Scientists at the National Institute of Dental and Craniofacial Research (NIDCR), one of the National Institutes of Health, and their colleagues have isolated human postnatal stem cells for the first time directly from the periodontal ligament, the fibrous, net-like tendon that holds our teeth in their sockets.
The scientists also say these cells have "tremendous potential" to regenerate the periodontal ligament, a common target of advanced gum (periodontal) disease. This enthusiasm is based on follow up experiments, in which the researchers implanted the human adult stem cells into rodents, and most of the cells differentiated into a mixture of periodontal ligament -including the specific fiber bundles that attach tooth to bone - and the mineralized tissue called cementum that covers the roots of our teeth.
"The stem cells produced beautifully dense, regenerated tissue in the animals," said Dr. Songtao Shi, a senior author on the paper and an NIDCR scientist. "That was when we knew they had great potential one day as a treatment for periodontal disease, and we're continuing to follow up on this promise with additional animal work." The results are published in the current issue of The Lancet.
Shi said scientists have suspected since the 1970s that the periodontal ligament might contain its own unique stem cells. But, for a variety of technical reasons, the search had come up empty, leaving some to wonder whether stem cells could be extracted from such a tiny bit of tissue known to contain a confusing mixture of cell types and subsets.
About two years ago, Shi said he and his colleagues decided to take a stab at the problem. They obtained 25 newly extracted third molars, or wisdom teeth, and gently tugged the ligament free from the root of the tooth. Through trial and error, the group successfully extracted, sorted, and cultured the various cells from the tissue, hoping they had isolated stem cells in one of their many Petri dishes.
The scientists didn't have to wait long to get their answer. They noticed numerous rapidly dividing colonies that had the general characteristics of stem cells. According to Dr. Byoung-Moo Seo, an NIDCR scientist and lead author on the study, the group confirmed their observation by detecting two proteins (STRO-1 and CD146/MUC18) that are known to reside on the surface of mesenchymal stem cells, the general type of postnatal stem cell from which those in the periodontal ligament would be developmentally derived.
Seo said they also detected an unusually high level of a gene-activating protein (Scleraxis) that is specific to cells in tendons, further suggesting they had periodontal ligament stem cells. "It's amazing how well these cells replicate," said Seo. "In fact, we had over 100 doubles of the stem-cell colonies in culture. That's similar to the replication rate of dental pulp stem cells, which are known to proliferate very rapidly."
After further validation of their findings, Shi said he and his colleagues decided to pursue the next big question: Could these stem cells actually form periodontal ligament and cementum when transplanted into mice?
Of the 13 transplants - each of which was derived from a distinct colony of stem cells cultured in the laboratory and loaded into a hydroxyapetite carrier - eight produced a dense mixture of cementum and periodontal ligament. Interestingly, the cells even produced fibrous structures similar to the so-called Sharpey's fibers, which insert into both cementum and bone to hold teeth in place. The other five transplants showed no signs of differentiation.
Shi said his group is now following up on this finding in larger animals. If successful, Shi said he would be eager to evaluate their regenerative ability in people with advanced periodontal disease, which can be extremely difficult to control with current treatments. "From a clinical point of view, what's nice is these stem cells are easily accessible," said Shi. "In theory, people could one day preserve, or bank, these stem cells when they have their wisdom teeth extracted, opening the door for biology-based regenerative treatment later in life of advanced periodontal disease."
The article is titled, "Multipotent Postnatal Stem Cells from Human Periodontal Ligament," and it was published in the July 10, 2004 issue of The Lancet. The authors are: Byoung-Moo Seo, Masako Miura, Stan Gronthos, Peter Mark Bartold, Sara Batouli, Jaime Brahim, Marian Young, Pamela Gehron Robey, Cun-Yu Wang, and Songtao Shi.