THIRD DENTITION IN HUMANS : FICTION OR REALITY ?

A couple of years ago I was flipping through magazine as a short note on human third dentition caught my attention. On those no more than five lines lay the hope of tooth renewal. ‘Fantastic!’, I said to myself, ‘but it is too good to be true.’ So, I decided to investigate. Here I share some of my findings.



The action of time
Aging affects all living organisms. Nothing escapes from the action of time. Natural processes make flowers blossom, whereas others make them wither some time later. Natural processes affect teeth similarly.

After teething, toddlers bear bright white pearl-like teeth. Years later, one by one, baby teeth fall to give place to stronger permanent teeth that are supposed to endure the rest of human lives. In the dawn of humankind, two dentitions were enough. Humans did not live long, so permanent teeth lasting 30 or 40 years would accomplish their function.



As human lifespan extended, human teeth could not keep pace with longevity. Although teeth do not wither like flowers, they show the signs of time, such as yellowness, wear, and lack of resistance, and, sometimes, they fall. With age, gums tend to recede, overexposing teeth that lose stability and can also fall. Certain life habits can potentialize this process, either an excessive aggressive brushing or the complete lack of brushing can be among possible causes of receding gums. When tooth loss is unavoidable, people can try expensive, painful, and lengthy procedures to implant false teeth or invest in removable dentures and bridges to be able to eat and smile properly for the rest of their lives.

Dental implants
The concept of dental implants is not a new trend in humans. There is evidence of implants in ancient Egyptians, Mayans, and Romans. Back then, instead of a titanium rod attached to an artificial tooth made of plastic or ceramic currently in use, they used iron and shells. Doubtless, we experienced a huge progress in this area, as modern artificial implants are attached directly to the bone and certainly grant more stability than shelves. However, problems such as bone loss around the implant are still common and can eventually lead to implant loss and permanently prevent users from undergoing a second procedure.

Dentition patterns in animals
Considering this scenario, wouldn’t it be wonderful if after losing teeth they just pop up again? This is the reality of many vertebrates. Sharks, snakes, and lizards have continuous tooth replacement. Sharks, in particular, have also several rows of teeth continuously replaced.

Unfortunately, as mammals evolved, they lost the capacity of multiple dentitions that extinct mammals had. Most mammals have only one dentition (monophyodonts), as in rodents, or two dentitions (diphyodonts), as in humans, during their life. There are exceptions, though, the kangaroo is among the few mammals that kept multiple dentitions (polyphyodonts). By comparative studies between humans and other animals, it is possible to collect information on molecular and cellular mechanisms related to tooth development.

Third dentition in humans
Mammals, and, therefore, humans still bear a precursor of a third dentition that could develop into permanent teeth. Now and again individuals bearing an extra set of teeth are reported, their prevalence in the population ranges from 0.2% to 0.8% in baby teeth and from 0.5% to 5.3% in the permanent dentition, with a higher incidence in males than females.

Besides, more than 20 human syndromes are associated with extra teeth. The study of those syndromes, among them, the cleidocranial dysplasia (CCD) and the familial adenomatous polyposis (FAP) shed some light on the process of inhibition of a third dentition in humans. Individuals suffering from those syndromes bear genetic mutations in two genes related to presence or absence of teeth.

In CCD, the Runx2 gene is affected. This gene is a cell growth inhibitor and controls the dental lamina (a structure where the primordium of the tooth develops) and the formation of successive dentitions. Unfortunately, affected individuals suffer from serious skeletal anomalies, such as scoliosis, osteoporosis, and underdeveloped collarbones.

In FAP, the APC gene can also induce the appearance of supernumerary teeth, but as it is a tumor suppressor gene, its malfunction causes several polyps in the large intestine and a predisposition to the development of colon cancer in affected individuals.

By studying those syndromes and the genes involved in them, scientists noticed that the repression or activation of a gene candidate could induce the development of a new tooth. Experiments successfully induced supernumerary teeth in genetically modified mice, using suppressed APC or Runx2 genes.

For the accomplishment of an extra dentition, gene therapy uses a gene-delivered technique to activate of repress locally endogenous dental cells. Basically, it inserts genes into cells using a vector to promote the desired effect, which in this case is tooth formation. Although the use of viral and non-viral vectors are possible, the former is more efficient. Among the most common viral vectors used are the retrovirus, adenovirus, and adenoassociated virus. Vectors can be inserted directly into cells (in vivo) or harvested cells removed from the individual (ex vivo). In the latter form, harvested treated cell undergo subsequent implantation. Due to the poor availability of ideal cells, this form is not very efficient.

In spite of the enormous leap in basic science, there is still a long way to go in practical application due to several reasons. First, the suppression of APC or Runx2 genes usually causes disastrous effects on humans. Second, there are more than 200 genes expressed in tooth development. Third, a mouse can be a good study model, but it differs from humans in many aspects. Further studies are required to advance the knowledge of these and other genes involved in tooth development to enable practical application in the future.

Until this technology is not available at the nearest dentist, we might have to count on implants. The good news is that there has been an outstanding progress in this area too.

Although the outcome of a perfectly managed implant resembles natural teeth more than dentures and bridges in external appearance, they are not yet able to accurately reproduce the tooth root. Once in the mouth, the natural movement of teeth during mastication cause friction and can commonly lead to bone resorption. Bone loss compromises implant stability and can eventually result in its fall. To make matters worse, the loss of bone may make a new implant impossible.

The biotooth
With this concern in mind, scientists started working on alternatives that could produce an artificial tooth more similar to the natural one. The idea came to life in the form of the biotooth, a concept of an artificial embryonic tooth primordium created from cultured cells using the own patient cells that could be transplanted back into the mouth as cell pellets and then develop into a functional adult tooth.

The process to achieve that goal is complicated, some specific tooth-inducing human cells are necessary, and they are not easy to find in adults. In one of the recent successful attempts, scientists grew separately in the laboratory human gingival epithelial cells willingly removed from dentists’ patients and mesenchyme tissue removed from mice embryo. After those cells had undergone enough expansion, scientists combined them by placing a thin layer of epithelial cells on the top of mesenchymal cells. The mesenchymal cells responded to epithelial tooth-inducing signals triggering tooth formation and the cell aggregation generated tooth-like structures (tooth primordia). The next stage was the transplantation of tooth primordia to kidney capsules of adult mice. After some time, 20% of the tooth primordia developed into whole functional teeth composed of dentin, enamel, and early stages of roots.

Sources of epithelial and mensenchymal tissue comprising inducing and responding cells are mandatory to induce tooth formation in this procedure. Human induced pluripotent stem cells could be a promising alternative to producing mesynchymal and epithelial tissue.

While the promise of a whole brand new third dentition remains a distant dream, the biotooth is a reality, as scientists successfully reproduced it in other mice, pigs, and dogs. It is very likely that the current generation takes advantage of this technology, which is claimed to be clinically available in 15 or 20 years.

I want it all, and I want it now
Although it seems to be a long time for waiting, it is a short period in science considering the creation of a new technology. A rushed transition process from basic science to its applications can lead to disastrous outcomes.

A good example to illustrate that is the case of thalidomide. In the late 1950s, that drug was largely prescribed to pregnant women for matinal sickness, supposedly provoking no side-effects. At that time, drug test procedures were not as rigid as they are today and tests in mice did not reveal the teratogenic effects of the medicine, as mice metabolized the drug differently from humans. As a result, thalidomide was released into the market causing countless cases of baby malformation.

Nevertheless, this tragedy also brought positive accomplishments. Nowadays new medication and medical procedures must undergo a series of controlled phases before market release. Each research phase, each achievement in science demands careful documentation so that peers can replicate it. Those peers analyze and criticize the study probably pointing out flaws or providing new ideas for the development of the research. Unthought aspects usually come to light in the process, and further research is required. The process goes on until the new technology is proven to be safe enough for regular use to assure no new thalidomide case appears in the future.

Even so, this does not mean approved new technology is side-effect free, but that scientists properly documented all side-effects they observed. In science, there is no universal truth. Science is as dynamic and ever changing as the world where people create it.

Science gives birth to science fiction ideas and makes them real. Organ transplantation, in vitro fecundation, and so many other achievements, were only present in older generation’s fiction but are common events now. In the next generation, biotooth or some marvelous novelty will be probably available at the next door dentist. Until then, we let science follow its course.

Suggestions for further reading:
Angelova Volponi A., Kawasaki M., Sharpe P. T., 2013. Adult Human Gingival Epithelial Cells as a Source for Whole-tooth Bioengineering, Journal of Dental Research, 92: 329–334.
Sartaj, R., & Sharpe, P., 2006. Biological tooth replacement. Journal of Anatomy, 209 (4): 503–509.
Takahashi K, Kiso H, Saito K, Togo Y, Tsukamoto H, Huang B, Bessho K. Feasibility of gene therapy for tooth regeneration by stimulation of a third dentition. In: Gene Therapy-Tools and Potential Applications. Rijeka, Croatia: InTech, 2013; pp.727-744.
Wang X.P, Jiabing Fan J., 2011. Molecular genetics of supernumerary tooth formation. Genesis 49 (4):261–277.
Whitlock J.A., Richman J.M., 2013. Biology of tooth replacement in amniotes. International journal of oral science 5 (2): 66-70.
Zhang Y., Chen Y., 2014. Bioengineering of a human whole tooth: progress and challenge. Cell Regeneration 3(1): 8.

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