Dr. Gardner is a board certified orthopedic surgeon from the Orthopedic Education and Research Institute. He discussed the emerging research in the field of tissue engineering, which allows engineered transplantation constructs to be designed for regeneration of human tissue. The fabrication of new and functional living tissues requires 3 main components: cells, signals and scaffolds.The required cells are referred to as pre-cursor cells, and these undifferentiated adult stem cells manufacture the extra cellular structural components (also known as the matrix) of the musculoskeletal tissue. These cells (i.e. pre-chondroblast, pre-osteoblast, and pre-fibroblast) can often be found in the tissues being reconstructed. An ideal source of these cells would be readily available, provide easy access to the cells, and have a capacity for self-renewal. In addition, these cells would have to differentiate relatively easily into the cell lineages of interest and they would need to have a minimal capacity to cause tumors or stimulate an immune response. One great example of these cells are mesenchymal cells. These are adult stem cells found in bone marrow and other tissues, such as bone and cartilage. Mesenchymal stem cells can differentiate into adipose tissue, bone, muscle, tendon, cartilage, and marrow stroma. These cells have the benefit of already being in the body, thus embryonic stem cells are not required for therapy.
The required signals are chemotactic, mitogenic, and anabolic to varying degrees. Extracellular signals include growth factors such as: the BMP family, FGF family, TGFb family, PGDF, VEGF, and IGF. BMPs (Bone Morphogenic Proteins) can bind to receptors on the extracellular surface of the cell membrane and stimulate a signal transduction pathway. The cascade of reactions eventually causes a protein in the nucleus to become phosphorylated, which causes the cell to make the matrix. Any cell has the capacity to be any other cell. The FGF family of growth factors was mostly thought of as an angiogenic factor which works on endothelial cells; however, FGF-1 is also a very potent signal for chondro (cartilage), osteo (bone), and fibro (tendon) cells. These signals are important in angiogenesis, osteogenesis, chondrogenesis, and fibrogenesis, and it is necessary in signaling pathways to build bone.
A scaffold is also necessary to create functional living tissue. These scaffolds can be made with synthetic compounds, such as hydroxyapatite or they can be biologic scaffolds. Some of the issues concerning the efficiency of scaffolds are pore size, biocompatibility, the 3-D nature of the scaffold, and load distribution. Some of the advantages to using a synthetic scaffold are ensured sterility, the use of a standardized product, can be used “off-the-shelf” and it can be customized. Some of the disadvantages are that it is difficult to mimic biologic scaffolds, they are expensive to manufacture, and they can have poor biocompatibility. The other option is to use a biologic scaffold such as xenografts (i.e. porcine collagen patch) or allografts (i.e. human bone putty). Some of the advantages to using biologic scaffolds are that they are inexpensive, biologically active, compatible with growth factors, and the pore size is helpful in vascular in-growth. Some of the disadvantages include a limited supply, cannot always ensure sterility, and they can be immunogenic. The process required to remove immunogenicity may decrease the physical properties of the scaffold. In addition, you need to ensure that there is an adequate blood and nutrient supply.
How do the signals, cells, and matrix interact? The precursor cell is signaled into the matrix, finds a home, and begins to work. Chemotactic and mitogenic signaling molecules (i.e. BMP, FGF, GDF, etc.) bind to the extracellular surface of the cell. Matrix signaling occurs through Integrin receptors, which create an intracellular signaling cascade. Gene expression for matrix synthesis occurs through intracellular actin filaments bound to the extracellular matrix proteins.
Bone, cartilage, tendon, and discs are four types of tissues that need engineering. One example of an issue concerning bone is severe fractures of the tibia (i.e. from motorcycle accidents). 43-100% of these injuries are non-union requiring reconstructive surgery and they often result in poor blood supply, bone loss, and infection. The standard treatment for such injuries would be debridement, fixation, and a bone graft when the tissue is stable. However, an alternative treatment suggested by Dr. Garner would be to add 1.5 mg/mL rhBMP-2 (signal) and absorbable collagen sponge (scaffold). The FDA approved rhBMP-2/ACS for use in April 2004. BMP also has uses in spine surgery. BMP-2 could be used for anterior fusion (Medtronic, “Infuse”) and BMP-7 could be used for posterior fusion (Stryker, OP-1).
Another example that Dr. Gardner discusses is injuries to the articular cartilage. These areas have poor repair potential and very few precursor cells. This cartilage receives its nutrition from subchondral bone and synovial fluid, and injuries to this cartilage can result in osteoarthritis. Full thickness defects can be fixed with drilling the subchondral defect, mosaicplasty, or with an osteochondral transplant. Full thickness injuries can also be fixed using autologous chondrocyte transplantation (ACT). In this procedure, ten million cells are grown in culture and are then injected back into the defect (Carticel “Genzyme”). Growth factors can also be used to fix local cells. FGF-2 stimulates new (de novo) cells at injury site in culture. In equine cells, it was found that there was a significant increase in progenitor cells after only seven days in culture. In addition, it has been found that liposome encapsulated TGFb healed partial-thickness defects though migration of local precursor cells.
Although orthopedic tissue regenerative studies are already in the process of developing products that are affecting millions of lives, there is still a lot of room for additional development in this area. A few of the future directions that Dr. Garner discusses are making bone products less expensive/more effective, further development of articular cartilage treatments and tendon and ligament engineering. The key point in his discussion is that no embryonic stem cells are necessary for these therapies.
