Mesenchymal stem cells in periodontics: new perspectives

Tissue engineering is a contemporary field of science, which aims to create conditions based on principles of cell and molecular biology, bioengineering and biomaterials to regenerate tissues. Mesenchymal stem cells present high proliferation rates and are able to differentiate into multilineages under certain conditions, suggesting that they have great potential to act in regeneration field. Tooth derived stem cells are a suitable alternative source of mesenchymal cells once they are easily accessible and have poor morbidity to the donor. Studies showed that they have been isolated and characterized from diverse tissues such as dental pulp, exfoliated deciduous teeth, periodontal ligament, gingiva, dental follicle and apical papilla. However studies show that there is heterogeneity among these populations and there is no standard method to select the most appropriate tooth derived stem cells for regenerative procedures. The aim of this review is to present the current perspective of the multiple types of tooth-derived stem cells and to discuss the basis for their use in periodontal tissue engineering. Indexing terms: Periodontics. Stem cells. Tissue engineering.


INTRODUCTION
Tissue engineering is an emerging field, which aims to guide formation, repair, and vascularization of organs by using biological and physical principals.The basic components for tissue engineering involve the interaction of three factors: scaffolds, signaling molecules, and cells.This interactive triad targets the production of functional and biocompatible conditions for tissue regeneration 1 .Stem cells (SCs) are an increasing subject once they are a way to regenerate injured tissues and should improve the treatment of some illness that so far has no resolution such as diabetes and Parkinson's disease.They can be obtained dental pulp [8][9][10][11][12] , exfoliated deciduous teeth [13][14][15][16][17] , periodontal ligament, dental follicle [18][19][20] , apical papilla [21][22][23][24] , periodontal ligament of deciduous teeth [14][15][25][26][27] and gingival tissue stem cells 28 . This rview article proposes to summarize the literature regarding the current knowledge about stem cells from dental tissue, and their potential in regenerative therapy.

TOOTH DERIVED STEM CELLS (TDSCS)
Variable methodologies are used to isolate and characterize TDSCs.A summary is represented in Table 1.
therapy proposed a minimal criteria to define mesenchymal stem cells phenotype, which include be plastic adherent in standard cutures, expression of CD 105, CD 73 and CD 90 but not CD45, CD34, CD14 or CD11b, CD79a or CD19; major histocompatibility complex class II surface molecules and the potential to differentiate into osteoblasts, adipocytes and chondroblasts.Also they must present fibroblast-like spindle shape in culture 7 .Dental tissues are a good alternative source of MSCs, once they are easily accessible with insignificant or no morbidity of the donor site.Various types of tooth-derived stem cells (TDSCs) have been isolated from dental tissues, which include

Dental Pulp Stem Cells (DPSCs)
In 2000, Gronthos and collaborators were the pioneers on isolation and characterization of DPSC, the first TDSCs 8 .When compared with human bone marrows stem cells (BMSCs), DPSCs showed higher proliferation rate and greater capacity to form mineral nodules, so they are more appropriate for regeneration of mineralized tissues than BMSCs.They are able to differentiate into osteoblast, smooth muscle cells, adipocyte-like cells, neuron, dentin, dentin-pup-like complex and endothelial like cells 8,29 .

Stem cells from human exfoliated deciduous teeth (SHED)
SHEDs are progenitor cells first isolated in 2003, from the remnant pulp of exfoliated deciduous teeth 13 .They showed a higher proliferative rate, when compared to BMSCs and DPSCs 13,15 , and a higher capability to differentiate in osteoblast and adipocyte-like cells when compared to DPSCs in vitro 15 .They also showed the capability do differentiate into odontoblast, neural cells 15,30, .

Stem cells from apical papilla (SCAPs).
SCAPS are cells isolated from apical papilla located on the root apex of developing teeth 31 .It is distinct from the pulp tissue 32 .They presented a higher proliferation, migration and telomerase activity.They are able to differentiate into osteoblastic, odontoblastic, adopocyte -like and neuron-like cells under specific induction 22 .A cDNA microarray profiled comparative analysis between SCAP and DPSCs concluded that genes such as CD24 and survivin were highly expressed in SCAPS 22 .

Dental follicle progenitor cells (DFPCs)
DFPCs are cells obtained from dental follicle which is a condensation of cells originated from the ectomesenchyma that surrounds the tooth germ in early stages of tooth formation.It contains a heterogenic cell population that forms the periodontium 18,33 .They can differentiate into osteoblast, adipocyte, chondrocyte and neuronal cells, but they present differences on proliferation and mineralization patters which suggests that they could commit in distinct lineages 33 .

Periodontal ligament stem cells (PDLSCs)
PDLSCs are a heterogeneous cell population with neural crest cell origin.They have higher proliferation rate but forms less mineralized nodules when compared to BMSCs.They present the ability to differentiate into osteoblasts, cementoblasts, adipocytes, chondrocytes and endothelial like cells.In vivo experiments confirmed the ability to form periodontal ligament and cementum-like tissue 34 .

Decidous periodontal ligament cells (DePDL)
Periodontal ligament cells also can be isolated from decidous teeth.It showed higher proliferative rate than PDLSCs, and share the same ability of differentiate into osteoblasts, cementoblasts, adipocytes and chondrocytes, but with a higher potential to differentiate into adipocytes 25 .

Gingival tissue Stem cells (GSCs)
GSCs are obtained from gingival connective tissue, so the sample must be deepithelialized, to leave only connective tissue.They are able to differentiate into osteogenic, chondrogenic and adipogenic lineages.It also present an immunemodulatory capacity 28 .

Induced pluripotent stem cells (iPSCs)
iPS cells are derived from somatic cells via transduction and expression of selective transcripition factors.They can differentiate into all derivatives of the 3 primary germ layers 35 .They can be obtained from stem cells of apical papilla, dental pulp, exfoliated deciduous teeth stem cells, gingival, periodontal ligament and buccal mucosa fibroblast.They have the ability to differentiate into mesenchymal stem cells, neural crest-like cells, ameloblastlike cells, odontoblast-like cells and osteoprogenitor like cells.Although iPS cells are an option without any ethical concerns and it has a great potential towards regeneration of periodontal ligament, alveolar bone, cementum and dentin-pulp complex, issues like epigenetic memory, viraltransduction, tumorgenesis and teratoma formation has to be further investigated 35 .

REGENERATIVE APPLICATIONS OF TDSCS
Basic components for tissue engineering includes scaffolds, signal molecules and cells.Scaffolds are tridimensional structures, which mimics the extracellular matrix and must have physical, chemical and biological characteristics to provide a microenvironment for cell signaling activation, and stimulation of cellular growth, differentiation, cell adhesion and migration.The cells provide synthesis of extracellular matrix and tissue regeneration.MSCs presents important characteristics such as high proliferation rates and ability to differentiate into multilineages, therefore they have a great potential into tissue engineering field.
Several studies demonstrated that TDSCs have successfully regenerated dental tissues such as dentin, pulp and periodontal ligament 12,22,[47][48][49] .In vivo experiments demonstrated that human PDLSCs and SCAP were able to generate periodontal ligament in minipigs 22 .Also DPSCs promoted complete pulp regeneration in dogs 12 .Combination of iPS cells with silk scalffold and enamel matrix promoted PDL regeneration in mouse periodontal fenestration defects 35 .
Although the literature confirms the potential of TDSCs in regeneration, there are some aspects that must be discussed.First, proliferation capacity, clonogeniticy and differentiation ability are different from each type of TDSCs lineages suggesting that it has an association with the type of original tissue.Even in the same population there are heterogeneous cell subpopulation with different behavior 50 .Notwithstanding that International Society for cell therapy defined a minimal phenotype criteria for MSCs, specific surface markers associated with TDSCs commitment are not established.

CONCLUSION
Interest in regeneration topic has increased inside scientific community.TDSCs are potential actors for regenerative procedures once they are an easy available source that presents almost no morbidity to the donor.Studies, which used TDSCs for regeneration, presented promising results.However, MSCs populations obtained from dental tissues are heterogeneous and, currently, there is no standard method to select the most appropriate TDSCs for regenerative procedures.Further studies must be designed to confirm TDSC-based therapies as safe, predictable and reproducible.

Collaborators
BR AMORIM, EA SALLUM, MZ CASATI, KGS RUIZ, RCV CASARIN, KR KANTOVITZ and FH NOCITI JUNIOR participated in all stages of the preparation of the manuscript.