biology Review Insight into the Role of Dental Pulp Stem Cells in Regenerative Therapy Shinichiro Yoshida 1,* , Atsushi Tomokiyo 1 , Daigaku Hasegawa 1, Sayuri Hamano 2,3, Hideki Sugii 1 and Hidefumi Maeda 1,3 1 Department of Endodontology, Kyushu University Hospital, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan; [email protected] (A.T.); [email protected] (D.H.); [email protected] (H.S.); [email protected] (H.M.) 2 OBT Research Center, Faculty of Dental Science, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan; [email protected] 3 Department of Endodontology and Operative Dentistry, Faculty of Dental Science, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan * Correspondence: [email protected]; Tel.: +81-92-642-6432 Received: 20 May 2020; Accepted: 5 July 2020; Published: 9 July 2020 Abstract: Mesenchymal stem cells (MSCs) have the capacity for self-renewal and multilineage differentiation potential, and are considered a promising cell population for cell-based therapy and tissue regeneration. MSCs are isolated from various organs including dental pulp, which originates from cranial neural crest-derived ectomesenchyme. Recently, dental pulp stem cells (DPSCs) and stem cells from human exfoliated deciduous teeth (SHEDs) have been isolated from dental pulp tissue of adult permanent teeth and deciduous teeth, respectively. Because of their MSC-like characteristics such as high growth capacity, multipotency, expression of MSC-related markers, and immunomodulatory effects, they are suggested to be an important cell source for tissue regeneration. Here, we review the features of these cells, their potential to regenerate damaged tissues, and the recently acquired understanding of their potential for clinical application in regenerative medicine. Keywords: dental pulp stem cells; stem cells from human exfoliated deciduous teeth; regenerative therapy; cell-based therapy; regenerative medicine 1. Introduction Mesenchymal stem cells (MSCs) have been reported to be isolated from various somatic tissues such as bone marrow [1], adipose tissue stromal vascular fraction [2], umbilical cord [3], and placenta [4]. They are noteworthy for their robust self-renewal capacity and multilineage differentiation potential including osteogenic, adipogenic, chondrogenic, myogenic, neurogenic, and tenogenic [5–8]. Thus, cell-based medicine using MSCs is expected to facilitate tissue regeneration in various somatic disorders. MSC-like populations have also been identified in dental tissues. Gronthos et al. initially identified dental stem cells from the human pulp tissue and termed dental pulp stem cells (DPSCs) [9]. Subsequently, dental MSC-like populations from other dental tissues were identified and characterized: Stem cells from human exfoliated deciduous teeth (SHEDs) [10], periodontal ligament stem cells (PDLSCs) [11], dental follicle precursor cells (DFPCs) [12], and stem cells from apical papilla (SCAPs) [13]. Among these stem cells, DPSCs and SHEDs are more attractive as cell sources for regenerative medicine than other MSC-like populations because of their easy accessibility. Dental pulp is an unmineralized connective tissue that is composed of stem cells as well as fibroblastic cells, capillary blood vessels, peripheral nerves, lymphatic elements, and extracellular matrices and odontoblasts on the periphery of pulp tissue. DPSCs are isolated from adult human Biology 2020, 9, 160; doi:10.3390/biology9070160 www.mdpi.com/journal/biology Biology 2020, 9, 160 2 of 24 Biology 2020, 9, x 2 of 24 pulpcharacterized tissue of impactedas cells with third intense molars, clonogenicity, orthodontic teeth, proliferative and supernumerary activity, and teeth, the andability have to beenform characterizedmineralized nodules as cells with [9] (Figure intense 1). clonogenicity,proliferative activity,and the ability to form mineralized nodules [9] (Figure1). Figure 1. Isolation of dental pulp stem cells (DPSCs) and stem cells from human exfoliated deciduous teeth (SHEDs). DPSCs and SHEDs isolated from pulp tissue of permanent teeth and deciduous teeth, respectively,Figure 1. Isolation have the of potentialdental pulp to bestem cell cells sources (DPSCs) in regenerative and stem cells medicine. from human exfoliated deciduous teeth (SHEDs). DPSCs and SHEDs isolated from pulp tissue of permanent teeth and deciduous teeth, Inrespectively, addition, DPSCshave the have potential been to clarified be cell sources to possess in regenerative multilineage medicine. differentiation potential and to express MSC-related markers [14–17]. Subsequently, the isolation of SHEDs was reported; these cells were characterizedIn addition, DPSCs as having have high been proliferative clarified to activity possess and multilineage the ability todifferentiation differentiate intopotential osteoblasts, and to adipocytes,express MSC-related odontoblasts, markers and [14–17]. neural cells Subsequently, [10] (Figure the1). isolation Like DPSCs, of SHEDs SHEDs was are reported; advantageous these cells in termswere characterized of their ease ofas having isolation high in aproliferative minimally activi invasivety and manner the ability compared to differentiate with MSCs into fromosteoblasts, other tissues.adipocytes, Specifically, odontoblasts, DPSCs and can easilyneural be cells obtained [10] (F fromigure wisdom 1). Like teeth DPSCs, and SHEDs premolars are from advantageous orthodontic in treatment,terms of their while ease SHEDs of isolation can be in isolated a minimally from primaryinvasive teethmanner that compared are lost duewith to MSCs the eruption from other of permanenttissues. Specifically, teeth. Given DPSCs the expressioncan easily be of obtained MSC-related from markers, wisdom multipotency,teeth and premolars origin, from and therapeuticorthodontic potentialtreatment, of while DPSCs SHEDs and SHEDs, can be they isolated have attractedfrom prim substantialary teeth attentionthat are inlost terms due ofto theirthe translationeruption of intopermanent clinical application.teeth. Given Becausethe expression of their of MSC-like MSC-rela characteristics,ted markers, multipotency, they are suggested origin, to and be therapeutic promising cellpotential sources of forDPSCs the regenerationand SHEDs, orthey reconstruction have attracted of substantial various somatic attention tissues, in terms including of their those translation in the cranio-maxillofacialinto clinical application. region. Because Thus, stemof their cell MSC-like biology usingcharacteristics, these cells they has becomeare suggested an important to be promising research fieldcell sources for understanding for the regeneration and implementing or reconstruction regenerative of various medicine. somatic tissues, including those in the cranio-maxillofacialThe aim of this review region. was Thus, to summarize stem cell biology and compare using these the features cells ha ofs become DPSCs andan important SHEDs, and research their potentialfield for understanding to regenerate damaged and implementing tissues, as wellregenerative as future medicine. prospects of cell reprogramming application for cell-basedThe aim regenerativeof this review therapies. was to summarize and compare the features of DPSCs and SHEDs, and their potential to regenerate damaged tissues, as well as future prospects of cell reprogramming 2.application Self-Renewal for cell-based and High regenerative Growth Capacity therapies. of DPSCs and SHEDs The ability to self-renew is one of the essential defining features of stem cells. This ability is divided2. Self-renewal into two and types High according Growth to Capacity how they of self-renew. DPSCs and One SHEDs involves generating daughter cells with developmentalThe ability potential to self-renew similar is to one that of of the originalessential stem defining cells (symmetricfeatures of division),stem cells. while This theability other is involvesdivided generatinginto two types both according undifferentiated to how cellsthey andself-renew. cells fated One to involves differentiate generating (asymmetric daughter division) cells [with18] (Figuredevelopmental2). Consequently, potential stem similar cells to canthat expand of the origin duringal development,stem cells (symmetric be maintained division), in somatic while the tissues, other andinvolves participate generating in tissue both regeneration. undifferentiated cells and cells fated to differentiate (asymmetric division) [18] DPSCs(Figure have2). Consequently, been clarified stem to show cells robustcan expand proliferative during development, activity. Gronthos be maintained et al. revealed in somatic that single-colony-derivedtissues, and participate strains in tissue of human regeneration. DPSCs could proliferate beyond 20 population doublings [14]. MokryDPSCs et al. alsohave measured been clarified population to show doublings robust ofprolif 11 dierativefferent activity. human DPSCGronthos lines et [19 al.]. Interestingly,revealed that allsingle-colony-derived lines were capable strains of growing of human beyond DPSCs Hayflick’s could proliferate limit and beyond the highest 20 popu populationlation doublings doubling [14]. numberMokry et was al. also 81. measured These results popula stronglytion doublings suggested of 11 the different high self-renewal human DPSC ability lines of [19]. human Interestingly, DPSCs. all lines were capable of growing beyond Hayflick’s limit and the highest population doubling number was 81. These