell application with minimal senescence [46]. The studies also attempt to identify the specific microenvironment conditions in which the cells are both taken from and placed for regenerative treatment and how it will impact the potential and effectiveness of the proliferation and differentiation capacities of the cells [47].
Nakashima
DPSC have also been successful in regenerating bone lost from periodontal disease [51, 52, 53]. Ferrarotti
Additionally, PDLSC have been used to treat periodontal disease in the regeneration of periodontal tissues. In a recent study by Iwata
12. Discussion of stem cell banking
Stem cell banking may be loosely defined as the collection, isolation, and preservation of stem cells in preparation for use in regenerative treatment and therapy. The instance in which the cells are available for harvest might not align with when the cells are needed; thus the cells must be stored and preserved in a manner that will maintain the functionality of the cells over a potentially long period of time. The process is unique because it provides the means for individualized autologous medicine that can be implemented throughout an individual’s lifetime [17]. Personalized advancements in medicine have the goal of increasing tolerance of the immune system and low rejection by host due to the host and the donor being the same individual. Specifically, dental stem cell banking utilizes the stem cells obtained from the oral region, and dental professionals, especially oral surgeons, are responsible for the collection of the cells. Dental stem cell banks were created in response to the identification of the oral region as a source of stem cells and current research investigates the best means of collecting, isolating, and preserving the cells so that the full potential of use of these cells may be reached in the advancement of personalized medicine.
The various oral sources of stem cells, as previously discussed, can be harvested during tooth extractions, the naturally occurring exfoliation of deciduous teeth, or through other minimally invasive surgeries. An advantage of utilizing orally derived stem cells is that there are fewer ethical concerns associated with their harvesting than with obtaining stem cells from embryological sources [17]. Additionally, the harvesting of DMSC is often simple and painless through a less invasive manner than stem cells harvested from other regions of the body as they are often a byproduct of a surgical procedure and would otherwise be discarded as waste. Dental stem cell banking, specifically using SHED, was found to be more cost efficient than banking stem cells obtained from cord blood [56]. If stem cells from cord blood were not obtained, stem cells from oral sources offer more opportunities in an individual’s life to harvest and bank stem cells [56]. Stem cell banking is a proactive form of personalized medicine as the individual chooses to bank their cells in preparation for their future medical needs.
Current research regarding the banking of stem cells focuses on determining the specific conditions for optimal collection, isolation, and preservation of dental stem cells. By identifying the specific condition in which to harvest and store the cells, the proliferation and differentiation capacities of the stem cells can be maintained.
13. Collection
Stem cell banking begins in the dental chair with the collection of the cells during a scheduled extraction or routine eruption of the tooth. The dental professional assisting in the process works with the stem cell bank to decide regarding the specificities of harvesting the cell and initial processing steps. Dental stem cell banks currently exist in Japan, the United States, India, the United Kingdom, Germany, Singapore, Mexico, India and Norway. Dental stem cell bank information of FDA accredited banks in the United States is presented below:
USA:
BioEDEN, USA (https://www.bioeden.com/us/).
StemSave, USA (https://www.stemsave.com/).
Store-a-Tooth by Provia Laboratories, LLC, USA (http://www.store-a-tooth.com/).
National Dental Pulp Library, LLC (https://ndpl.net/).
Tooth Bank, USA (https://www.toothbank.com/).
It is important that the tooth from which the cells are collected is a normal healthy tooth with little to no decay due to pulpitis. Tsai
The cells are immediately placed in a container of sterile saline solution to maintain the vitality of the cells and to keep them from drying out during transport. The time between when the cells are collected and when the cells are processed is an important factor to determine the usefulness of the cells as the tissue may begin to degrade [58]. Perry
Perry
14. Isolation
Isolation refers to the processing of the cells in preparation for preservation by the stem cell bank that is preserving them. As previously mentioned, isolation of cells is possible up to 5 days after the tooth containing the cells is extracted or removed from its source [59]. The pulp from the harvested tooth is disinfected, and the cells are isolated and cultured in MSC media. The focus of current research is identifying the optimal conditions in which the cells are allowed to proliferate and colonize. Factors to be taken into consideration are the process through which the cells are isolated, the extent to which the cells are digested, the means of cellular attachment during culturing, and the media, serum, and supplements that are used to enhance cellular growth and function during the culturing process. Perry
15. Preservation - cryopreservation and magnetic field programmed freezing
Cryopreservation is the process of cooling cells to an eventual temperature below 150°C by liquid nitrogen vapor. The cells must freeze carefully and quickly as to not form ice crystals which would result in cell death [60]. This is the main cellular damage that a cell may experience as a result of cryopreservation as well as mechanical stress from ice formed outside of the cell [61]. A cryoprotective agent (CPA) can be used in the vitrification process to maintain cell functionality during preservation, but the cryoprotectants are often harmful to the cells or even can cause cell death [62]. Through cryopreservation, cells are able to maintain their cellular markers, gene expression, and differentiation potential [59]. Perry
In 2010 Kaku
Interestingly, viable stem cells have not only been obtained from post preservation isolated cell populations but also from teeth that were preserved intact [59, 65, 67]. This confirms that immediate processing post extraction is not a crucial factor necessary to effectively bank stem cells which would in turn result in lowered costs associated with banking stem cells as the processing of the cells is the most costly part of the process [58, 59, 63].
16. Conclusion
With the discovery and research of dental stem cells come great opportunities to utilize these cells in regenerative medicine and dental tissue repair. The aspects of each source of stem cells must be analyzed so that the best candidate for stem cell banking and the specific regenerative treatment may be used in personalized medicine. The eight sources of stem cells and their ability to successfully aid in regeneration of tissues have been analyzed: human exfoliated deciduous teeth, dental pulp stem cells, periodontal ligament stem cells, dental follicle stem cells, tooth germ progenitor cells, gingival-derived mesenchymal stem cells, alveolar bone mesenchymal stem cells, and stem cells from the apical papilla. The relatively recent developments in stem cell biology make banking dental stem cells a feasible option for regeneration of tissues and other oral structures. As with any new developments in research, however, there exist certain limitations associated with achieving a consistent application in a clinical setting. Despite the many advancements that have been made in the field, the cellular conditions in which the stem cells may be collected, isolated, and preserved must be improved and perfected before clinical application is regularly implemented. The viability and stability of the preserved stem cells must be considered further so that the preservation efforts do not result in loss. Appropriate long term double-blind randomized clinical trials must be perfected before stem cell therapy may become a normalcy in a clinical setting. Additionally, current stem cell research is successful animal models but must be further applied to human models before clinical application may occur. The opportunity for immune rejection exists, but as is in the case with stem cells, immune rejection is limited due to the autologous nature of the cells. With continued research and developments in the field of stem cell biology, the dental stem cells support the continued development and betterment of the stem cell banking industry.
Abbreviations
MSCs | mesenchymal stem cells |
SHED | stem cells from human exfoliated deciduous teeth |
DPSC | dental pulp stem cells |
PDL | periodontal ligament |
PDLSC | periodontal ligament stem cells |
DFSC | dental follicle stem cells |
TGPC | tooth germ progenitor cells |
GMSC | gingival-derived mesenchymal stem cells |
ABMSC | alveolar bone mesenchymal stem cells |
SCAP | stem cells from the apical papilla |
BM-MSCs | bone marrow derived mesenchymal stem cells |
DMSC | dental-derived mesenchymal stem cells |
HA-TCP | hydroxyapatite/tricalcium phosphate |
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