A team of scientists from Tufts University and the University of Florida designed a device that can induce partial hindlimb regeneration in adult frogs by “kick-starting” tissue repair at the amputation site. The researchers used adult aquatic African clawed frogs (Xenopus laevis) for their study. They created a wearable bioreactor attached to the wound site of the frogs to deliver progesterone locally for a 24-hr period and observed that it had a lasting beneficial effect on tissue regrowth, allowing the frogs to partially regenerate their hindlimbs.
“At best, adult frogs normally grow back only a featureless, thin, cartilaginous spike,” says senior author Michael Levin, developmental biologist at the Allen Discovery Center at Tufts University. “Our procedure induced a regenerative response they normally never have, which resulted in bigger, more structured appendages. The bioreactor device triggered very complex downstream outcomes that bioengineers cannot yet micromanage directly.”
The African clawed frog can actually regenerate limbs, but only in its youth during the tadpole and froglet stages. As they become adult, they lose this ability altogether. When adult aquatic African clawed frogs regrow limbs, the result is usually simple, small, and spike-like, lacking the complicated structures of its original limb. With this research, the scientists wanted to explore how new limbs can be generated in organisms that have lost, or never had, the capability.
The bioreactor was 3D printed with silicon and filled with hydrogel – a sticky glob of polymers. The researchers then laced the hydrogel with hydrating silk proteins that promote healing and regeneration, then added progesterone – a hormone well known for its role in preparing the uterus for pregnancy, but has also been shown to promote nerve, blood vessel, and bone tissue repair.
The frogs were divided into 3 groups: Experimental, Control, and Sham. For the experimental and sham group, the team sutured the device on the frogs immediately after limb amputation. In the experimental group, the bioreactor released progesterone onto the amputation site. In all cases, the devices were removed after 24 hours.
When the researchers observed the frogs in the experimental group at different time points over 9 ½ months, they noticed that the bioreactor appeared to trigger a degree of limb regeneration not observed in the other groups. Instead of a typical spike-like structure, normally seen in these types of frogs, the bioreactor treatment resulted in a paddle-like formation closer to a fully formed limb and were still more functional than the spike-like protrusion.
“The bioreactor device created a supportive environment for the wound where the tissue could grow as it did during embryogenesis,” says Levin. “A very brief application of bioreactor and its payload triggered months of tissue growth and patterning.”
Moving forward, the researchers hope to expand the treatment to mice, while improving their cocktail and bioreactor, before setting their sights on human subjects.
“We believe [this system] will help develop a solution to limb loss. Someday, humans might wear a ‘bio-dome’ and induce regenerative repair of missing, damaged, or malformed limbs,” said Levin.