What if you could regrow a hand?

Your body is good at fixing itself. A scrape on your knee heals up in a few days.

But what if your body could grow a whole new hand?

Think about the last time you scraped your knee. A few days later, new skin had grown over it, as if by magic. Your body is always repairing itself in small ways.

Now imagine something much bigger: what if you could regrow a whole hand, with bones, muscles, skin and fingernails? Some animals can do something like that. In this lab we meet the axolotl, a salamander that can regrow its legs, find out what our own bodies can regrow, and see what scientists are learning from these amazing animals.

Regeneration, according to the National Institute of General Medical Sciences (NIGMS), is the process of replacing or restoring damaged or missing cells, tissues, organs and even entire body parts to full function. All living things can regenerate to some degree, but the range is enormous.

This thought experiment asks what it would take for a human to regrow a hand. It follows three threads: how axolotls rebuild a limb, what human bodies can and cannot regenerate, and what research is revealing. The biology comes from NIGMS, the San Diego Zoo Wildlife Alliance and Harvard University; human limb regrowth itself remains an idea, not a prediction.

Axolotls live in lakes in Mexico. They have feathery gills and a little smile.

They can grow back legs, and even parts of their hearts and brains!

The axolotl is a salamander that keeps its baby features all its life: feathery gills, a tail and a fin along its back. In the wild, it now lives in just a few isolated places in the southern part of Mexico City, where it is critically endangered.

Axolotls are famous for regrowing body parts. The San Diego Zoo Wildlife Alliance says they can regenerate their limbs, lungs, heart, jaws, spine and even parts of their brain. A new leg comes back with all its parts, skin, bone, cartilage and muscle, in a few weeks, without a scar.

Ambystoma mexicanum is neotenic: it retains larval features such as external gills, a tail fin and the lack of movable eyelids into adulthood and remains aquatic. Wild populations now survive at just three isolated sites in southern Mexico City, and the species is critically endangered (San Diego Zoo Wildlife Alliance).

NIGMS lists the axolotl's regenerative range as brain, heart, jaws, limbs, lungs, ovaries, spinal cord, skin and tail. The San Diego Zoo Wildlife Alliance notes that a regrown limb replaces every tissue (skin, bone, cartilage, muscle) within weeks and without scarring. Its genome, at 32 billion base pairs, is about 10 times larger than ours (NIGMS).

When an axolotl loses a leg, a little bump of busy cells grows there.

The cells divide again and again. Slowly, they build a whole new leg!

When an axolotl loses a leg, something special happens. A little bump of cells, called a blastema, gathers where the leg used to be. NIGMS explains that these cells, including stem cells, divide rapidly and build everything the new leg needs: skin, muscle, bone and cartilage.

Harvard University's Whited Lab, which studies axolotls, describes the blastema as made from cells of the stump that turn back into a more flexible, 'young' state, together with stem cells. Step by step, they rebuild a leg that works just like the old one.

Salamander regeneration begins with a blastema: in NIGMS's description, a mass of many types of dividing cells, including adult stem cells, that forms at the injury and then divides rapidly to produce the skin, muscle, bone and cartilage of a new limb or tail.

The Whited Lab at Harvard describes the axolotl blastema as a collection of dedifferentiated cells and stem cells derived from stump tissues, which orchestrate limb regeneration, and notes that the detailed molecular mechanisms are still being worked out. If a human were to regrow a hand, a blastema-like structure, or something that does its job, would presumably be needed; that is our reasoning, not a known pathway.

Our bodies can regrow skin and hair. Broken bones can mend.

But people can't grow a new hand. Our bodies make a scar instead.

Human bodies regenerate too, just less dramatically. NIGMS lists the things we can do: grow new skin and hair, build new bone to knit a broken bone back together, and form scars to heal wounds. One organ is a real star: if part of the liver is removed, the rest grows back to its original size.

So why not a hand? NIGMS explains that most mammals, including humans, don't form blastemas. Harvard's Whited Lab suggests this difference may be why we can't regrow limbs. Instead, our repairs rely on stem cells that replace only the kind of tissue they belong to.

NIGMS lists human regenerative abilities as forming thick scars to heal wounds and burns, building new tissue to knit bone after a fracture, and regrowing hair and skin. Some organs show compensatory growth: the remaining liver regrows to its original size after partial removal; the pancreas, thyroid, kidneys, adrenal glands and lungs compensate in more limited ways.

Most mammals don't form blastemas; their repair relies on adult stem cells that differentiate into the cell types of their own tissue. The Whited Lab notes that mammals do not respond to most amputations by creating blastemas, and that this difference may underlie their inability to regenerate. Repair capacity also declines with age, and the brain, spinal cord, heart and joints regenerate least.

Scientists study axolotls to learn their secrets.

Someday, what they learn might help people heal better.

Scientists around the world study axolotls to understand how regeneration works. NIGMS describes researchers testing how different molecules affect an axolotl's ability to regrow its tail, then investigating whatever stops it working. They've also read the axolotl's genome, its full set of genetic instructions, which is about 10 times larger than ours.

The goal isn't to turn people into salamanders. It's to learn enough to help people heal wounds better, repair damaged organs and perhaps, someday, replace lost limbs. That last step is far off, and nobody knows exactly when, or if, it will happen.

NIGMS describes several research directions: identifying the cell pathways that detect tissue loss and build a blastema, understanding the systems that suppress regeneration and how they might be switched off, and explaining why regeneration declines with age. Axolotl researchers screen molecules for effects on tail regeneration, and the sequenced 32-billion-base-pair genome lets them look for conserved genes linked to regenerative ability.

The field is called regenerative medicine. NIGMS frames its hopes carefully: better treatment of age-related conditions, clues for repairing damaged organs, and 'even, someday, replacing lost limbs.' Human limb regeneration is therefore a long-term aspiration, not a forecast.