Many patients seen by orthopedic surgeons would benefit from tissue regeneration. In the U.S., over 33 million injuries occur every year, and about half involve damage to ligaments, tendons and cartilage. Bone has a high capacity for self-repair, but soft tissues do not. Instead, you get scar tissue, arthritis and limited mobility. It is felt that stem cells would provide better tissue regeneration without scarring and related problems. The stems cells used are called mesenchymal, and are harvested from adults, not fetuses.
Bone marrow stem cells can be used to regenerate broken or missing bones. Animals with birth defects affecting their large bones have been successfully treated this way. Other sources of mesenchymal stems cells (MSC) include fat tissue and skeletal muscle. Scientists have found that the stromal vascular fraction of adipose tissue has exciting potential for regeneration procedures, due in part for its ability to build blood vessels as the bone grows. Human bone morphogenetic protein-2 shows great promise for promoting bone ingrowth and storage of osteoinductive molecules. To help this protein generate higher amounts of osteoinductive molecules, scientists have been able to apply gene-splicing techniques that cause overproduction of the desired molecules.
Cartilage is a lot trickier than bone when it comes to regeneration. It has poor intrinsic healing capacity, making it a prime target for stem cell-based regeneration. One technique is to stimulate MSC from the marrow to differentiate into cartilage and heal the defect. The results are only so-so, since the repair doesn’t have a lot of strength. Surgeons now prefer replacement over stimulation, using native tissue from nearby healthy locations. However, replacement presents its own set of problems, including:
- Long-term complications
- Damage to the donor site
- Scarcity of replacement tissues
- Tissue mismatches
Other tissues, such as meniscus, ligament and tendon are also under study. The basic idea is to add MSC-loaded scaffolds to the damaged tissues. Some success has been seen with this approach, especially with respect to meniscal repair. Scientists don’t fully understand all of the functions provided by MSC, such as their immunomodulatory and tropic capacities. Researchers are excited by the MSC secretions that contain high amounts of cytokines and growth factors. These cells help suppress the body’s natural immune defense, thereby decreasing incidence of tissue inflammation and rejection. At present there are almost 40 clinical trials occurring worldwide to study MSCs as a means for tissue regeneration, with many more in the works.