Scientists have created the world's first lab-grown limb in a mouse model that could lead to a method of creating complex tissue to be transplanted into humans.
A team of Massachusetts General Hospital (MGH) used an experimental approach to create rat forelimbs that contain functioning vascular and muscle tissue, and demonstrated the same approach could likely be used in primates.
"The composite nature of our limbs makes building a functional biological replacement particularly challenging," said Harald Ott of the MGH Department of Surgery and the Center for Regenerative Medicine, senior author of the paper. "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."
In this new technology, 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 generate organs such as kidneys, livers, hearts and lungs in the past, but this is the first time it has been used to create complex limb tissues.
To create this tissue, the researchers 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 vasculature and nerve matrix in a process that took about a week. The team grew muscle and vascular cells in a culture and injected them into the forelimb matrix, which was also being 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 an analysis confirmed presence of vascular cells along blood vessel walls and muscle cells. Electric stimulations of muscle fibers proved to cause them to contract with a strength of 80 percent of what would be seen in a newborn 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.
The team also successfully decellularized baboon forearms, suggesting the process could be applicable to human patients. The next challenge there researchers will tackle is to grow nerves within a limb graft and reintegrate them into a patient's immune system.
"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," Ott concluded.
The findings were published in a recent edition of the journal Biomaterials.
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