Indian Journal of Dental Research

REVIEW ARTICLE
Year
: 2012  |  Volume : 23  |  Issue : 4  |  Page : 558-

Stem cells of the dental pulp


K Ranganathan1, Vidya Lakshminarayanan2,  
1 Department of Oral Pathology, Ragas Dental College and Hospital, Chennai, India
2 Department of Oral Pathology, Chennai Dental Research Foundation, Chennai, India

Correspondence Address:
K Ranganathan
Department of Oral Pathology, Ragas Dental College and Hospital, Chennai
India

Abstract

Stem cells of the dental pulp are a population of postnatal stem cells with multilineage differentiation potential. These cells are derived from the neural ectomesenchyme, similar to most craniofacial tissues, and specific niches in the pulp have been identified. Since the isolation of dental pulp stem cells (DPSC) and stem cells from exfoliating deciduous teeth (SHED), numerous studies have attempted to define and characterize these cells, and embryonic stem cell features have been reported in both DPSC and SHED. These cells have a vast repertoire of differentiation - osteogenic, odontogenic, myogenic, adipogenic, neurogenic, and melanocytic, and have even demonstrated transdifferentiation to corneal cells and islet cells of pancreas. The combined advantages of multipotency/pluripotency and the relative ease of access of pulp tissue for autologous use render DPSC/ SHED attractive options in regenerative dentistry and medicine. This review gives a bird«SQ»s eye view of current knowledge with respect to stem cells from the dental pulp.



How to cite this article:
Ranganathan K, Lakshminarayanan V. Stem cells of the dental pulp.Indian J Dent Res 2012;23:558-558


How to cite this URL:
Ranganathan K, Lakshminarayanan V. Stem cells of the dental pulp. Indian J Dent Res [serial online] 2012 [cited 2019 Nov 21 ];23:558-558
Available from: http://www.ijdr.in/text.asp?2012/23/4/558/104977


Full Text

The human body performs a variety of functions essential for its survival and healthy existence, made possible by the ability of the tissues to undergo renewal or regeneration following trauma or disease. This renewal or regeneration of tissues is possible due to the existence of a unique set of unspecialized cells - the stem cells. [1] These cells are unique because of their:



Potential ability for unlimited replicationCapacity for differentiation into other cell types from the same embryonic germ layer or to transdifferentiate into cells from a different germ layer [2]

The replication of stem cells is referred to as asymmetric, where one daughter cell retains stem cell properties and the other daughter cell undergoes differentiation, thus maintaining the stem cell population in the tissue [1]

The different types of stem cells are classified based on their capacity for differentiation, as follows [Table 1]: [3] {Table 1}

Totipotent cells: These are stem cells capable of generating an entire organism; this property is exhibited by embryonic stem cells.

Pluripotent stem cells: These are found in embryonic, fetal and, to some extent, in adult tissues; they can potentially differentiate into cells of ectodermal, mesodermal, and endodermal origin (germ layers).

Multipotent stem cells: These differentiate into cells of different lineages, usually derived from the same germ layer.

Induced pluripotent cells: These are created by retroviral transcription of genes like Oct4, Nanog, Sox2, Klf4, and c-myc from multipotent stem cells or adult somatic cells; these cells have properties similar to that of pluripotent cells. [4]

Somatic cell nuclear transfer: This is a cloning technique where an adult somatic cell nucleus is introduced into a de-nucleated ovum; this ovum then divides to form the entire organism. This procedure could be used to generate human stem cell lines for therapeutic purposes (therapeutic cloning). [5]

Stem cells are also classified based on the tissue of origin [3] as hematopoietic stem cells, umbilical cord blood stem cells, bone marrow mesenchymal cells, adipogenic stem cells, and stem cells from the dental pulp. Stem cells from the dental pulp exhibit predominantly mesenchymal stem cell properties. Dental pulp stem cells (DPSC) from permanent teeth were first isolated by Gronthos et al. in 2000. [6] Subsequently, Muira et al., in 2003, isolated stem cells from exfoliated human deciduous teeth (SHED). [7] Though other sources of dental stem cells have been identified [8],[9],[10] such as apical papilla, periodontal ligament, and the dental follicle, DPSC/ SHED have a distinct advantage owing to their volume, ease of access, and their 'immature' nature, which give them a vast repertoire of differentiation [Figure 1].{Figure 1}

Current research in regenerative medicine is directed towards using stem cells to treat diabetes mellitus, impaired vision, extensive burns, cardiomyopathies, neurodegenerative diseases (Parkinson disease, Alzheimer disease), muscular dystrophy, baldness, and pulp/tooth regeneration. Hematopoietic stem cells and umbilical cord stem cells are being used in the treatment of leukemias and lymphoma. Dental stem cells are novel targets as a source of stem cells for research as:

The stem cell populations in the pulp may be the most viable alternative to the current population since it is only now that umbilical cord blood cryopreservation is gaining momentum;Retrieving the pulp is simple by means of extraction or extirpation of pulp from permanent teeth or exfoliating teeth in children and carries no associated morbidity;A high percentage of stem cells are present in pulp tissue;Dental stem cells are capable of differentiating or transforming into varied cell types such as bone, dentin, cartilage, fat, nerve, and muscle. [11] Dental stem cells have also demonstrated compatibility and attachment to various biomaterials. [12]

 Embryology and Stem Cell Niche in the Dental Pulp



Tooth development is a complex process and begins as early as sixth to eight week in utero. Teeth are formed from the oral ectoderm and ectomesenchyme (derived from the neural crest). The ectoderm forms the enamel organ, which gives rise to the enamel. The ectoderm also interacts with the ectomesenchyme, which results in the formation of the dentin, cementum, and periodontal ligament. As the teeth are formed, a part of the primitive ectomesenchyme is enclosed within the prospective teeth to form the pulp, which can be the future source of stem cells. The formation of the third molar at around the sixth to seventh year is probably the only representation of de novo postnatal development in humans. [13],[14]

In animals like mice, which have continuously erupting incisors, Notch signaling was used to identify a source of stem cells (not available in humans) called epithelial stem cells of the tooth. Notch is a signaling molecule essential for regulating stem cell fate, and its expression is now used to identify stem cells niches in the pulp. [15] The niche is a microenvironment sustaining the stem cells. The cellular signals in the niche determine the fate of the stem cell with respect to its replication/ renewal or differentiation. [16] In animal models, Notch-1 expression was observed in the subodontoblastic layer adjacent to the mature odontoblasts of the pulp and among the perivascular cells. Notch-2 overexpression was observed in the pulpal stroma, and Notch-3 was limited to the perivasular region. Thus, it may be stated that multiple stem cell niches are present within the pulp [Figure 2], which include the cells in perivascular region, the undifferentiated cells in the subodontoblastic layer, and the isolated pockets in the pulpal stroma. [17],[18] {Figure 2}

 Properties of Pulp Stem Cells



Dental stem cells share similarities with bone marrow stem cells (BMSC) by exhibiting similar surface markers [Table 2] and matrix proteins associated with formation of mineralized tissue (alkaline phosphates, osteocalcin, and osteopontin). [6],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30] DPSC have higher proliferation rate (~30%) when compared to BMSC and this has been attributed to the expression of cell cycle mediators like cyclin-dependent kinase-6 and insulin-like growth factor in DPSC. [6] Stem cells from the pulp are capable of differentiating into odontoblastic, osteoblastic, neurogenic, adipogenic, myogenic, chondrogenic, and melanocytic lineages in vitro and of forming mineralized matrix similar to dentin or bone when transplanted into immunocompromised mice; they also exhibit neurotrophism. [6],[7],[11],[30],[31],[32],[33],[34],[35] {Table 2}

SHED cells differ from DPSC on account of their relatively higher proliferation rate, 'rounded' cell morphology, and osteoinductive capacity in vivo. SHED cells are also capable of neurogenic and adipogenic differentiation and, when injected into the hippocampus of immunocompromised mice, have been shown to be capable of surviving the microenvironment. [8]

DPSC have been reprogrammed to iPS which were indistinguishable from ES by the formation embryoid bodies and expressed markers similar to ES. [36] This opens up more avenues for the use of DPSC with ES-like properties but without the ethical, moral, and legal implications associated with the use of ES.

 Potential Applications of Stem Cells from the Pulp



The prospective multipotent properties of DPSC and the stem cell property of transdifferentiation make DPSC ideal targets of research in medicine as well. However, for obvious reasons, experiments to reliably reproduce results in animal models are necessary before translation to human studies.

Govindasamy et al. have recently explored and accomplished transdifferentiation of DPSC to islet cells aggregates (ICA) similar to pancreatic islet cells. The DPSC-derived ICA express dithiozone-positive staining, C-peptide, Pdx-1, Pax4, Pax6, Ngn3, and Isl-1 and have also been observed to release insulin in a glucose-dependent manner, thus confirming functionality in vitro. These initial results show the promising role of DPSC in the treatment of diabetes. [37]

Nesti et al. observed that when DPSC were co-cultured with mouse dopaminergic cells, the dopamine uptake by the mesencephalic cells in mice was not hampered in the presence of rotenone (a pesticide linked to the development of Parkinson disease in humans). This has been attributed to factors like brain-derived neurotrophic factor and nerve growth factor released by DPSC. [38]

Gandia et al. utilized the multilineage potential of DPSC in the treatment of myocardial infarction. Myocardial infarction was induced in nude rats. DPSC were expanded in vitro and improvements in cardiac functions were observed within 4 weeks of intramyocardial injection of the DPSC. [39]

In vitro studies have shown that limbal stem cells and SHED cells express similar markers, and successful repair of scarred rabbit cornea was observed when transdifferentiated corneal cells from dental stem cells were transplanted onto damaged ocular surface. Transplanted dental stem cells may have potential to be used in humans with bilateral corneal damage and/ or total limbal stem cell deficiency. [40],[41]

Kerkis et al. have demonstrated that human SHED cells expanded in vitro and injected into the peritoneum of Golden Retriever dogs with muscular dystrophy (the closest model for Duchenne muscular dystrophy in humans) persist in the host, successfully engraft into host tissue, and differentiate into muscle tissue. The authors concluded that SHED has potential in the management of Duchenne muscular dystrophy. [42]

In attempts at tooth regeneration, epithelial and mesenchymal stem cells seeded sequentially on a collagen scaffold ex vivo have been implanted into the cavity of adult mice. All dental structures were observed with this technique. [43],[44] Though these findings are promising, direct translation to restorative dentistry is not yet feasible since a reliable source of EpSC is yet to be identified in humans, and tooth substitute using stem cell therapy will need to balance the benefits to the patient with the inherent costs and the need of stem cells for other purposes.

Dental pulp revascularization in regenerative endodontics is a procedure where a necrotic immature pulp is first treated by a triple-antibiotic regimen followed by induction of bleeding to help regenerate pulpal tissue, presumably from the stem cell niche, with the blood clot acting as a scaffold. Moreover, it has been shown that DPSC (cultured in vitro) seeded on scaffolds in root fragments formed pulp-like tissue lined by dentin-like tissue subsequent to transplantation into mice. Thus, it may be possible to potentially 'revitalize' root canal-treated teeth. [45],[46],[47]

One of the most promising uses of DPSC is likely to be in bone regeneration. Autologous human DPSC, expanded in vitro on collagen biosponges, have been used to repair mandibular alveolar bone defects following surgical removal of third molars. [48]

 Conclusion



Stem cells of the pulp appear to hold the key to various cell-based therapies in regenerative medicine, but most avenues are in experimental stages and many procedures are undergoing standardization and validation. With the rising involvement of DPSC in regenerative medicine, other concerns that need to be addressed simultaneously include long-term preservation, immunogenic potential, and in vitro expansion.

Long-term preservation of SHED cells or DPSC is becoming a popular consideration, similar to the banking of umbilical cord blood. Studies by Lee et al., Woods et al., and Perry et al. are promising, with about 73%-85% DPSC being isolated from cryopreserved cultures, especially from early passages. It may still be necessary to explore the long-term (>15 years) effect of cryopreservation on the post-thaw yield of DPSC/ SHED in these cultures. [49],[50],[51]

Another area of interest is the immunogenic potential of DPSC/ SHED and their allogenic utilization. It has been shown that MSC have immunomodulatory properties and suppress T-cell proliferation through secretion of factors such as transforming growth factor-β, hepatic growth factors, prostaglandin-E2 and interleukin-10.[52],[53] Studies exploring MSC-natural killer (NK) cell interactions have shown activation of NK cell receptors, including Nkp30, NKG2D, and DNAM-1, with resulting lysis of MSC. [54] Thus, better understanding of the roles of MSC in development and immune modulation are required before non-autologous use can be implemented in therapy. The harvest of dental pulp from the extracted/ exfoliated tooth is currently established protocol. But the process is not inherently without pitfalls. Culture protocols vary from lab to lab and the standardization procedure itself, though cumbersome, needs to be addressed; indigenous standardization of protocols are also needed. DPSC promise attractive possibilities in the realms of restorative dentistry and regenerative medicine. It would be simplistic to believe that novel therapeutic strategies are near at hand. A pragmatic approach, fuelled by better understanding of DPSC biology, cellular interactions, and immunogenic status, along with simultaneous attempts to overcome the challenges in methodology and clinical translation, will lead to significant benefits in the not too distant future.

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