Skin has been re-engineered using several different approaches with varying degrees of success. The body's own attempts at repair are imperfect and result in scar formation rather than replacement of an organized epidermis and dermis. In skin, the regenerative cell is the keratinocyte, and sheets of these cells have been produced in culture for use as grafts to treat burns and ulcers. Keratinocytes are very versatile in that they don't express the histocompatibility proteins that can trigger immune rejection, and they can be stored for immediate availability. However, a major disadvantage is that the keratinocyte sheets lack a dermal component, which affects graft stability and cosmetic outcome. Great strides have been made in creating an artificial skin with both epidermal and dermal components. Apligraf is one of the most advanced organ constructs to date. The material is a bi-layered skin equivalent, with a keratinocyte sheet that covers a dermal layer formed from fibroblasts that have been cultured in a collagen matrix. Dermagraft is another engineered skin substitute that has been approved by the Food and Drug Administration and shows promise in treating diabetic ulcers. Dermagraft is a single-layered material made from fibroblasts embedded in a dissolvable mesh.
Oral mucosa is a much more forgiving tissue than skin when it comes to wound healing. The oral mucosa appears to be a "privileged site" in the adult that retains fetal-like ability to heal without scar formation. Mucosal keratinocytes can be grown in culture and have been shown to retain the "memory" of the donor site -- attributes that could prove extremely valuable for bioengineering tissues of the mouth and other mucosal body areas. Mucosal cells removed from a patient's mouth and cultured in the laboratory could have applications for repairing surgical damage from periodontal treatment or large mucosal defects resulting from cancer surgery.
Virtually every soft tissue in the body is a potential target for bioengineering and progress is occurring rapidly on many fronts. In the works is the first lab-grown cornea, made from corneal epithelial cells seeded into a polyvinyl alcohol hydrogel. Functional heart valves have been made by molding a synthetic mesh into the proper shape and then seeding it with cells ( fibroblasts and endothelial cells) isolated from blood vessels. The fibroblasts form the structural matrix of the valve and the endothelial cells form the surface coating. A similar approach is being used to make "tubular scaffolds" that show great promise for blood vessel replacement.
In a recent advance in this area, scientists at the Massachusetts Institute of Technology seeded a tubular polyglycolic acid scaffold with bovine smooth muscle cells and grew it for 8 weeks in a culture chamber called a bioreactor. Not just any bioreactor -- this one subjected the developing vessel to a pulsating pressure of 165 beats a minute, mimicking the em-bryonic heartbeat. The final product, after an internal coating with endothelial cells, was a vessel that functioned well in dogs and resisted the clotting that has plagued other cultured vessels. The pulsing made the tissue stronger and more pliable by increasing the cells production of collagen.
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Synthetic meshes are also being seeded with specialized cells in an attempt to go one step further to not only create a living structural replacement, but a metabolically functional tissue. In addition to skin, this approach has been applied to smooth and skeletal muscle, liver, kidney, pancreas, intestines, and even neural tissues. Harvard University scientists were the first to synthesize a whole functioning organ. It was a softball-sized bladder made from a polyglycolic acid "ball" seeded with dog smooth muscle cells and bladder endothelial cells. After being cultured with the right mix of growth factors, the bladders were transplanted into beagle dogs. Remarkably the bladders worked -- the scaffolding eventually disappeared, leaving a leak-proof, functioning bladder that linked up with blood vessels and even became wired to nerves. The same Harvard scientists are growing human bladders in the lab and are seeking approval for clinical trials.
Tissues of any size need a blood supply to bring in nutrients and carry out waste products. Engineers are addressing this problem by building matrices that house growth factors that attract blood vessel ingrowth. What may be an even better approach involves substituting growth factors with progenitor cells that would sprout vessels from within the body of the matrix. Internally derived vessels would need only to link with surrounding vessels to establish a ow-through circulation.
The "Holy Grail" of the tissue engineer is to grow large, vascularized solid organs such as the liver. Such structures require very complex scaffolds filled with channels that would allow uid to ow and blood vessels to develop inside the tissue. Scientists at MIT are using a three-dimensional printing system to construct scaffolds a layer at a time, complete with a complex network of internal channels. Early results look promising structures seeded with liver endothelial cells and cultured for 5 weeks set up shop and not only looked like liver tissue but made albumin just like real liver.
Other novel approaches are being directed to making more "intelligent" matrices. One NIDCR grantee is experimenting with a technique called "cytoscription." This method is based on using highly specific monoclonal antibodies to isolate specific cell types and then applying a computer model to microposition the cells in an "anatomically correct" tissue template. The method is being developed for cells of the periodontium and oral mucosa, but has application for virtually any tissue in the body.
Exciting progress is being made with some of the least regenerative tissues in the body -- the brain and peripheral nerves. Injuries to the peripheral nervous system can result in partial or total paralysis of the affected tissues. Nerve injuries can result from trauma, disease, and certain surgical procedures. For example, surgery to remove tumors from the base of the skull often requires cutting through the facial nerve. Peripheral nerves can regenerate to a degree if the cut ends are surgically reconnected or joined by a nerve graft. However, current clinical treatments have led to incomplete recovery of motor and sensory function. Animal studies are under way looking at nerve "conduits" to enhance regeneration of severed peripheral nerves. An NIDCR grantee is working with a model based on a degradable collagen tube filled with a collagen-glycosaminoglycan matrix.
Neural stem cells have been isolated from rat and mouse brains and have shown encouraging results for repairing damaged spinal cords. Even the brain itself shows promise for tissue regeneration. Embryonic brain cells taken from human or pig fetuses have been transplanted into the brains of patients with Parkinson's disease, resulting in new nerve growth and signs of clinical improvement. Not long ago, the first laboratory grown adult human nerve cells were transplanted into the brain of a 62-year-old stroke victim in an attempt to reverse the woman's brain damage. Interestingly, the cells were not derived directly from brain tissue, but from a line of testicular tumor cells. The tumor cells were treated with chemicals and induced to differentiate into nerve cells. These early studies are laying the groundwork for tissue engineering approaches for restoring function to the brain and the myriad nerves that radiate to the face, jaw, body, and limbs.
The tissues of the head and face offer particularly challenging targets for bioengineers. The craniofacial region is powered by a complex network of muscles and nerves that are fed by a rich supply of blood vessels. The head and face are also home to a number of unique organs and tissues. In addition to teeth and periodontal tissues, there is the most complex joint in the body -- the temporomandibular or jaw joint and specialized structures like the tongue and salivary glands.
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