Maxillofacial tissues are important to individuals. People might have much trouble living confidently when suffering maxillofacial defects, usually caused by congenital anomalies, infections, trauma, and tumor section. Effective reconstruction or regeneration of the damaged part would be beneficial to patients both physiologically and psychologically (Mao and Prockop, 2012; Mertens et al., 2016). Flap transplants combined with dental implants or obturators are currently the main therapy methods to rehabilitate the defected tissues, however, the functions and esthetics still remain to be improved (Mertens et al., 2016; Okay et al., 2016; Wijbenga et al., 2016).

Adult stem cells, also called somatic stem cells or postnatal stem cells (Tatullo et al., 2015), are harvested directly from adults. They are multipotent cells, only differentiating into limited types of cells or mesenchymal origin (Caplan, 1991). Dental tissues including bone marrow, dental pulp, dental follicle, periodontal ligament, apical papilla, and gingiva are good sources of adult stem cells. These cells commonly express specific molecular markers and can be isolated (Table 2). Recently, oral cavity have been paid more and more attention to gaining somatic stem cells as it's more flexible for the surgeons to harvest the tissues without undue trauma to the patients (Mao and Prockop, 2012).

There are different types of somatic stem cells in oral and maxillofacial compartments or “stem cell niches” (Figure 2). They are stem cells of the apical papilla (SCAP), inflammatory periapical progenitor cells (iPAPCs), dental follicle stem cells (DFSCs), dental pulp stem cells (DPSCs), periodontal ligament stem cells (PDLSCs), bone marrow stem cells (BMSCs), tooth germ progenitor cells (TGPCs), salivary gland stem cells (SGSCs), stem cells from human exfoliated deciduous teeth (SHED), oral epithelial stem cells (OESCs), gingival-derived mesenchymal stem cells (GMSCs), and periosteal derived stem cells (PSCs; Liao et al., 2011; Egusa et al., 2012). Most adult stem cells we could get in oral and maxillofacial areas are mesenchymal stem cells (MSCs). Pre-clinical studies of their utilization in treatment of several diseases have been extensively conducted in animal models (Table 2).

iPS cells are the pluripotent stem cells induced through reprogramming somatic cells genetically by introducing defined factors (Takahashi and Yamanaka, 2006). IPS technology is considered to be one of the most significant discovery and may make great contribution to the treatment of diseases. It brings hope to solve the ethical problems encountered by the application of human ES cells and limited material source for both ES cells and adult stem cells. Subsequently, a lot of studies have reported that defined transcription factors can directly induce one cell type converting to another without going through a pluripotent state, further promoting iPS cell technology (Zhou et al., 2008; Szabo et al., 2010; Vierbuchen et al., 2010; Huang et al., 2011; Kim E. Y. et al., 2016). During 10 year studies of iPS cells, extensive studies are focused on the multiple sides of iPS cells including method optimization, clinical application exploration, and the mechanisms of iPS cell reprogramming (Karagiannis and Eto, 2016). Most notably, iPS cell technology have made significant progresses in regenerative medicine that is to replace damaged organ part by establishing a normal functional cells, tissues or the whole organ (Mason and Dunnill, 2008).

In the field of dentistry, for the regeneration of periodontal tissues, alveolar bone, and tooth are urgently needed. As iPS cell technology is considered to have great potentials, researchers are interested in the dental-tissue-derived iPS cells and the application of iPS cells in dental tissue regeneration. It was shown that three kinds of human dental stem cells: DPSCs, SHED, and SCAP, could be induced to generate iPS cells by introducing 3–4 factors with higher efficiency than human fibroblasts (Figure 3; Yan et al., 2010). Mesenchymal stromal cells derived from human third molars also can be reprogramed into iPS cells with high efficiency in a similar strategy (Oda et al., 2010). However, cells such as dental pulp and apical papilla are not convenient to isolate from patients for the donor tissues cannot regenerate. Human deciduous teeth might be discarded after exfoliation, so human deciduous tooth dental pulp cells (HDDPCs) are considered as better iPS cell source (Tamaoki et al., 2010). Using gingival fibroblasts as iPS cell source is another wise choice. Gingiva can be easily acquired from patients with no need of tooth or pulp extraction surgery and have relatively high reprogramming efficiency (Egusa et al., 2010; Wada et al., 2011; Arakaki et al., 2013).

Stem cells are applied without scaffolds in most clinical trials (32/44 of total trials; Tables 3, 4). There are 5 clinical trials with reported results, 6 clinical trials completed without reported results, and 21 trials ongoing without results. Among the clinical trials with results, the MSCs, PDLSCs, and ADSCs are used.

Oral MSCs have been used to promote the dental pulp and periodontal tissue regeneration. In an ongoing pilot study to determine whether MSC injections directly into mucosal lesions in patients with oral cGVHD are able to alleviate the symptoms and facilitate the reparative process, it states that 55% aGVHD patients who failed front-line steroid treatment responded to MSC infusion in the finished phase II clinical trial (NCT02055625). Intravenously infused allogeneic MSCs suppressed autoimmunity by directing T cells toward Treg and Th2, while suppressing Th17 and Tfh responses, and restored salivary gland secretory function in both mouse models and Sjögren's Syndrome patients who have been resistant to multiple standard treatments (NCT00953485). The study suggests that allogeneic MSC treatment may provide an effective therapy for Sjögren's Syndrome patients (Xu et al., 2012). Although results are not available yet, the other three trials with MSCs focused on osseointegration (NCT02731586), and regeneration of bones (NCT02755922, UMIN000011286).

Human PDLSCs, as well as DPSCs, SHEDs, and bone marrow stem cells have been studied to regenerate periodontal tissues (Seo et al., 2004; Ding et al., 2010; Khorsand et al., 2013; Du et al., 2014; Fu et al., 2014). Clinical trial (NCT01357785) identified no clinical safety problems that could be attributed to the investigational PDLSCs and found a significant increase in the alveolar bone height (decrease in the bone-defect depth) in each group but no statistically significant differences between the PDLSC group and the control group (Chen et al., 2016). Another clinical trial showed reduction of pocket probing depth and clinical attachment gain, while greater linear bone gain at PDLSC sheets/β-TCP treated sites with no adverse reaction in the finished four cases of autologous transplantation of PDLSC sheets to the denuded root surface during the modified Widman flap procedure for the treatment of periodontitis (UMIN000005027, https://www.researchgate.net/publication/266802767). Allogeneic human DPSCs are injected in local infected periodontal tissue for the treatment of chronic periodontal disease (NCT02523651) and a clinical trial UMIN000002050 aims to establish human dental pulp stem cell bank for dental diseases that extraction of tooth is necessary for treatment. A clinical trial is ongoing to clarify the efficiency of autologous SHED to regenerate pulp and periodontal tissue in the patients with immature permanent teeth and pulp necrosis (NCT01814436).

ADSCs represent an attractive and ethical cell source for stem cell therapy and have been used in six clinical trials (Table 3). Intramuscular autologous transplantation of ADSCs enhance the survival of fat grafted into the face in patient with progressive hemifacial atrophy (NCT01309061), suggesting ADSC injection may be used to treat Parry-Romberg disease without the need for microvascular free flap transfer (Koh et al., 2012). The three ongoing clinical trials of ADSCs without scaffolds aim to treat periodontitis, radiation induced xerostomia, and cranial nerve dysfunction.

For the application of iPS cells in disease treatment, the first step is to get iPS cells with defined factors. Four factors Oct4, Sox2, Nanog, and Lin28 are sufficient to induce pluripotent stem cells from neonate fibroblasts (Yu et al., 2007). Masato and colleagues found that some family proteins of the four factors Oct3/4, Sox2, Klf4, and c-Myc are capable to reprogram mouse embryonic fibroblasts (MEFs) to iPS cells, and they also found that iPS cells can be generated without Myc although the generation of iPS cells takes more time than with Myc (Nakagawa et al., 2008). More than that, it has been shown that iPS cells can be induced from primary human fibroblasts with Oct4 and Sox2 or be acquired from mouse neural stem cells by introducing only one factor, Oct3/4 (Huangfu et al., 2008; Kim et al., 2009). Nevertheless, the four factors play important roles in the reprogramming, but unique factors may be needed for different cell type.

Four factors Oct4, Sox2, Nanog, and Lin28 or Oct3/4, Sox2, Klf4, and c-Myc carried by viral vectors can reprogram three different dental stem cells: SHED, SCAP, and DPSCs (Yan et al., 2010). It has also been demonstrated that three factors without c-Myc can reprogram adult wild-type mouse gingival fibroblasts (GFs), but generation of iPS cells from primary human gingival fibroblasts need four factors (Egusa et al., 2010; Wada et al., 2011). In dentistry, researchers prefer to adopt the classical Yamanaka iPS cell method. But dental tissues are very different from fibroblasts, and there may be new transcription factors involved in the induction. It was shown that miRNA-720 expressed in DPSCs participates in the regulation of the pluripotency factor Nanog and was involved in reprogramming (Hara et al., 2013; Eguchi and Kuboki, 2016). MSC-like populations are rich in dental tissues, which suggest that dental tissues have great potential to be directly reprogrammed into other tissues using defined transcription factors cultured in specific condition. More studies are needed to optimize iPS cell method for dental tissue reprogramming.

Many endeavors have been made with iPS cells to produce new cementum, alveolar bone, ameloblast, and so on (Figure 3; Duan et al., 2011; Yoshida et al., 2015). More and more researchers are interested in the possibility of tooth regeneration from iPS cells. The first trial to generate tooth-like structure from iPS derived neural crest like cells was failed (Otsu et al., 2012). Shortly, tooth-like structures, with bone-like and dental pulp-like areas, were generated from mouse iPS cells by mixing iPS cells with both embryonic dental epithelial and mesenchymal cells (Wen et al., 2012). Furthermore, it was shown that tooth-like structures, of which chemical composition are similar with human teeth, can be generated from patient's own urine induced pluripotent stem cells (Cai et al., 2013). But generation of whole teeth from iPS cells is still a challenge (Wen et al., 2012). The formation of a tooth germ need the interaction between ectoderm-derived epithelial cells and neural-crest-derived mesenchymal cells, so iPS cells are supposed to differentiate into two lineages, and one of them must be odontogenic (Ning et al., 2010). However, it was showed that ES cell or iPS cell derived dental epithelial cells did not have odontogenic potential (Arakaki et al., 2012). Although, mouse iPS cells have potential to differentiate into odontogenic mesenchymal cells through neural crest-like cells, but no signal of tooth generation was shown (Otsu et al., 2012). More work should be done to regenerate a whole tooth.

Using retroviruses to generate iPS cells is simple and reproducible, and is the first choice in vitro studies. Integrating viruses such as retroviruses or lentiviruses to deliver transcription factors may cause insertional mutagenesis and induce tumorigenesis. New techniques including transduction of plasmids, proteins, and episomal expression vectors which will disappear during culture are emerging (Okita et al., 2008; Eggenschwiler and Cantz, 2009; Yu et al., 2009; Zhou et al., 2009). However, virus-free methods are far less effective. In dentistry, retroviruses and plasmids are favored methods. For clinical application, integration-free methods are essential and more effective integration-free methods are needed.

It was shown that 30% for eight different human iPS cell lines differentiated into tooth-like structures successfully, although it was less efficient compared with 100% differentiation efficiency of mouse embryonic dental epithelium and dental mesenchyme (Cai et al., 2013). It has been questioned always that the iPS cells showed poorer differentiation capacity than ESCs. Neural differentiation of human iPS cells showed lower efficiency than ESCs (Hu et al., 2010) while other groups demonstrated that the ability of human iPS cells to differentiate into motor neurons has no difference with ESCs (Boulting et al., 2011). Although there must be inherent differences among ESCs, MSCs, and iPS cells, culture condition including types of feeder cells, medium, etc. are equally important to successful differentiation. All the conditions should be taken into account to optimize method. Ten years of iPS cell studies are full of surprises and challenges. Tooth regeneration was considered to be an important test ground for application of iPS cells and will meet new surprises and challenges.

Epigenetic modifications including modification of histone, DNA methylation, and regulation of micro RNAs are established to affect the maintenance and differentiation of stem cells. DNA methylation and histone modifications are two primary mechanisms to regulate transcriptions of genes in adult stem cells. Generally, DNA methylation of CpG is associated with transcriptional repression, while DNA demethylation activate gene transcription. The modifications of post-translational histone including methylation and acetylation are major epigenetic regulation in dental pulp stem cells. The functions of miRNAs in epigenetic regulation of stem cells are extensively studied recently. Regulation of miRNA mainly rely on translational inhibition by degradation of targeted mRNA. Several miRNAs such as miRNA-204/211, miR-138, miR-125b, and miR-21 are important regulators involved in the osteogenic or adipogenic differentiation of bone marrow stem cells (BMSCs; Eskildsen et al., 2011; Lu et al., 2013; Hu et al., 2014; Wu et al., 2015). miR-720 was observed to promote odontogenic differentiation in dental pulp stem cells (Hara et al., 2013). Alteration of epigenetic modifications by inhibitors can affect the reprogramming of somatic cells or stem cells fate. It has been demonstrated that inhibition of DNA methyltransferase can increase the reprogramming efficiency of somatic cells dramatically (He et al., 2010). The balance between acetylation and deacetylation can be altered by histone deacetylases inhibitors (HDACi). HDACi including trichostatin A (TSA) and valproic acid VPA can promote proliferation and odontoblast differentiation of hDPSCs (Jin et al., 2013; Paino et al., 2014; Duncan et al., 2016).

Growing evidence shows that epigenetic modifications play a crucial role in the induction of iPS cell. As well as four defined factors, reprogramming of histone modifications are required for iPS cell induction. The situation of histone modification in iPS cells and ESCs in pluripotency associated genes are in the same, but in contrast to the differentiated cells. However, the epigenetic reprogramming in iPS cells is not complete since numerous differences in DNA methylation between human ESCs and iPS cells were observed (Lister et al., 2011). These differences are generated by reprogramming as well as epigenetic memories from origin tissues, and will affect the differentiation and application of iPS cells (Kim et al., 2010). For few knowledge about the epigenetic differences between iPS cells and ESCs, complete reprogramming can't be achieved now. Further efforts should be drawn to this field.

Because of the progresses made recently, it is expected to be no problem to get enough desirable stem cells in near future. The most challenging issue is how to precisely control the differentiation of adult stem cells and iPS cells in vitro and in vivo. To address the issues, the researches could focus on three directions. One is uncovering the differentiation pathways of stem cells to a specific cell type. This step is most important and difficult. The second is to precisely drive the differentiation of stem cells in vitro with the target molecules, cytokines, or transcription factors identified in the clarified differentiation pathways. This step can also check or update the previous findings. The last one is controlling stem cells to develop accurately in vivo. This step, unlike the in vitro differentiation, is very complicated due to the body responses and undefined micro-environment. One ignored issue in wound repair and organ regeneration with stem cells is that the stem cells may migrate from the target area to elsewhere, thus reducing the therapy efficiency. Moreover, the cytokines or differentiating factors used to control stem cell fate will distribute everywhere in the body, which exaggerate the difficulty of precisely direct stem cell differentiation. The 3D print technology and nanotechnology may help solve these issues.

Nanotechnology is defined as the manipulation of matter with at least one dimension sized from 1 to 100 nanometers. Nanomaterials have unique optical, electronic, or mechanical properties and thus show special bioeffects. In a recent study with DPSCs, the ROS-scavenging events of cerium nanomaterials (CeNMs) were related to the aspect ratio-dependent cellular internalization, suggesting the promising use of CeNMs to protect stem cells from the ROS-insult environments and ultimately improve the stem cell potential for tissue engineering and regenerative medicine (Mahapatra et al., 2017). In another study, it is reported that enhanced rapid bone regeneration and complete mature bone-structure formation was obtained when using the physiological electric potential and the fabricated nanocomposite membrane mimicking the endogenous electric potential (Zhang et al., 2016). It provides another strategy to compose novel scaffolds for dental stem cells. And Heo et al. found that gold nanoparticles (GNPs) coated titanium could enhance the osteogenic differentiation in human adipose-derived stem cells (Heo et al., 2016). What's more, nano-thin polymeric shells constructed and modified by nanostructure involvement were used to immobilize the DPSCs with a layer-by-layer technique to ensure the layers stability and integrity as well as separation from bacterial cells (Grzeczkowicz et al., 2015).

3D bioprinting, the combination of 3D printing technology, biology, and materials science, is the process of direct printing of biological materials (cells, proteins, etc.) into a confined space, where biological function and viability of biomaterials are preserved within the printed construct (Murphy and Atala, 2014). One of the three major concepts in 3D bioprinting is the type of bio-ink, which is made up of poly (ethylene glycol; Arcaute et al., 2006; Chan et al., 2010), fibrinogen (Cui and Boland, 2009; Lee et al., 2009) and/or alginate (Fedorovich et al., 2012; Hockaday et al., 2012) and helped to maintain both the biological function and viability of biomaterials and the dimensions of printed objects (Shafiee and Atala, 2016). The combination of nanotechnology and 3D printing technology, allows the creation of sophisticated materials with exquisite fine structural detail and provides fantastic therapy strategies. Figure 5 outlines the strategies used potentially in oral disease treatment. For example, bone stem cells can be administrated in a 3D printed biocompatible and biodegradable scaffold which is made of nanomaterials and matches the bone wound. Stem cells, cytokines and cytokine expressing plasmids can be either used as bioink to directly print an organ-like structure with a bioprinter or loaded on magnetic nanoparticles (Sun et al., 2011) and then precisely immobilized in the target area with magnetic field. The design of novel nanostructured materials, such as biomimetic matrices and scaffolds for controlling cell fate and differentiation, and nanoparticles for diagnostics, imaging and targeted treatment, is a novel direction for stem cell therapy. The administration to patients of dynamic biological agents comprising stem cells, bioactive scaffolds and/or nanoparticles will certainly increase the regenerative impact of dental pathological tissues (Mitsiadis et al., 2012).

In summary, there are endogenous/autologous and exogenous/allogeneic adult stem cells and iPS cells for oral disease treatment. As compared in Table 5, these cells have their own pros and cons features. The features suggest the main research direction of stem cell therapy. For autologous/endogenous stem cells, there are two main research directions: try to expand the stem cells in vitro without stemness loss or function change; try to speed up self-renewal of the in situ stem cells and precisely control their differentiation. For allogeneic stem cells, the in vitro expand technology is also very important and the technology to lower down the immune rejection is urgently needed. Besides the immune suppression drugs, regulatory immune cells such as Treg and Breg (Sun et al., 2015; Xu et al., 2016) may be used to overcome the immune rejection and enhance the efficiency of stem cell therapy. For iPS cells, the biosafety is the main issue that should be addressed due to their high tumorigenesis. The advances of technologies in these directions determine which stem cells could easily reach a desirable number and show highest biosafety and lowest immunogenicity, and therefore would be clinically used successfully.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer VD, VT and handling Editor declared their shared affiliation, and the handling Editor states that the process nevertheless met the standards of a fair and objective review.