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Stem cells’ source, differentiation potential and potency impact their use in the spine
Stem cells have the ability to generate an unlimited number of cells and the potential to differentiate into cell types needed for disc therapy or spine fusion.
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Preclinical and clinical studies using stem cells to enhance spine fusion and treat disc degeneration are underway in the United States and internationally. Cells used in these cell therapy techniques can come from various sources, however each type of cell and its properties have pros and cons for spine and other clinical applications.
According to S. Tim Yoon, MD, PhD, member of the Orthopedics Today Editorial Board, this area of stem cell therapy is expanding. “Bone marrow transplantation is probably the most widely used cell therapy at the moment to treat a variety of cancers, but [cell therapy] may have a bright future in spine fusion and disc therapy, as well,” he said at the North American Spine Society 2009 annual meeting, in San Francisco.
During a symposium, he presented stem-cell terminology and discussed the potential and limitations of various types of stems cells.
Source matters
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Stem cells used to treat diseases come from several sources. Autologous cells, such as those used in autologous chondrocyte implantation, a cartilage repair technique, are from a patient’s own tissue. Allogeneic cells come from another human, while cells originating from another species are called xenographic cells.
“Trans-differentiated cells are … autologous cells that have been trans-differentiated into a different type of cell line,” Yoon said, using the example of autologous hepatocytes that are trans-differentiated to produce insulin.
Autologous cells show real promise, according to Yoon. “Autologous cells are non-immunogenic and they may really be the best cells, if you can transplant them.”
Their downsides include morbidity that may occur at the donor site with cell harvest and an often required period of ex vivo cell expansion, which can add time and cost to their use.
By comparison, allogeneic cells have no associated harvest morbidity and can be mass produced in a laboratory and stored until needed, but they present immunogenicity and disease transmission risks.
Stem cells: pros and cons
“Stem cells are undifferentiated cells,” with almost a limitless capacity to self-renew, divide and recreate themselves, Yoon said.
An obstacle to their more widespread use is the ethical concern over harvesting embryonic stem cells. Adult stem cells, one alternative, are obtainable from bone marrow, fat, muscle, skin, discs and other sites.
Mesenchymal stem cells (MSCs) can differentiate into various types of cells, such as osteoblasts, chondrocytes and endothelium, thus avoiding any ethical problems.
“The advantages of MSCs, even allogeneic stem cells, are they are less immunogenic because of [an] immunosuppressive micro-environment they can create. Therefore, it is a nice target for commercial use of allogeneic stem cells,” Yoon said.
He discussed cell potency and differentiation, noting that tissue regeneration using pluripotent embryonic stem cells is appealing because, under the right conditions, they grow and differentiate into MSCs, hematopoietic or neural stem cells, and many other cells types, but for adult MSCs to differentiate depends on the presence and actions of “cueing agents.” Transforming growth factor beta, for example, is an agent needed for MSCs to differentiate into chondrocytes. — by Susan M. Rapp
Reference:
- Yoon TS. Basic science of stem cells: What are they, where do they come from? Presented at the North American Spine Society 24th Annual Meeting. Nov 11-14, 2009. San Francisco.
- S. Tim Yoon, MD, PhD, can be reached at 59 Executive Park South, Suite 3000, Atlanta, GA 30345; 404-778-7155; e-mail: tim.yoon@emoryhealthcare.org.
