Bioengineering of a human whole tooth: progress and challenge
© Zhang and Chen; licensee BioMed Central Ltd. 2014
Received: 27 June 2013
Accepted: 25 April 2014
Published: 30 April 2014
A major challenge in stem cell-based bioengineering of an implantable human tooth is to identify appropriate sources of postnatal stem cells that are odontogenic competent as the epithelial component due to the lack of enamel epithelial cells in adult teeth. In a recent issue (2013, 2:6) of Cell Regeneration, Cai and colleagues reported that epithelial sheets derived from human induced pluripotent stem cells (iPSCs) can functionally substitute for tooth germ epithelium to regenerate tooth-like structures, providing an appealing stem cell source for future human tooth regeneration.
Stem cell-based tissue engineering is a promising approach to replace or repair lost or damaged tissues or even organs in humans. Two major types of stem cells currently used in tissue engineering research and clinical application are embryonic stem cells (ESCs) and adult stem cells. However, ethical concerns in using ESCs and difficulties in isolation, expansion, and differentiation of adult stem cells limit their clinical application. The advent of iPSCs has provided a promising alternative source of stem cells for tissue bioengineering .
Whole tooth bioengineering has given hope to dental replacement and regenerative therapy [2, 3]. In the last decade, techniques have been established to bioengineer a whole tooth crown from embryonic tooth germ cells in several animal models including mouse, rat, pig, and dog [4–11]. Remarkably, it was shown that re-aggregates of mouse embryonic tooth germ cells can develop into fully functional tooth replacement with revascularization and root formation in the lost tooth site of adult mice [12–14], demonstrating the feasibility of implanting a bioengineered tooth germ in adults for functional tooth replacement.
Tooth development relies on reciprocal tissue interactions between ectoderm-derived dental epithelium and cranial neural crest-derived mesenchyme . In mice, the dental epithelium prior to embryonic day 12 (E12) possesses odontogenic potential (tooth-inducing capability) and is capable of inducing tooth formation when confronted with mesenchymal tissue of non-dental origin . After E12, this odontogenic potential shifts to the dental mesenchyme that becomes capable of inducing tooth formation when recombined with non-dental epithelium. Thus, in vitro generation of a bioengineered whole tooth, which should follow the principles of tooth development, will also require epithelial and mesenchymal cells with either of them possessing the tooth-inducing capability. From the point view of clinical therapy, it is ideal to use patients’ own cells to generate bioengineered replacement teeth. At least five types of human postnatal mesenchymal stem cells of dental origin have been isolated thus far , including dental pulp stem cells (DPSCs), stem cells from exfoliated deciduous teeth (SHED), periodontal ligament stem cells (PDLSCs), dental follicle progenitor cells (DFPCs), and stem cells from the apical papilla (SCAP). However, epithelial stem cells of dental origin do not exist, because ameloblasts are lost via apoptosis upon tooth eruption .
Attempts have been made to identify alternative sources of human postnatal stem cells as the epithelial component for human whole tooth regeneration. It has been demonstrated that keratinocytes isolated from human foreskin and gingival epithelial cells isolated from patients’ gingival tissue are able to differentiate into enamel-secreting ameloblasts when recombined with mouse embryonic molar mesenchyme that possesses odontogenic potential [19, 20]. However, in these studies, epithelial cells were harvested from different individuals. The age, genetic variation, health condition, and even sex of each patient may influence the ultimate outcomes. A consistent cell source as the epithelial substitute would avoid these variations. In a recent issue of Cell Regeneration (2013, 2:6), Cai and colleagues reported their studies on ameloblastic differentiation capability of integration-free human urine-derived iPSCs (ifhU-iPSCs) . They first differentiated ifhU-iPSCs into epithelial cells with differentiation medium, followed by tissue recombination of the induced epithelial sheets with E14.5 mouse molar mesenchyme. Intact tooth-like structure formed in the recombinants after 3-week in subrenal culture in nude mice. Histological and molecular marker analyses demonstrated differentiation of ameloblasts from ifhU-iPSC derived epithelial cells as well as enamel formation. Interestingly, Nano-indentation and Raman spectroscopy analyses further revealed that the enamel deposited by ifhU-iPSC derived epithelial cells had similar physical properties and about 1/3 of hardness as compared to that in normal adult human teeth, suggesting the formation of functional enamel. Considering prolonged normal tooth development and differentiation in humans (about 400 days from the initiation of tooth development to tooth eruption), the formation of such structurally comparable enamel in the bioengineered teeth within 3 weeks indicates an adoption of accelerated developmental and differentiation programs in the human epithelial cells in response to the induction by the mouse embryonic dental mesenchyme, suggesting potential clinical application of accelerated differentiated tissues/organs from human iPSCs. While iPSC-derived cells have been widely tested for their capability in directed differentiation of various cell types, this study by Cai and colleagues appears to be the first in whole organ bioengineering with human iPSCs, indicating a realistic cell source for future in vitro generation or assembly of implantable tooth germs in dental practice.
We thank Dr. Xuefeng Hu for her help with the illustration. Researches in the authors’ laboratories are supported by a “973” Program Project grant from the Ministry of Science and Technology of China and by grants from the National Natural Science Foundation of China (to YZ), and by the NIH (to YC).
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