1Senior lecturer. Department of Conservative Dentistry and Endodontics. MGS Dental College and Research Center, Sri Ganganagar, Rajasthan, India.
2Professor and Head. Department of Conservative Dentistry and Endodontics. MGS Dental College and Research Center, Sri Ganganagar,Rajasthan, India.
3Reader. Department of Conservative Dentistry and Endodontics. MGS Dental College and Research Center, Sri Ganganagar, Rajasthan,India.
4Senior lecturer. Department of Oral and Maxillofacial Surgery. MGS Dental College and Research Center, Sri Ganganagar , Rajasthan, India.
5Senior lecturer. Department of Conservative Dentistry and Endodontics. SDD Hospital and Dental CollegeBarwala, Panchkula, Haryana, India. ADDRESS FOR CORRESPONDENCE:
Dr. Alka Arora
Senior lecturer. Department of Conservative Dentistry and Endodontics MGS Dental College and Research Center, Sri Ganganagar,Rajasthan,India
Email id- email@example.com
ABSTRACT: Stem cells constitute the source of differentiated cells for the generation of tissues during development and for regeneration of tissues that are diseased or injured postnatally. The dental pulp is considered a rich source of mesenchymal stem cells that are suitable for tissue engineering applications and having potential to differentiate into several cell types. In the last few years lots of studies and demonstrations have been carried out which show that stem cells and tissue engineering are giving rise to a separate branch named “Regenerative Dentistry” that will have its own position in future dental clinical practice. This review discusses the therapeutic potential of stem cells in regenerative dentistry as seen in studies and demonstrations carried out by different workers.
Human dental tissue has limited potentials to regenerate but the discovery of dental stem cells have developed new and surprising scenario in regenerative dentistry. The most valuable ongoing research in regenerative dentistry is the study on stem cells. Regeneration of the dental tissues offers an eye-catching alternative to more conventional restorative approaches because the infected tissue is replaced by natural tissue, which forms an essential part of the tooth.
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.1 Research on stem cells is providing advanced knowledge about how an organism develops from a single cell and how healthy cells replace damaged ones in adult organisms.2
Stem cell has two important characteristics that distinguish them from other types of cells. First, they are unspecialized cells that renew themselves for long periods through cell division. The second is that under certain physiologic or experimental conditions, they can be induced to become cells with special functions.3 Dental stem cells have drawn attention in recent years because of their accessibility and high proliferative ability.
CLASSIFICATION OF STEM CELLS
I. Based on the origin, the stem cells are classified as follows:4
a)Early embryonic stem cells: The first step in human development occurs when the newly fertilized egg or zygote begins to divide, producing a group of stem cells called an embryo. These early stem cells are totipotent, i.e. possess the ability to become any kind of cell in the body.
b) Blastocyst embryonic stem cells: Five days after fertilization, the embryo forms a hollow ball-like structure known as a blastocyst. The embryonic stem cells in the blastocyst are pluripotent, i.e. having the ability to become almost any kind of cell in the body. Scientists can induce these cells to replicate themselves in an undifferentiated state for very long periods before stimulating them with appropriate signaling molecules to create specialized cells. However, the sourcing of embryonic stem cells is controversial and associated with ethical and legal issues.
c)Fetal stem cells: Stem cells in the fetus are responsible for the initial development of all tissues before birth. Like embryonic stem cells, fetal stem cells are pluripotent.
d)Umbilical cord stem cells:. Blood from the umbilical cord contains stem cells that are genetically identical to the newborn baby. Umbilical cord stem cells are multipotent, i.e. they can differentiate into a limited range of cell types. Umbilical cord stem cells can be stored cryogenically after birth for use in future medical therapy.
e)Adult stem cells: The term Postnatal Stem Cells is preferable, because infants and children also have stem cells. These stem cells reside in tissues that have already developed, directing their growth and maintenance throughout life. These cells are also multipotent.Adult stem cells typically generate the cell types of the tissue in which they reside. However, some experiments over the last few years have raised the possibility of a phenomenon known as plasticity, in which stem cells from one tissue may be able to generate cell types of a completely different tissue.
Postnatal stem cells have been found in almost all body tissues, including dental tissues where they make up only 1-4% of cells, which may include progenitor cells (Smith et al, 2005).5 To date, five types of human dental stem cells have been isolated and characterized: i) Dental pulp stem cells (DPSCs), ii) Stem cells from human exfoliated deciduous teeth (SHED) and immature dental stem cells (IDPSC) iii) Stem cells from apical papillae (SCAP), iv) Periodontal ligament stem cells (PDLSCs), and v.) Dental follicle progenitor cells (DFPC).6 as shown in Figure 2.
f)Progenitor cells: Stem cells generate intermediate cell types before they achieve their fully differentiated state. The intermediate cell is known as a precursor or progenitor cell and lacks the ability to self replicate. Generally, undifferentiated cells are considered to be progenitor cells until their multi tissue differentiation and self-renewal properties are demonstrated and they become designated as stem cells.4
Embryonic stem cells generate the same sort of immune response as a transplanted organ; however since one has access to them before differentiation, it may be possible to modify them genetically to reduce or eliminate immune incompatibility.7 However, there are at least two large obstacles to their use. The first is a technical hurdle - difficulty in manipulating the cells to reproduce and predictably differentiate into the desired tissue. Another equally challenging question that must be resolved is one of law and ethics. This explains why many researchers are now focusing attention on developing stem cell therapies using postnatal stem cells donated by the patients themselves or their close relatives.1
II 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:
(a) Totipotent stem cells: each cell is capable of developing into an entire organism, including extraembryonic tissues.
(b) Pluripotent stem cells: cells from embryos (embryonic stem cells) that when grown in the right environment in vivo are capable of differentiating into any of the three germ layers; endoderm, mesoderm or ectoderm.5 as shown in Figure 1.
(c) 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 the third category (Roboy 2000).5
Figure1.: Applications Of Pluripotent Stem Cells.
III Stem cells are categorized on the basis of donor :1
a)Autologous stem cells: The most practical clinical application of a stem cell therapy would be to use a patient’s own donor cells. Bone marrow harvesting of a patient’s own stem cells and their reimplantation back to the same patient represents one clinical application of autogenous postnatal stem cells.Autologous stem cells have the fewest problems with immune rejection and pathogen transmission. Harvesting the patient’s own cells makes them the least expensive to obtain and avoids legal and ethical concerns.To accomplish endodontic regeneration, the most promising cells are autologous postnatal stem cells, because these appear to have the fewest disadvantages that would prevent them from being used clinically.
b)Allogenic cells: Allogenic cells originate from a donor of the same species examples of donor allogenic cells include blood cells used for a blood transfusion; bone marrow cells used for a bone marrow transplant. There are some ethical and legal constraints to the use of human cell lines to accomplish regenerative medicine. The use of preexisting cell lines and cell organ cultures removes the problems of harvesting cells from the patient and waiting weeks for replacement tissues to form in cell organ-tissue cultures. However, the most serious disadvantages of using preexisting cell lines from donors to treat patients are the risks of immune rejection and pathogen transmission.
c)Xenogenic cells: These are those cells that are isolated from individuals of another species. Duailibi MT et al (2004)8 showed that pig tooth pulp cells have been transplanted into mice, and these have formed tooth crown structures. The harvesting of cells from donor animals removes most of the legal and ethical issues associated with sourcing cells from other humans. However, many problems remain, such as the high potential for immune rejection and pathogen transmission from the donor animal to the human recipient.
STEM CELLS OF THE DENTAL ORIGIN:
1.DENTAL PULP STEM CELLS (DPSCS):
DPSCs were isolated from the human pulp tissue for the first time in 2000 by Gronthos et al (2000).9 These cells were isolated from the human adult third molars with enzyme treatment of pulp tissues. Pulp tissue from exfoliated deciduous teeth was also used as a source of DPSCs.10 These cells have gene expression profiles and differentiation capacity similar to BMSCs(bone marrow stem cells). They showed a higher proliferation capacity compared with osteogenic cells, had the ability to differentiate into odontoblast-like cells which express the early odontoblast cell marker, dentine sialophosphoprotein, and formed a dentine–pulp complex when transplanted in vivo.4
Laino et al 2006,11 isolated a selected subpopulation of DPSCs known as Stromal Bone-producing Dental Pulp Stem Cells (SBP-DPSCs).These were described as multipotential cells that were able to give rise to a variety of cell types and tissues including osteoblasts, adipocytes, myoblasts, endotheliocytes, and melanocytes, as well as neural cell progenitors (neurons and glia), being of neural crest origin. This stem cell behaviour occurs following cryopreservation, signifying the potential use of frozen tissues for stem cell isolation.12
Pulp cells are able to proliferate and differentiate into odontoblast-like cells with processes, extending into dentinal tubules when in contact with chemo-mechanically treated dentine surfaces in an in vitro situation, which is a requirement for the secretion of new dentine.5 A study was conducted to characterize human adult dental pulp cells isolated and cultured in vitro and to examine the cell differentiation potential grown on dentin. Pulp cells, after being seeded onto mechanically and chemically treated dentin surface, appeared to establish an odontoblast like morphology with a cytoplasmic process extending into a dentinal tubule revealed by scanning electron microscopy analysis. It was concluded that isolated human pulp stem cells may differentiate into odontoblasts on dentin in vitro.13 Huang et al(2008)14 conducted a study to source hDPSCs from complicated crown-fractured teeth requiring root canal therapy. hDPSCs were harvested from the pulp tissues for two complicated crown-fractured teeth requiring root canal therapy, retaining the teeth for subsequent prosthodontic rehabilitation, in a 41-year-old woman who had suffered a motorcycle accident. Pulp tissue from the left lower deciduous canine of a healthy 10-year-old boy (the positive control) was also removed because of high mobility and cultured for hDPSCs. The results indicated that the hDPSCs derived from the two complicated crown-fractured teeth and the deciduous tooth were able to differentiate into adipogenic, chondrogenic, and osteogenic lineages and also expressed stem cells markers and differentiation markers, which indicated their stem cell origin and differentiation capability. In addition, hDPSCs from both the complicated crown fractured teeth and the deciduous tooth showed high expression for bone marrow stem cell markers including CD29, CD90, and CD105 and exhibited very low expression of markers specific for hematopoietic cells such as CD14, CD34, and CD45. It was concluded that pulp exposed in complicated crown-fractured teeth might represent a valuable source of personal hDPSCs.
When isolated at the stage of crown development, DPSC are more proliferative than later on. (Takeda et al. 2008)15
The ability of young and old teeth to respond to injury by induction of reparative dentinogenesis suggests that a similar population of competent progenitor cells still may exist within the dental pulp, which can later differentiate into odontoblastoids cells.1The stem cell population in the pulp is very small; approximately 1% of the total cells (Smith et al. 2005)16 and the effect of aging reduce the cell pool available to participate in regeneration which reflects the better healing outcomes seen in younger patients.
STEM CELLS FROM HUMAN EXFOLIATED DECIDUOUS TEETH (SHED)
Stem cells from human exfoliated deciduous teeth were isolated for the first time in 2003 by Miura et al. who confirmed that they were able to differentiate into a variety of cell types to a greater extent than DPSCs, including neural cells, adipocytes, osteoblast-like and odontoblast-like cells, and can also be retrieved from a tissue that is disposable and readily accessible. The main task of these cells seems to be the formation of mineralized tissue, which can be used to enhance orofacial bone regeneration.The ethical constraints associated with the use of embryonic stem cells, together with the limitations of readily accessible sources of autologous postnatal stem cells with multipotentiality, have made stem cells from human exfoliated deciduous teeth an attractive alternative for dental tissue engineering. Thus, they are ideally suited for young patients at the mixed dentition stage who have suffered pulp necrosis in immature permanent teeth as a consequence of trauma.4
Miura et al (2003)17 also demonstrated the inability of SHED to generate complete dentin-pulp like tissue as did hDPSCs, indicating that perhaps they are immature cells. Nor JE (2006)18 showed that SHED seeded onto synthetic scaffolds seated in pulp chamber of a thin tooth slice and implanted into immunocompromised mice generated odontoblast like cells against existing dentin. Cordeiro et al (2008)19 conducted a study to evaluate morphologic characteristics of the tissue formed when SHED seeded in biodegradable scaffolds prepared within human tooth slices are transplanted into immunodeficient mice. They observed that the resulting tissue presented architecture and cellularity that closely resemble those of a physiologic dental pulp. Ultrastructural analysis with transmission electron microscopy and immunohistochemistry for dentin sialoprotein suggested that SHED differentiated into odontoblast-like cells in vivo. Notably, SHED also differentiated into endothelial-like cells, as demonstrated by B galactosidase staining of cells lining the walls of blood containing vessels in tissues engineered with SHED stably transduced with LacZ. It was concluded that exfoliated deciduous teeth constitute a viable source of stem cells for dental pulp tissue engineering.
Using similar approach (i.e. tooth slice/scaffold model), Casagrande et al (2010)20 showed that SHED differentiated into odontoblast-like cells, expressing three putative markers of odontoblastic differentiation (DSPP, DMP1, MEPE).Furthermore, the formation of a well-organized pulp tissue has recently been observed inside the root canal of opened-apex maxillary first molars, using SHED with a self-assembled injectable scaffold. SHED differentiated into functional odontoblast capable of generating new dentin. (Rosa V, 2010)6 These promising results suggest that primary teeth constitute a rich source of stem cells.
Figure 2.Human Dental Stem Cells
STEM CELLS FROM THE APICAL PAPILLA (SCAP):
A new unique population of mesenchymal stem cells (MSCs) residing in the apical papilla of permanent immature teeth, known as stem cells from the apical papilla (SCAP), were discovered by Sonoyama et al (2008).21 They characterized the apical papilla tissue and stem cell properties of SCAP using histologic, immunohistochemical, and immunocytofluorescent analyses. It was found that the apical papilla is distinctive to the pulp in terms of containing less cellular and vascular components than those in the pulp. Cells in the apical papilla proliferated 2-to 3-fold greater than those in the pulp in organ cultures. Both SCAP and DPSCs were as potent in osteo/dentinogenic differentiation as MSCs from bone marrows, whereas they were weaker in adipogenic potential. The immunophenotype of SCAP was found to be similar to that of DPSCs on the osteo/dentinogenic and growth factor receptor gene profiles. Double-staining experiments showed that STRO-1 coexpressed with dentinogenic markers such as bone sialophosphoprotein, osteocalcin, and growth factors FGFR1 and TGF_RI in cultured SCAP. Additionally, SCAP expressed a wide variety of neurogenic markers such as nestin and neurofilament M upon stimulation with a neurogenic medium. It was concluded that SCAP were similar to DPSCs but a distinct source of potent dental stem/progenitor cells.
Abe et al (2008), also suggested that SCAP are an effective source of cells for regeneration of hard tissue.4
The soft tissue on the exterior of the apical foramen area expresses markers for STRO-1 and CD24, a surface marker for SCAP, which is lost during odontogenic differentiation. Compared with DPSC, SCAP have greater numbers of STRO-1 positive cells, faster proliferation, a greater number of population doublings and increased capacity for in vivo dentine regeneration. Unlike DPSC and other MSC, SCAP are positive for telomerase activity which is present in embryonic stem cells and suggests a very immature source of cells available for hard tissue regeneration which has been demonstrated by the use of SCAP to engineer bioroots in minipigs (Sonoyama et al. 2006, 2008, Yang et al.2008).5
Recent studies have shown that SCAP have the capacity to produce vascularized pulp-like tissue in vivo into 5-6 mm-long root canals. Furthermore, SCAP appear to undergo odontogenic differentiation as measured by expression of DSP, BSP, ALP, and CD105, indicating that the pulp-like tissue resembles human pulp tissue (Huang GT et al, 2010).22
PERIODONTAL LIGAMENT STEM CELLS (PDLSCS)
Earlier researchers hypothesized that cementoblasts, alveolar bone cells and PDL cells may be derived from a single population of immature cells which were capable of migrating from endosteal spaces into the PDL where they express osteoblast or cementoblast phenotypes (McCulloch 1985, Melcher 1985, McCulloch et al. 1987). Recently, isolation and characterization of a stem cell population within the PDL has been confirmed (Seo et al. 2004).5 Periodontal ligament stem cells are more proliferative than BMSSC, with a longer lifespan, and higher number of population doublings in vitro. The potential of PDLSC to develop into other cell lineages and obtain periodontal ligament-like characteristics has been established by the ability of cultured PDLSC to differentiate into cementoblast-like cells, adipocytes and collagen-forming cells in vitro and the capacity to generate a cementum/PDL-like structure in vivo.5
The presence of MSCs in the periodontal ligament is also supported by the findings of Trubiani et al (2005)4 who isolated and characterized a population of MSCs from the periodontal ligament which expressed a variety of stromal cell markers, and Shi et al (2005)23who demonstrated the generation of cementum-like structures associated with PDL-like connective tissue after transplanting PDLSCs with hydroxyapatite/tricalcium phosphate particles into immunocompromised mice. The clinical potential for the use of PDLSCs has been further enhanced by the demonstration that these cells can be isolated from cryopreserved periodontal ligaments while maintaining their stem cell characteristics, including the expression of MSC surface markers, single-colony strain generation, multipotential differentiation and cementum/periodontal-ligament-like tissue regeneration,thus providing a ready source of MSCs.4 Using a minipig model, autologous SCAP and PDLSCs were loaded onto hydroxyapatite/tricalcium phosphate and gelfoam scaffolds, and implanted into sockets in the lower jaw, where they formed a bioroot encircled with periodontal ligament tissue and in a natural relationship with the surrounding bone. Although there was no evidence for PDLSC forming pulp-like structures.6
Recently, Trubiani et al (2008) suggested that PDLSCs had regenerative potential when seeded onto a three dimensional biocompatible scaffold, thus encouraging their use in graft biomaterials for bone tissue engineering in regenerative dentistry,24 whereas Li Y et al (2008) have reported cementum and periodontal ligament-like tissue formation when PDLSCs are seeded on bioengineered dentin.4 Dental pulp stem cells, SHED and PDLSC have similar gene expression profiles for extracellular matrix proteins, growth factors, receptors and adhesion molecules, suggesting the existence of a common origin and molecular pathway regulating the formation of dentine, cementum and bone, but as yet no genes are exclusively expressed by either cell population (Shi & Gronthos 200325, Shi et al. 2005).23
PROCEDURE OF STEM CELL USE: 1)STEM CELL IDENTIFICATION: Stem cells can be identified by four commonly used techniques:
(a) staining the cells with specific antibody markers and using a flow cytometer, in a process called fluorescent antibody cell sorting (FACS);
(b) immunomagnetic bead selection;
(c) immunohistochemical staining; and
(d) physiological and histological criteria, including phenotype (appearance), chemotaxis, proliferation, differentiation, and mineralizing activity.
FACS together with the protein marker CD34 is widely used to separate human stem cells expressing CD34 from peripheral blood, umbilical cord blood, and cell cultures. Different types of stem cells often express different proteins on their membranes and are therefore not identified by the same stem cell protein marker. The most studied dental stem cells are those of the dental pulp. Human pulp stem cells express von Willebrand factor CD146, alpha-smooth muscle actin, and 3G5 proteins. Human pulp stem cells also have a fibroblast phenoptype, with specific proliferation, differentiation, and mineralizing activity patterns.1
2) CULTURING OF STEM CELLS
The capacity to expand stem cells in culture is an indispensible step for regenerative medicine and considerable effort has been made to evaluate the consequences of the cultivation on stem cell behavior.26
Dental pulp stem cells can be cultured by two methods; the first is the enzyme digestion method in which the pulp tissue is collected under sterile conditions, digested with appropriate enzymes, and then the resulting cell suspensions are seeded in culture dishes containing a special medium supplemented with necessary additives and incubated. Finally, the resulting colonies are subcultured before confluence and the cells are stimulated to differentiate.
The second method for isolating dental pulp stem cells is the explants outgrowth method in which the extruded pulp tissues are cut into 2-3 mm cubes, anchored via microcarriers onto a suitable substrate, and directly incubated in culture dishes containing the essential medium with supplements. Ample time (up to 2 weeks) is needed to allow a sufficient number of cells to migrate out of the tissues. Haung et al (2006) compared both methods and found that cells isolated by enzyme-digestion had a higher proliferation rate than those isolated by explants outgrowth method.4
a) Differentiation of stem cells
Generation of specialized cells from unspecialized stem cells is a process known as differentiation, and is triggered by signals inside and outside the cells. The internal signals are controlled by the genes of one cell, which are interspersed across long strands of DNA, and carry coded instructions for all the structures and functions of a cell. The external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighboring cells, and certain molecules in the microenvironment.4
Cultured dental pulp stem cells can be stimulated to differentiate to more than one cell type according to the contents of the culture medium. Osteo/dentinogenic medium contains dexamethasone, glycerophosphate, ascorbate phosphate and 1,25 dihydroxy vitamin D in addition to the basic elements.9 Adipogenic medium (Song Let al 2004) contains dexamethasone, insulin and isobutyl methylxanthine, whereas for neurogenic induction ( Miura M et al 2003) cells are cultured in the presence of B27 supplement, basic fibroblast growth factor, and epidermal growth factor.4
b) Cell lines: Culturing of stem cells is the first step in establishing a stem cell line, which is a propagating collection of genetically identical cells that can be used for research and therapy development. Once a stable stem cell line has been established, stem cells can be triggered to differentiate into specialized cell types. Odontoblasts are postmitotic terminally differentiated cells, and thus cannot be induced to undergo further differentiation. The major proteins synthesized by fully differentiated odontoblasts are type I collagen, which forms the scaffold for mineral deposition and provides strength to the mineralized dentin, and two major noncollagenous proteins (NCPs) considered to have mineralization-regulatory capacities, namely dentin phosphophoryn (DPP; or DMP-2) and dentin sialoprotein (DSP). DPP and DSP are encoded by a single gene, DSPP or DMP-3, which specifically defines the phenotypic characteristics of dentin.
Another important non-collagenous protein is dentin matrix protein-1 (DMP- 1), which is found primarily in dentin and bone and has been implicated in the regulation of mineralization, being considered to act as a growth factor to induce the differentiation of DPSCs.In order to explore the pulp wound-healing mechanism and to develop a therapeutic strategy for pulp regeneration, development of an odontoblast cell line is very important. Upto now, however, odontogenic differentiation has not been well characterized due to two major limitations: The first is the paucity of differentiation markers, which is now being overcome by the characterization of odontoblast specific markers (DMP-1, DMP-2, and DMP-3) that can indicate the presence of a true odontoblastic cell line. The second is the limited life span of the primary cells, which is being addressed by trials of several methodologies including cell cloning and immortalization.4
TOOTH-BANKING- PRELIMINARY STEP FOR FUTURE TISSUE REGENERATION:
Extracted teeth are traditionally thought to be medical waste. With advances in tissue engineering, dental stem cells have shown their potential in regenerating odontoblasts, dentin/pulp like structure, and dentin. Furthermore, dental Stem cells can differentiate into adipocytes and neurons, and promote the proliferation and differentiation of endogenous neural cells. lt is also possible that myocardial infarction and liver dysfunction could be treated with dental stem cells in the near future. Thus, the therapeutic capability and clinical benefits of dental stem cells are not limited to dental use but can also be used for regenerative medicine.2
Because of the opportunity to preserve dental stem cells for medical applications, the term "tooth bank" was first raised in 1966. Several attempts to preserve dental SCs have also been reported by other groups. However the absence of appropriate preservation methods for teeth and/or dental SCs remains a significant limitation. With the rapid development of advanced cryopreservation technology, the first commercial tooth bank was established as a venture company at National Hiroshima University in Japan in 2004. By systematic organization, an increasing number of teeth have been cryopreserved for future generative medicine.2
With the documented discovery of SHED in 2003 by Dr. Songtao Shi, an accessible and available source of stem cells was identified which can be easily preserved and used for future cure of ailments. SHED appear at the 6th week during the embryonic stage of human development. Scientists believe that these cells behave differently than the postnatal stem cells. SHED cells multiply rapidly and grow much faster than the adult stem cells, suggesting that they are less mature, so they have the potential to develop into a wider variety of tissue types.
Obtaining stem cells from SHED is simple and convenient, with little or no trauma. Every child loses primary teeth, which creates the opportunity to recover and store this convenient source of stem cells. Stem cell scan also be recovered from developing wisdom teeth and permanent teeth. Individuals have different opportunities at different stages of life to bank these valuable cells. It is best to recover stem cells when a child is young and healthy and the cells are strong and proliferative.27SHED is turning into a favourite choice for commercial stem cell banks where autologous stem cell sources may be stored for future use.
Collection, Isolation and preservation of SHED for Tooth Banking27
Step 1: Tooth Collection: SHED banking is a proactive decision made by the parents, so, the first step as informed to them is to put tooth fulfilling in sterile saline solution and give a call to tooth bank or attending dentist of the bank. The tooth exfoliated should have pulp red in color, indicating that the pulp received blood flow up until the time of removal, which is indicative of cell viability. If the pulp is gray in color, it is likely that blood flow to the pulp has been compromised, and thus, the stem cells are likely necrotic and are no longer viable for recovery. Teeth that become mobile, either through trauma or disease (e.g. Class III or IV mobility), often have a severed blood supply, and are not candidates for stem cell recovery. This is why recovery of stem cells from primary teeth is preferred after an extraction than the tooth that is “hanging on by a thread” with mobility. Pulpal stem cells should not be harvested from teeth with apical abscesses, tumours or cysts.
In the event of a scheduled procedure, the dentist visually inspects the freshly-extracted tooth to confirm the presence of healthy pulpal tissue and the tooth or teeth is transferred into the vial containing a hypotonic phosphate buffered saline solution, which provides nutrients and helps to prevent the tissue from drying out during transport (up to four teeth in the one vial). Placing a tooth into this vial at room temperature induces hypothermia. The vial is then carefully sealed and placed into the thermette a temperature phase change carrier, after which the carrier is then placed into an insulated metal transport vessel. The thermette along with the insulated transport vessel maintains the sample in a hypothermic state during transportation. This procedure is described as Sustentation.
Store-A-Tooth, a company involved in tooth banking uses the Save-A-Tooth device same as that used for transportation of avulsed teeth for transporting stem cells from the dental office to the laboratory.The viability of the stem cells is both time and temperature sensitive, and careful attention is required to ensure that the sample will remain viable. The time from harvesting to arrival at the processing storage facility should not exceed 40 hours. The same steps are performed by the attending assistant of the tooth bank if it is not a scheduled extraction for the collection of specimen.
Step 2: Stem Cell Isolation27: When the tooth bank receives the vial, the following protocol is followed.
A) Tooth surface is cleaned by washing three times with Dulbecco’s Phosphate Buffered Saline without Ca++ and Mg++ (PBSA).
B) Disinfection is done with disinfection reagent such as povidone iodine and again washed with PBSA.
C) The pulp tissue is isolated from the pulp chamber with a sterile small forceps or dental excavator. Stem cell rich pulp can also be flushed out with salt water from the center of the tooth.
D) Contaminated Pulp tissue is placed in a sterile petridish which was washed at least thrice with PBSA.
E) The tissue is then digested with collagenase Type I and Dispase for 1 hour at 37ºC. Trypsin- EDTA can also be used.
F) Isolated cells are passed through a 70 um filter to obtain single cell suspensions.
G) Then the cells are cultured in a Mesenchymal Stem Cell Medium( MSC) medium which consists of alpha modified minimal essential medium with 2mM glutamine and supplemented with 15% foetal bovine serum (FBS),0.1Mm L- ascorbic acid phosphate, 100U/ml penicillin and 100ug/ml streptomycin at 37ºC and 5% CO2 in air. Usually isolated colonies are visible after 24 hrs.
H) Different cell lines can be obtained such as odontogenic, adipogenic and neural by making changes in the MSC medium.
I) If cultures are obtained with unselected preparation, colonies of cells with morphology resembling epithelial cells or endothelial cells can be established.
Usually cells disappear during course of successive cell passages. If contamination is extensive, three procedures can be performed:
1) Retrypsinizing culture for a short time so that only stromal cells are detached because epithelial or endothelial like cells are more strongly attached to culture flask or dish.
2) Changing medium 4-6 hrs after subculture because stromal cells attach to culture surface earlier than contaminating cells.
3) Separate stem cells using Fluorosence Activated Cell Sorting (FACS), in which STRO-1 OR CD 146 can be used. This is considered most reliable.
Confirmation of the current health and viability of these cells is given to the donor’s parents.
Step 3: Stem Cell Storage27 :Two approaches are used for stem cell storage.
b) Magnetic freezing
a)Cryopreservation: It is the process of preserving cells or whole tissues by cooling them to sub-zero temperatures typically -196 degree Celsius. At these freezing temperatures, biological activity is stopped, as are any cellular processes that lead to cell death. SHED can be successfully stored long-term with cryopreservation and still remain viable for use. These cells can be cryopreserved for an extended period of time, and when needed, carefully thawed to maintain their viability. Cells harvested near end of log phase growth (approximately. 80–90% confluent) are best for cryopreservation. The sample is divided into four cryo-tubes and each part is stored in a separate location in cryogenic system so that even in the unlikely event of a problem with one of storage units, there will be another sample available for use. The cells are preserved in liquid nitrogen vapour at a temperature of less than -150ºC. This preserves the cells and maintains their latency and potency. In a vial, 1-2x 106 cells in 1.5 ml of freezing medium is optimum. Too low or high cell number may decrease recovery rate.
Papaccio G et al (2006) studied the differentiation and morpho-functional properties of cells derived from stem cells after long-term cryopreservation to evaluate their potential for long-term storage with a view to subsequent use in therapy. They concluded that dental pulp stem cells and their osteoblast-derived cells can be long-term cryopreserved and may prove to be attractive for clinical applications.27
b)Magnetic freezing: Masato et al (2007) described long-term tooth cryopreservation on using a programmed freezer with a magnetic field, the so called Cell Alive System (CAS). Using the CAS method, the PDL showed good cell viability and differentiation capability after cryopreservation. ln support of this, further experiments by Temmerman et al (2009) demonstrated the successful preservation of human pulpal tissues, when the cryoprotectant encompassed the entire pulp.2 Hiroshima University uses magnetic freezing rather than cryogenic freezing. This technology is called CAS and exploits the little known phenomena that applying even a weak magnetic field to water or cell tissue will lower the freezing point of that body by up to 6-7 degrees Celsius. The idea of CAS is to completely chill an object below freezing point without freezing occurring, thus ensuring, distributed low temperature without the cell wall damage caused by ice expansion and nutrient drainage due to capillary action, as normally caused by conventional freezing methods. Then, once the object is uniformly chilled, the magnetic field is turned off and the objects snap freezes. The Hiroshima University company is the first expression of this new technology. Using CAS, Hiroshima University claims that it can increase the cell survival rate in teeth to a high of 83%.This compares to 63% for liquid nitrogen (-196 degrees C), 45% for ultra-cold freezing (-80 degrees C), and just 21.5% for a household freezer (-20 degrees C). Maintaining a CAS system is a lot cheaper than cryogenics and more reliable as well.27
In addition to the activities at Hiroshima University, the sister school, Taipei Medical University (TMU) has recently completed a cooperative system and established a second tooth bank in 2008 (the TMU Tooth Bank). After consecutive experimental studies using the CAS, the TMU Tooth Bank has successfully expanded from cryopreservation for autotransplantation to long-term preservation of dental SCs. Now, patients who store teeth in the TMU Tooth Bank will have teeth for autotransplantation and also for SC isolation from thawed dental pulp tissue.2
CONCLUSION: The future dentistry will be more of regenerative based, where patients own cells can be used to treat diseases. Stem cell therapy has got a paramount role as a future treatment modality in dentistry. On the other hand, stem cells should be differentiated to the appropriate cell types before they can be used clinically, otherwise it might lead to deleterious effects.
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