The periodontal tissue is composed of four major tissues: periodontal ligament (PDL) tissue, cementum, alveolar bone, and gingival tissue. PDL tissue is composed of heterogeneous cell populations, various extracellular proteins including collagens, elastic system fibers, fibronectin, osteocalcin, proteoglycans, vitronectin, and matricellular proteins, nerve fibers, and blood vessels [1]. PDL cell population includes fibroblasts that are principal cells in PDL tissue [2,3], PDL stem cells (PDLSCs), epithelial cell rests of Malassez that are descendants of the Hertwig’s epithelial root sheath (HERS), endothelial cells, and others. Especially, PDLSCs have the potentials to differentiate into PDL fibroblasts, cementoblasts and osteoblasts [4-6]. Aged PDLSCs decrease tissue regenerative potential, but they can be restored by the extrinsic microenvironment, suggesting the retention of their plasticity [7]. Thus researchers have tried to characterize PDLSCs for generation and regeneration of the periodontal tissue in the last decade.

The periodontal tissue is composed of four major tissues: periodontal ligament (PDL) tissue, cementum, alveolar bone, and gingival tissue. PDL tissue is composed of heterogeneous cell populations, various extracellular proteins including collagens, elastic system fibers, fibronectin, osteocalcin, proteoglycans, vitronectin, and matricellular proteins, nerve fibers, and blood vessels [1]. PDL cell population includes fibroblasts that are principal cells in PDL tissue [2,3], PDL stem cells (PDLSCs), epithelial cell rests of Malassez that are descendants of the Hertwig’s epithelial root sheath (HERS), endothelial cells, and others. Especially, PDLSCs have the potentials to differentiate into PDL fibroblasts, cementoblasts and osteoblasts [4-6]. Aged PDLSCs decrease tissue regenerative potential, but they can be restored by the extrinsic microenvironment, suggesting the retention of their plasticity [7]. Thus researchers have tried to characterize PDLSCs for generation and regeneration of the periodontal tissue in the last decade.

The Cephalic neural crest (CNC) cells are principal in fabricating many mesenchymal structures during craniofacial development, including teeth, bones, muscles, and neurons [8,9]. Some reports described a hierarchy of periodontal lineage segregated from migratory CNC cells through intermediate dental follicle (DF) cells [10-12].

Dental tissue-derived mesenchymal stem cells, such as PDLSCs and dental follicle progenitor cells (DFPCs) were characterized with respect to their feasibility of PDL-like and cementum-like formations [13] . Cementogenesis is indispensable in PDL tissue formation and reformation.

Sonoyama et al. developed the ‘bio-root’ by using PDLSCs and stem cells from apical papilla with tooth root-shaped hydroxyapatite tricalcium phosphate (HA/TCP), and showed the formation of PDL tissue on the surface of the ‘bio-root’ when transplanted in swine [14]. Although they did not show apparent cementum deposition on the HA/TCP, this group improved the ‘bio-root’ by utilizing the PDLSC sheet, showing the defined periodontal tissue structure including cementum-like formation [15].

In addition, PDLSC pellet, PDL cell sheet, PDLSCs cultured with conditioned medium from developing apical tooth germ cells have been introduced to construct cementum/PDL complex in vivo [1618]. A recent study described the pivotal role of HERS that guides root formation in cementogenesis as well as root formation [19].

On the other hand, multifunctionality of signal molecules in fabricating cementum/PDL tissue complex is requisite. The innate features of many factors have been discovered, including activin A [20], angiotensin [21], basic fibroblast growth factor (bFGF or FGF2) [22], bone morphogenetic proteins (BMPs) [23-27], brain-derived neurotrophic factor (BDNF) [28] , calcium [29], connective tissue growth factor (CTGF/CCN2) [30,31], enamel matrix derivative (EMD) [32,33], epidermal growth factor (EGF) [34,35], glial cell-line derived neurotrophic factor (GDNF) [36], growth/differentiation factor-5 (GDF-5/BMP-14) [37], insulin-like growth factor-1 (IGF-1) [27,38], interleukin-11 [39], platelet-derived growth factor (PDGF) [40,41], transforming growth factor-β (TGF-β) [1,42-44], and BMP-2 + nerve growth factor (NGF) [45]. Researchers need to exploit properties of these factors in a single or combined use for periodontal generation and regeneration.

On the other hand, multifunctionality of signal molecules in fabricating cementum/PDL tissue complex is requisite. The innate features of many factors have been discovered, including activin A [20], angiotensin [21], basic fibroblast growth factor (bFGF or FGF2) [22], bone morphogenetic proteins (BMPs) [23-27], brain-derived neurotrophic factor (BDNF) [28], calcium [29], connective tissue growth factor (CTGF/CCN2) [30,31], enamel matrix derivative (EMD) [32,33], epidermal growth factor (EGF) [34,35], glial cell-line derived neurotrophic factor (GDNF) [36], growth/differentiation factor-5 (GDF-5/BMP-14) [37], insulin-like growth factor-1 (IGF-1) [27,38], interleukin-11 [39], platelet-derived growth factor (PDGF) [40,41], transforming growth factor-β (TGF-β) [1,42-44], and BMP-2 + nerve growth factor (NGF) [45. Researchers need to exploit properties of these factors in a single or combined use for periodontal generation and regeneration.

DF is a mesenchymal tissue sac surrounding tooth germ and appears only during tooth development [47]. DF cells show selfrenewal and multipotency to differentiate into osteoblasts, adipocytes, and neurocytes in vitro [48,49]. In addition, DF reportedly possesses various intermediate progenitors that differentiate into PDL fibroblasts, cementoblasts, and osteoblasts [11,12]. Therefore, researchers have pursued potentials of these cells in generating and regenerating cementum/PDL complex.

Kemoun et al. reported cementoblastic differentiation of human DF cells cultured with enamel matrix derivatives [50]. Guo et al. demonstrated the cementum/PDL construction by rat DF cells on the surface of EDTA-treated dentin matrix in vivo using a rat model [51]. Recently an interesting study was reported, showing that when rat adipose tissue-deprived stem cells (ADSCs) were cultured in DF cell-conditioned medium supplemented with an inhibitor of the Wnt pathway, ADSCs differentiated into cementoblast-like cells, suggesting contribution of Wnt/β-catenin pathway in cementogenesis [52]. By contrast, another study showed that HERS regulates osteogenic differentiation of DF cells through the Wnt pathway, and that Wnt3a enhanced the differentiation of DF cells [53]. Thus the roles of the Wnt signal pathway in constructing cementum/PDL complex remain to be solved.

In this context, Oshima et al. reported the revolutionary study concerning the development of the ‘bio-hybrid dental implant’ that is a hydroxyapatite-coated titanium covered by murine DF tissue at ED 18.5, which was detached from developing tooth germ [54]. They demonstrated the development of functional cementum/PDL tissue complex including the formation of the Sharpey-fibers, and innervation around this implant, which allowed to response to orthodontic force, and to restore vertically lost bone defect, indicating features very similar to natural tooth.

The longevity rate in many countries of the world is getting longer. In such circumstances, to avoid tooth loss and to retain ones’ own teeth as many and long as possible lead to maintaining their quality of life. Of course, replacement therapy with osseo-integrated implants is current mainstream, but a statistical study performed in the USA showed that many people desire to keep and use their own teeth during their lifetime even though patients who experienced successful implant treatment [55]. As described above, a report by Oshima et al. opened the way for generation and regeneration of periodontal tissue by using DF cells from tooth germ at temporally-limited stage of development, while it was restricted to murine specimen [54]. Therefore considering clinical practice in patients for structuring human periodontal tissue with much larger size, researchers need to clarify the characteristics of DF cells at 18.5, comparing with those at other stages or PDLSCs, and how they can obtain such DF-like cells from bone marrow mesenchymal stem cells (BMMSCs), induced pluripotent stem (iPS) cells or other mesenchymal stem cell sources (MSCs) because the quantity of PDLSCs localized in PDL tissue is too small to use in clinical use while PDLSCs are naturally eligible (Figure 1). In addition, which bioactive scaffolds and multifunctional signal molecules support the differentiation of these stem cells also remains to be determined. Sufficient assessment of outcomes from integration of these elements from various aspects is needed for restoration of periodontal function.