Scientists regrow frog’s leg, bringing limb regeneration a leap closer

 By Mark Waghorn via SWNS

Scientists have regrown a frog's lost leg - bringing limb regeneration a leap closer.

In the movie "Spider-Man," scientist Curt Connors injects himself with a serum based on lizard DNA - and re-grows his amputated arm.

The phenomenon had remained the province of salamanders and superheroes - until now.

A US team used a cocktail of drugs applied in a wearable dome and the elixir was sealed over the stump for just 24 hours.

It set in motion an 18-month period of regrowth that restored a functional leg.

Frogs - like humans - are naturally unable to regenerate limbs.

For millions of patients who have lost limbs for reasons ranging from diabetes to trauma, the possibility of natural regeneration remains out of reach.

The groundbreaking therapy described in Science Advances gives them fresh hope.

First author Dr. Nirosha Murugan, of Tufts University, said: "It's exciting to see the drugs we selected were helping to create an almost complete limb.

"The fact it required only a brief exposure to the drugs to set in motion a months-long regeneration process suggests frogs and perhaps other animals may have dormant regenerative capabilities that can be triggered into action."

Other creatures that can regenerate limbs include starfish, crabs and lizards.

Flatworms can even be cut up into pieces - with each reconstructing an entire organism.

Unfortunately humans are only capable of closing wounds with new tissue growth.

Our livers have a remarkable, almost flatworm-like capability of regenerating to full size after up to half the organ is destroyed.

But loss of an arm or leg cannot be restored by any natural process of regeneration in humans or mammals.

We tend to cover major injuries with a mass of scar tissue, protecting it from further blood loss and infection and preventing further growth.

On the other hand, if a newt or salamander loses its leg, it will grow another.

The wound will close and, over time, it will create new bones, muscles, nerves and skin.

Performing the feat in our own bodies has been a dream for centuries. Such healing powers were first discovered in 1740.

Swiss naturalist Abraham Trembley discovered a green pond animal could regenerate its tentacle-crowned head if it was amputated.

He called it Hydra after the head-renewing monster from Greek mythology.

The researchers triggered the regenerative process in African clawed frogs by enclosing the wound in a silicone cap - called a BioDome.

It contained a silk protein gel loaded with five drugs - each fulfilling a different purpose.

They dampened inflammation and inhibited production of collagen which causes scarring.

They also encouraged new nerve fibers, blood vessels and muscle.

The bioreactor provided a local environment and signals that tipped the scales away from the natural tendency to close off the stump - and toward the regenerative process.

Dramatic growth of tissue was observed 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 counterpart's.

They also had a richer complement of internal tissues, including neurons.

Several 'toes' grew from the end of the limb - although without the support of underlying bone.

The regrown limb moved and responded to stimuli such as a touch from a stiff fiber.

The frogs were able to make use of it for swimming through water - moving much like a normal peer.

The researchers explored the mechanisms by which the brief intervention could lead to long-term growth.

Within the first few days after treatment, they detected the activation of known molecular pathways that are normally used 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.

This is similar to how it occurs in an embryo - rather than require ongoing therapeutic intervention over the many months it takes to grow the limb.

Animals naturally capable of regeneration live mostly in an aquatic environment.

The first stage of growth after loss of a limb is the formation of a mass of stem cells at the end of the stump called a blastema.

It's used to gradually reconstruct the lost body part. The wound is rapidly covered by skin cells within the first 24 hours after the injury - protecting the reconstructing tissue underneath.

Co-author Professor David Kaplan, also from Tufts, said: "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.

"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."

Previous work by the same group showed a significant degree of limb growth triggered by a single drug, progesterone, with the BioDome.

But the resulting limb grew as a spike and was far from the more normally shaped, functional limb achieved in the current study.

The five-drug cocktail represents a significant milestone toward the restoration of fully functional frog limbs.

It suggests further exploration of drug and growth factor combinations could lead to limbs that are even more functionally complete.

They would have normal digits, webbing - and more detailed skeletal and muscular features.

Corresponding author Prof Michael Levin, director of the Allen Discovery Center at Tufts, said: "We'll be testing how this treatment could apply to mammals next.

"Covering the open wound with a liquid environment under the BioDome, with the right drug cocktail, could provide the necessary first signals to set the regenerative process in motion.

"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."