Current clinical dentistry is predominantly focused on restorative approaches involving the replacement of teeth with inert materials, but tissue regeneration is an attractive alternative because artificial materials eventually fail and do not provide the same functionality as tissue. David Mooney and his colleagues at Harvard School of Engineering and Applied Sciences (Cambridge, MA) and the Wyss Institute for Biologically Inspired Engineering (Boston, MA) collaborated with biomedical researchers to demonstrate how low-power light can be used to trigger stem cells inside teeth to regenerate dentin, the hard tissue that is similar to bone and makes up the bulk of the teeth. Because lasers are already used in dentistry, it is possible that such light-based treatment could be used in dental regeneration in people. “Our treatment modality does not introduce anything new to the body, and lasers are routinely used in medicine and dentistry, so the barriers to clinical translation are low,” explained Mooney.

Anecdotal evidence suggests laser therapy can reduce pain and inflammation and promote regeneration in cardiac, skin, lung and nerve tissues, effects that are collectively termed 'photobiomodulation.' But a direct link between laser treatment and stem cell biology has not yet been clearly demonstrated.

Mooney and his team drilled holes in the molars of rats. They treated the tooth pulp inside the molars, which contains abundant endogenous adult dental stem cell populations, with non-ionizing, low-power laser exposure and applied temporary caps to the molars. After 12 weeks, x-ray imaging and microscopy confirmed that the laser treatment triggered dentin formation in the molars (Sci. Transl. Med. 6, 238ra69; 2014). The dentin was similar in composition to normal dentin, although with slightly different morphology.

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Next, they carried out cell culture–based experiments to unveil the molecular mechanism responsible for the regenerative effects of the laser treatment. The laser induced reactive oxygen species, which activated the latent transforming growth factor beta-1 (TGF-beta1) complex. TGF-beta1 in turn triggered the dental stem cells to differentiate into odontoblasts, the bone cells that form dentin. This finding is consistent with recent studies highlighting the role of TGF-beta1 as a master regulator of stem cell differentiation. Mice lacking TGF-beta1 or treated with a TGF-beta inhibitor did not form dentin in response to the laser therapy.

The study lays the foundation for other clinical applications in restorative dentistry and regenerative medicine. “We are also excited about expanding these observations to other regenerative applications with other types of stem cells,” said the study's first author Praveen Arany.