Healing is slow in part because tendons, ligaments and cartilage lack the interlaced blood vessels that other organs rely on for quick delivery of cells involved in repair and growth factors that encourage cells to thrive and divide. Under these circumstances, the rationale for stem cell therapy seems straightforward: bathe the injury in a healing, concentrated wave of tissue-mending cells, and the body will repair itself that much more quickly.
As so often happens in biology, however, applying a simple idea can quickly become more complicated once you start dealing with the details. For starters, the term “stem cell” describes several different types of cells with different capabilities. Embryonic stem cells, for example, can readily give rise to any type of cell found in the body under the right set of circumstances—and when plied with the right set of biochemical signals. In contrast, a more specialized group of stem cells found in the bone marrow generally begets blood cells and immune cells exclusively. These so-called hematopoietic stem cells (from Greek meaning “to make blood”) have been reliably used over the past 40 years to seed bone marrow transplants in the treatment of some cancers and immune disorders.
By the 1970s researchers had recognized another type of progenitor cell in the adult bone marrow, which they named the mesenchymal stem cell. In a laboratory dish—and, presumably, also inside the body—these cells readily multiply and give rise to various structural cells, such as fat, muscle, bone, tendon, ligament and cartilage. Studies in animals suggest that mesenchymal stem cells play important roles in the body's ability to heal after an injury, although researchers are still working out the signals and steps required to steer their differentiation into one type of tissue or another.
It turns out, for example, that mesenchymal stem cells do not regenerate tissue in isolation. They depend on other cells and growth factors that may or may not be present in a particular region of inflamed tissue, says Rocky Tuan, who directs the Center for Cellular and Molecular Engineering at the University of Pittsburgh School of Medicine. “You can inject all the best cells,” Tuan says, “but if you don't have the right combination of healing goodies around them, it's useless.”
Lab studies are also finding that mesenchymal stem cells extracted from different parts of the body can have different attributes. Those found in fat, for example, though relatively plentiful and easy to extract, do not seem to form cartilage as readily as those that come from bone marrow, Tuan says. Other studies suggest that mesenchymal stem cells also modulate the immune system and may have some part in the spread of tumors. Gathering much more basic information about how these cells behave is a vital first step before any safe and broadly reliable treatment can be developed for people, Tuan and other leading stem cell scientists argue.
Galloping Ahead of the Science
Undeterred, advocates for the immediate use of stem cell therapy in human athletes point to successes with racehorses as the best evidence that the treatment works. Yet some experts say that the same hype that makes human testimonials unreliable has enveloped company-sponsored studies of competitive horses, too.
In the July 2012 issue of Equine Disease Quarterly, Wesley Sutter of Lexington Equine Surgery and Sports Medicine in Kentucky cautioned: “To date, no published controlled clinical studies show efficacy in use of stem cell treatment for any of the conditions being treated.”
Carol Gillis, a longtime veterinarian and researcher who specializes in soft-tissue injuries in racehorses, says that the more than 22,000 ultrasound images she has captured in her studies and clinical practice have convinced her that with a tightly regimented exercise program, tendons and ligaments will heal, producing strong, well-organized fibers—all without the use of stem cells. The reason that many soft-tissue injuries end a horse's racing career, Gillis explains, is because most owners allow the animal to run free too soon, when the pain from the initial injury has faded but the tissue is still fragile.