But, about four years ago, this fundamental idea received a serious jolt when scientists reported inducing a small percentage of adult stem cells from the brain to switch into cells from the blood. This led to a series of confirmatory findings that seemed to have settled the question, when two teams of researchers last year dropped the small bombshell that, in their laboratory studies, some stem cells physically fused with other cells. This finding suggested that maybe the brain-to-blood and other stem cell switches, a process that scientists call transdifferentiation, weren't what they seemed. Maybe the stem cells had fused with cells from other tissues all along.
Now, in the current issue of The Lancet, a team of scientists at the National Institutes of Health offers a key piece of new evidence to advance the debate. In a study of five women who had received bone marrow transplants from their brothers several years earlier, the team reports finding cheek cells that contained the male Y chromosome, a sign that some transplanted stem cells had differentiated into cheek cells. Moreover, the group found almost no evidence of fusion among the cells in the cheek.
Because the researchers analyzed cells extracted directly from patients, which previous studies did not for various technical reasons, these data offer strong evidence that transdifferentiation does occur. The study also shows the power of using oral tissues to pursue complex biological questions, an idea that is gaining wider favor among biologists.
"Being so accessible, the mouth is one of the best 'laboratories' in the body to study many issues in human biology that go beyond dental research," said Dr. Bruce Baum, a scientist at the National Institute of Dental and Craniofacial Research (NIDCR) and an author on the study. "This is clearly an excellent case in point."
While the pros and cons of transdifferentiation may sound of interest to academicians only, the issue potentially has profound public health implications. If transdifferentiation is a biological reality, scientists would have a potential inroad to therapeutically manipulate adult stem cells, the long-lived "progenitor" cells that produce the myriad specialized cells in our tissues.
In theory, scientists could gather the most easily obtainable adult stem cells, such as those from the blood, switch them into another type of adult stem cell, then prompt them to produce large amounts of tissue-specific cells. This harvest of specialized cells could be transplanted to heal wounds more efficiently or even possibly to construct replacement tissues, such as a salivary gland or a tooth.
As appealing as this scenario sounds, scientists continue to grapple with the question: How does one conclusively prove that transdifferentiation is possible in adult stem cells? After all, the current data suggest that the phenomenon occurs in only a small percentage of the cells, raising the additional question of whether transdifferentation would be robust enough for therapeutic purposes.
Leaders in the field recently established scientific criteria of proof to guide subsequent experiments. Yet, even with these established criteria as their guide, scientists have struggled with several technical difficulties. For one, much of the work, pro and con, has occurred in cell culture. That is, cells have been extracted from the body and placed into an artificial medium, where they behave differently than in their natural environment, a little like trying to study a fish out of water.
Secondly, the other studies involved cells from tissue sections, which literally provide scientists with only part of the picture. "What happens with other tissues, such as the liver or muscle, is you must slice the tissue into sections to obtain samples," said Dr. Simon Tran, an NIDCR scientist and the lead author on the study. "This means you must overlay parts of the cell to reconstruct them or just analyze a portion of the cell."
Tran said he and his colleagues at NIDCR, in particular Dr. Stanley Pillemer, reasoned that the mucosal cells of the cheek, which help to form the moist tissue that lines the inside of the mouth, might solve these technical problems. He said the cells replicate frequently, meaning a ready supply exists, and they can be collected non invasively from patients. Just as importantly, because the cheek cells haven't been sliced in two during processing, scientists can analyze the entire cell under a microscope.
To test this idea, Tran said he found, through his collaborators at NIH's National Heart, Lung, and Blood Institute, five women who had received bone-marrow transplants several years earlier from their brothers. Tran gathered the cheek cells from each of the women, then returned to his laboratory to confront two basic questions: Could he identify cheek cells that contained both an X and Y chromosome, an indication that the transplanted bone-marrow stem cells had differentiated into cheek cells? If so, could he also show that these cells were indeed functioning as cheek cells?
Tran said that's where he ran into technical difficulties of his own. He needed to create a two-in-one assay that could detect both the Y chromosome and the structural protein cytokeratin, a standard identifier of mucosal cells, such as those from the cheek. "It's been done before," said Tran, "but never on cheek cells."
Tran said he turned for help first to Dr. Eva Mezey, who studies adult stem cells at NIH's National Institute of Neurological Disorders and Stroke, and later to Dr. Amalia Dutra, a researcher at NIH's National Human Genome Research Institute, and Dr. Michael Brownstein, a scientist at NIH's National Institute of Mental Health. "I would have never been able to do this study if I had been in another type of setting," said Tran, highlighting the unique multidisciplinary environment at NIH that allows more complex studies to be undertaken.
Once the assay was up and running, the scientists discovered that all five women had cheek cells that contained both X and Y chromosomes. The range was from 0.8 percent in one woman to 12.7 percent in another. Because some of the women had sons, the scientists performed an additional DNA analysis that ruled out the possibility of the cells originating from their male offspring. The cells also tested positive for cytokeratin.
Interestingly, of the 9,700 cells that were examined in the study, only two showed signs of possible fusion. In the previous reports of cell fusion from celll culture studies, the rate was also extremely low, ranging from one every 100,000 to one million adult stem cells.
While Tran said this paper does not provide the definitive answer to the transdifferentiation debate, it does offer a higher level of evidence. "I hadn't studied stem cells previously," said Tran, who is employing tissue engineering techniques to develop an artificial salivary gland. "But I saw an opportunity to help forward the field with a model that perhaps some had overlooked. I think these data will be helpful to many."
Journal
The Lancet