Researchers have created rat forelimbs that contain vascular and muscle tissue, and demonstrated that the same approach could be used for creating human limbs.
Harald Ott of the MGH Department of Surgery and the Center for Regenerative Medicine and the senior author of the study said, “The composite nature of our limbs makes building a functional biological replacement particularly challenging, 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.”
For the creation of the limb, living cells are stripped from a donor organ using a detergent solution, the remaining matrix is then repopulated with progenitor cells specific to the desired organ. The method has been used to regenerate organs such as kidneys, livers, lungs and hearts in the past, but this is the first time it has been used to create a complex limb tissue.
To create the tissue, researchers have used the same decellularization process used in the whole-organ studies. All of the cellular material was stripped from the forelimbs of deceased rats, leaving behind the primary vascular and nerve matrix in a process that took about a week.
The team then grew muscle and vascular cells in a culture and injected them into the matrix of forelimb, which was also cultured in a bioreactor. Inside the bioreactor, the matrix was injected with muscle progenitors at the sheaths that define muscle positioning. Two weeks later, the grafts were removed and the presence of vascular cells along blood vessel wall and muscle cells was confirmed by an analysis.
Electric stimulations of muscle fibers prove to cause them to contract with the strength of 80 percent of what would be seen in a new born animal. After being transplanted into the recipient animals, the vascular system of the limb filled with blood and electric simulation of the muscles caused the limb to flex.
Researchers have also successfully decellularized baboon forearms, suggesting the process could be applicable to human patients.
The next challenge to the researchers will be to grow nerves within a limb graft and reintegrate them into patient’s immune system.
Ott concluded, “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.”