Massachusetts General Hospital grows world’s first biolimb
Team of Massachusetts General Hospital (MGH) investigators make the first steps towards bioartificial replacement limbs suitable for transplantation, offering hope for amputees.
A team of Massachusetts General Hospital (MGH) have reported using an experimental approach previously used to build bioartificial organs to engineer the forelimbs of rats. The limbs had functioning vascular and muscle tissue, and the team provided evidence that the same approach could be applied to the limbs of primates.
In the USA over 1.5 million people have lost a limb. Prosthetic devices have many functional limitations, and amputees with prostheses also encounter problems such as osteoarthritis and dermatologic problems. Another alternative is donor transplants, but this requires life-long immunosuppressive therapy.
This new approach by the MGH team involved removing all cellular materials from the forelimb of a deceased rat and repopulating the limb with the required cells; a method that various teams have carried out to regenerate organs from animal models such as kidneys, livers, hearts and lungs.
“The composite nature of our limbs makes building a functional biological replacement particularly challenging,” explains senior author of the paper Dr Harald Ott (MGH Department of Surgery and the Center for Regenerative Medicine, MA, USA). “Limbs contain muscles, bone, cartilage, blood vessels, tendons, ligaments and nerves – each of which has to be rebuilt and requires a specific supporting structure called the matrix. We have shown that we can maintain the matrix of all of these tissues in their natural relationships to each other, that we can culture the entire construct over prolonged periods of time, and that we can repopulate the vascular system and musculature."
Ott and colleagues perfused a detergent solution through the vascular system to remove all cellular materials from forelimbs removed from deceased rats in order to retain the cell-free primary vasculature and nerve matrix. While this was being carried out, populations of muscle and vascular cells were grown in culture.
The research team then injected a suspension of muscle progenitor cells into the cell-free matrix of a decellularized rat limb and cultured the forelimb in a bioreactor, which provides a nutrient solution and electrical stimulation to support and promote the growth of new tissues. In the bioreactor vascular cells were injected into the limb’s main artery for regeneration of the vasculature of the forelimb and muscle progenitors were injected into the matrix sheaths of each muscle.
After 5 days in culture, electrical stimulation was applied to the potential limb graft to further promote muscle formation, and after 2 weeks, the grafts were removed from the bioreactor and underwent functional analysis.
Electrical stimulation of muscle fibers caused them to contract with 80% of the strength observed in newborn animals, and the vascular systems of the bioengineered forelimbs quickly filled with blood which continued to circulate when transplanted into recipient animals. Furthermore, electrical stimulation of muscles within transplanted grafts resulted in flexion of the wrists and digits.
The research team also successfully decellularized baboon forearms to confirm the feasibility of using this approach on the scale that would be required for human patients. According to Ott, the next challenge is successfully regrowing and integrating nerves with the receipient of the transplant, but the experience of patients who have received hand transplants is encouraging: “In clinical limb transplantation, nerves do grow back into the graft, enabling both motion and sensation, and we have learned that this process is largely guided by the nerve matrix within the graft. We hope in future work to show that the same will apply to bioartificial grafts. Additional next steps will be replicating our success in muscle regeneration with human cells and expanding that to other tissue types, such as bone, cartilage and connective tissue.”