Three transforming advances will be highlighted: (1) saliva as a diagnostic and informative fluid, (2) tissue engineering, and (3) tooth regeneration.
SALIVA AS A DIAGNOSTIC AND INFORMATIVE FLUID
During the last decade, enormous progress has been made in identifying informative biomarkers found in saliva, and also in identifying oral buccal epithelial cells from which DNA can be extracted for human genomic studies and diagnostics.
Today, saliva is being used to harvest oral buccal epithelial cells and to thereby isolate genomic DNA representing 23,000 human genes and to be able to ascertain single nucleotide polymorphisms informa- tive for complex human diseases. Beyond genomics, saliva is also informative for biomarkers that indicate viral infections such as measles, mumps, rubella, hepatitis A and B and, of course, HIV-1 and -2. Direct antigen detection is also available for influenza A and B, streptococcus group A (N-acetylglucosamine), sali- vary estradiol, several breast cancer biomarkers (e.g., CA-15, epidermal growth factor receptor (EGFR), cathepsin-D and Waf 1), zinc-binding cystic fibrosis antigen, and several markers for type 2 diabetes. Beyond the hundreds of different types of oral bacteria species as well as yeast organisms, many hormones and drugs such as aldosterone, cortisol, estrogens, insulin, melatonin, progesterone, testosterone, carbamazepine, lithium, methadone, phenytoin antipyrine, threohil- line, caffeine, cocaine, continine, ethanol, marijuana, and various opiates are now identified and measured in saliva. And the list continues with the addition of informative messenger RNAs (so-called ‘‘transcrip- tome’’ representing human saliva).
Further, miniature devices for ‘‘point-of-care’’ are being developed for salivary diagnostics using micro- fluidics, microcapillary electrophoresis, and other nanobiotechnology advances.
TISSUE ENGINEERING
As clinicians, it is our opportunity to translate the fundamental discoveries of science and technology, as are now rapidly occurring in stem cell biology, molecular biology, genomics, functional genomics,
and biomaterials/biotechnology/information technol- ogy, into concrete advancements for our patients and communities. This approach provides a bridge over which clinicians can traverse to learn how to apply scientific advances in tissue engineering and regenera- tion to their clinical practices.
The convergence between stem cells and biotech- nology has made it possible to envision the design and fabrication of biological tissues applicable to a large number of human diseases and disorders. Three dec- ades ago, the first tissue engineering proof of principle was established with bone marrow transplantation for myelogenous leukemia. Less than 10 years ago, the first cartilage tissue engineering product was approved by the FDA and is being used for a variety of knee applications.
Tissue engineering is a contemporary field of bio- medical therapy focused at developing and improving procedures and complementary biomaterials for the design and fabrication of new tissues to replace damaged tissues and is based upon the principles of cell biology, developmental biology, molecular biol- ogy, and bioengineering. Tissue engineering is pre- sently focused on tissue replacements such as bone, cementum, dentin and enamel as well as periodontal ligament, oral mucosa, muscle and nerves. Recent progress has literally been astonishing.116–136, 139–143
TOOTH REGENERATION
The prospects for tooth regeneration are enormously promising.144During the past 50 years, a foundation has been provided for understanding the evolution, developmental biology, histogenesis and differentia- tion, and extracellular matrix-mediated biominerali- zation of vertebrate tooth organs. Based upon the knowledge of biological principles, it is becoming readily feasible to design and fabricate tooth organs in the not too distant future.144
Is the knowledge base and techniques presently avail- able sufficient to design and fabricate tooth organs for tooth replacement applications in clinical dentistry? Does the knowledge gained from experimental embryology, autotransplantation of teeth, human genetics, adult stem cell biology, and the identification of human and mouse morphoregulatory genes for tooth morphogenesis lead to biomimetic designs and fabrication for tooth regeneration? The answer to these questions would appear to be affirmative. For example, the National Institutes of Health recently released a Request for Applications to establish Centers for Excellence in Regenerative Medicine with tooth organ regeneration being one of the suggested models. It is becoming readily apparent that despite
many of the recent clinical advances in mechanical solu- tions to tooth restoration and dental implants, or the advancements of autotransplantation of human teeth for tooth replacement, the twenty-first century provides a number of unique opportunities to enable biological solutions to biological problems including tooth organ regeneration. The recent convergence of the human genome completion,136–138 scientific advances toward understanding the molecular regulation of tooth morphogenesis,144 stem cell biology,125,126,140,141 nano- technology, and biotechnology offer unprecedented opportunities to realize tooth regeneration. One of the next critical steps will be to apply the knowledge of molecular regulation of tooth morphogenesis to manip- ulate adult stem cells in becoming odontogenic pheno- types.
Significant progresses have been made in stem cell biological research, which has advanced our under- standing in the area of hematopoiesis, tissue engineering (e.g., bone, cartilage, and muscle), and biomaterials. Recently, studies have shown that adult stem cells have a much higher degree of developmental potential than previously thought, and this has prompted considera- tions to explore the potential of stem cell-mediated muscle, bone, cartilage, and dentin regeneration.
Stem cells are truly remarkable. They have the potential to grow into an array of specialized cells and hold great promise for treating medical and den- tal conditions, such as missing teeth. Conventionally, these pluripotent cells are divided into embryonic and adult stem cells. The difference is mainly in the num- ber of types of differentiated cells that can be pro- duced by the stem cell. When exploring the potential of stem cell-mediated tissue regeneration, recent stu- dies have focused on the application of adult bone marrow because it is readily accessible and contains both hematopoietic and stromal fibroblast stem cells.120–129 Systemically injected mouse bone mar- row-derived cells have given rise to muscle, cartilage, bone, liver, heart, brain, lung alveolar epithelium, intestine, and, of course, hematocytes. Although most of these animal studies now serve as precursors for future human clinical trials in treating certain medical and dental conditions, the basic scientific principles learned from these current analyses have certainly advanced our understanding of the biological regula- tion of tissue engineering. In contrast to previous paradigms, postnatal stem cells have a much greater potential when it comes to tissue regeneration.
To date, there is no study demonstrating bone marrow-mediated tooth regeneration. This might be attributed to an as yet incomplete understanding of proper manipulation of adult somatic stem cells and
several other confounding issues. The knowledge of molecular regulation of tooth morphogenesis will sig- nificantly enhance the focus on the importance of signaling pathways when studies begin to explore the regulatory issues of progenitor cell differentia- tion.144–147 Accordingly, as studies begin to discover the precise molecular regulation of stem cell prolifera- tion and differentiation, bone marrow-derived pro- genitors hold great promise in tooth regeneration.116
Meanwhile, odontogenic stem cells have been identi- fied, isolated, and tested from adult dental pulp tissue.117,130,133,148After being transplanted into immu- nocompromised mice, dental pulp stem cells regenerate dentin-like tissue; curiously, the quantity of regenerated dentin tissue appears to far exceed the dentin tissue generated in situ during the lifetime of a particular tooth organ. Thus, odontogenic stem cells isolated from adult dental pulp can serve as a resource to regenerate large amount of dentin tissue that can be used in tooth repair. Collectively, a new paradigm is emerging, which suggests an understanding of common mechanisms that are critical in regulating the fate of stem cells. These discoveries also reveal several signaling net- works that underlie the specific regulatory properties of tooth morphogenesis. Indeed, there remain many questions and technical obstacles before stem cells will be used to regenerate teeth. However, there are several significant lines of evidence to consider, which are as follows:
1. Enamel organ epithelia and dental papilla mesenchymal tissues contain stem cells during postnatal stages of life
2. Late cap stage and bell stage tooth organs contain stem cells
3. Odontogenic adult stem cells respond to mechan- ical as well as chemical ‘‘signals’’.
4. Presumably, adult bone marrow as well as dental pulp tissues contain ‘‘odontogenic’’ stem cells 5. Epithelial–mesenchymal interactions are prerequi-
site for tooth regeneration.
The authors express ‘‘guarded enthusiasm,’’ yet there is a little doubt that adult stem cell-mediated tooth regeneration will be realized in the not too distant future. The advances in basic, translational, and pre-phase 1 clinical trials are impressive. In tan- dem, the changing demographics within industrial nations indicate an increasing societal demand for ‘‘body parts’’ to address the quality of life issues for aging populations. The prospects for tooth regenera- tion could be realized in the next few decades and could be rapidly utilized to improve the quality of human life in many nations around the world.
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