After suffering a wound, the human body automatically goes into “fix-it” mode. It works to grow new tissue and typically leaves a layer a scar tissue to protect the injured area. While the scar tissue is meant to prevent further injury, which it does, it also prevents further growth.
At the moment, that’s a moot point since humans and mammals cannot regenerate limbs should an injury occur. But, in new research at Tufts University, scientists have made monumental strides in regrowing the legs of a frog, moving humans a step closer to the goal of regenerative medicine.
The researchers stopped the growth of scar tissue and triggered the regenerative process in African clawed frogs by enclosing the wound in a wearable bioreactor loaded with drugs, which they call the BioDome.
“Mammals and other regenerating animals will usually have their injuries exposed to air or making contact with the ground, and they can take days to weeks to close up with scar tissue,” said David Kaplan, professor of engineering at Tufts and co-author of the study. “Using the BioDome cap in the first 24 hours helps mimic an amniotic-like environment which, along with the right drugs, allows the rebuilding process to proceed without the interference of scar tissue.”
The Biodome contains a silk protein gel loaded with a specific five-drug cocktail. Each drug has its own purpose, from tamping down inflammation to inhibiting the production of collagen which would lead to scarring, and encouraging the new growth of nerve fibers, blood vessels and muscle.
According to the study results, published in Science Advances, the researchers observed dramatic growth of tissue in many of the treated frogs, re-creating an almost fully functional leg. The new limbs had bone structure extended with features similar to a natural limb’s bone structure, a richer complement of internal tissues (including neurons), and several “toes” grew from the end of the limb, although without the support of underlying bone.
The regrown limb also responded to stimuli, and the frogs were able to use it effectively for swimming—moving much like an uninjured frog would.
In previous work, the same team of researchers demonstrated significant limb regrowth using the BioDome loaded with just one drug—progesterone. However, the limb grew as a spike and was not functional.
“It’s exciting to see that the drugs we selected were helping to create an almost complete limb [in the new study],” said Nirosha Murugan, research affiliate at the Allen Discovery Center at Tufts and first author of the paper.
Interestingly, the BioDome and drug cocktail was placed on the injured limb for only 24 hours before it successfully triggered a months-long regeneration process. Murugan and her team believe this suggests frogs—and perhaps other animals—have dormant regenerative capabilities that can be spurred into action.
Delving into that hypothesis, in the days post-BioDome treatment, the researchers noted the activation of known molecular pathways that are normally operating in a developing embryo to help the body take shape. Activation of these pathways could allow the burden of growth and organization of tissue to be handled by the limb itself—similar to how it occurs in an embryo—rather than require ongoing therapeutic intervention over the many months it takes to grow the limb.
Next, the research team will be testing how the treatment could apply to mammals, as well as tinkering with the five-drug cocktail to see if different combinations can lead to even more complete limbs, with normal digits, webbing and more detailed skeletal and muscular features.
“It’s a strategy focused on triggering dormant, inherent anatomical patterning programs, not micromanaging complex growth, since adult animals still have the information needed to make their body structures,” concluded Michael Levin, director of the Allen Discovery Center at Tufts and associate faculty member of the Wyss Institute.