The short answer: many animals can regrow lost body parts, but the ability varies dramatically across species. The champions of regeneration include planarian flatworms, which can rebuild an entire body from a small fragment, and axolotls, which regrow complete limbs including bones, muscles, nerves, and skin.
Starfish regrow arms, zebrafish regrow heart tissue and fins, and newts can even regenerate their lenses and retinas. The mechanisms behind these abilities are conserved across species, and scientists study them with the hope of one day applying the lessons to human medicine.
Regeneration is not a single ability. It exists on a spectrum, from simple wound healing to complete rebuilding of complex structures. Some animals can replace nearly any part of their body. Others can manage only a limb or a tail. Understanding which animals can do what, and how, is a major focus of modern biology.
The extreme regenerators
Planarian flatworms
Planarians are small freshwater flatworms, usually just 2 to 3 centimeters long. They are arguably the most impressive regenerators in the animal kingdom .
A single planarian can regenerate a complete, fully functional individual from a fragment as small as 1/279th of the original body. Even a piece containing just a small section of the body can rebuild the brain, nervous system, digestive tract, skin, and all other organs . This is possible because their bodies are packed with a population of adult stem cells called neoblasts.
Neoblasts are pluripotent, meaning they can generate essentially every cell type in the adult animal, including more stem cells . When a planarian is cut, these cells migrate to the wound site, proliferate, and form a mass called a blastema. From this blastema, the missing structures are rebuilt .
The direction of regrowth is controlled by chemical signals. The Wnt signaling pathway plays a central role. When Wnt signaling is active, a fragment regenerates a tail. When it is inhibited, the fragment regenerates a head .
Axolotls and salamanders
The axolotl is a salamander native to Mexico, and it is one of the most studied vertebrates in regeneration research. It can regrow entire limbs, including bone, muscle, nerve, and skin, without forming scar tissue .
If an axolotl loses a limb, the cells near the wound site undergo a process called dedifferentiation. Mature cells, such as muscle and bone cells, revert to a less specialized state. They then proliferate to form a blastema, a bud of undifferentiated cells. The blastema grows and eventually re-differentiates into all the missing tissues in the correct positions .
The regenerated limb is not just a crude replacement. It is correctly proportioned. If the limb is cut at the wrist, a hand regrows. If it is cut at the shoulder, an entire arm regrows. The cells retain positional information that tells them what to build .
Axolotls can also regenerate their tail, which includes the spinal cord, backbone, and muscles . Newts, another type of salamander, can regenerate their lenses and retinas from pigmented epithelial cells in the eye .
Starfish
Starfish have remarkable regenerative abilities. They can regrow entire arms following loss, whether through self-amputation or injury . In some species, a single arm that still contains a portion of the central disc can regenerate into a complete new starfish .
The regenerative process in starfish involves the formation of a blastema at the wound site. Over several weeks, the new arm develops, including the radial nerve cord, muscles, and other tissues . Research on the red starfish Echinaster sepositus has documented the cellular and molecular details of this process, including the re-growth of the radial nerve cord that restores mobility .
Zebrafish
Zebrafish are small freshwater fish that have become a major model for studying regeneration in vertebrates. They can regenerate several organs and tissues, including the heart, pancreas, liver, jaw, spinal cord, and fins .
Heart regeneration in zebrafish is particularly important because mammals, including humans, cannot regenerate heart muscle after a heart attack. When a zebrafish heart is injured, the muscle cells re-enter the cell cycle and proliferate to replace the damaged tissue. Remarkably, the immediate response to injury is similar in zebrafish and mammals: both form a scar. But in zebrafish, the process continues, and new contractile heart muscle cells are formed .
Research has identified specialized fibroblasts that play a key role in this process. These cells produce collagen-12, which appears to signal for regeneration. When these fibroblasts were genetically switched off in experiments, heart regeneration failed .
Animals with more limited regeneration
Not all regenerating animals can rebuild entire limbs. Some have narrower abilities.
Lizards
Many lizards can regrow their tails when they are lost. However, the new tail is not a perfect replacement. It is made of cartilage rather than bone, and it lacks the complex structure of the original tail. This is a form of partial regeneration .
Frogs
Some frogs, particularly the African clawed frog Xenopus laevis, show regenerative abilities at certain life stages. Froglets can heal skin wounds without scarring and regenerate skin structures like exocrine glands . However, adult frogs generally cannot regrow limbs.
Newts
Newts are salamanders and share the limb regeneration abilities of axolotls. They have an additional ability: they can regenerate their lenses after the original lens is removed. This happens through the dedifferentiation of pigmented epithelial cells in the iris, which then redifferentiate into new lens cells .
How regeneration works
Despite the diversity of regenerating animals, some common principles apply.
Most regenerating animals use a structure called a blastema. This is a mass of cells that forms at the wound site and serves as the source of new tissue . The cells in the blastema come from dedifferentiation of mature cells near the wound, or from resident stem cells.
Nerve supply is important for many types of regeneration. In axolotl limbs, if the nerve supply to the wound is cut, the blastema fails to form or cell division stops. The nerve provides signals that are required for regeneration to proceed .
Positional information guides what gets built. Cells in the blastema retain information about their location in the body. This is why an axolotl regrows only the missing part of a limb, not a whole new limb when only the hand is lost . Signaling molecules like FGF8, SHH, and Wnt proteins coordinate this positional identity .
What this means for human medicine
Humans have very limited regenerative abilities compared to these animals. We can regrow parts of our liver, but not our limbs or heart muscle . When a human suffers a heart attack or loses a limb, the body forms scar tissue rather than regenerating the lost structure .
Scientists study regenerating animals to understand the molecular and cellular rules that make regeneration possible. The hope is that one day this knowledge can be used to encourage human tissues to repair themselves. Researchers have already shown that mammals, including mice, retain a latent capacity for regeneration that can be partially activated . The challenge is to understand how to safely turn that capacity on without causing cancer or uncontrolled growth.
The bottom line
The animal kingdom contains a remarkable range of regenerative abilities. Planarians can rebuild their entire bodies from tiny fragments. Axolotls regrow perfect limbs. Zebrafish repair their hearts. Starfish regrow arms. Each of these animals uses a combination of stem cells, dedifferentiation, and precise signaling to accomplish what humans cannot. Studying them is not just a biological curiosity. It is a path toward understanding whether human regeneration might one day be possible.
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