Regeneration, the ability to regrow lost or damaged body parts, is scattered across the animal kingdom far more widely than most people realize. Planarian flatworms can be sliced into dozens of pieces, each of which grows into a complete new worm. Salamanders regrow entire limbs with functioning nerves, bone, and muscle. Zebrafish rebuild damaged heart tissue. Even some mammals can pull off feats that seem to defy the limits of their biology. The range of what animals can restore, from a severed head to a punctured heart, raises a question that has occupied researchers for centuries: why can some animals do this and others cannot?
The Invertebrate Champions
If regeneration were a competition, the winners would be invertebrates, and the gold medal would go to planarian flatworms. Cut one in half crosswise, and each half produces a complete worm. Cut one into more than a hundred fragments, and every fragment can regenerate a whole animal. The secret lies in a population of adult stem cells called neoblasts, which are spread throughout the flatworm’s body. Some of these cells, known as clonogenic neoblasts, are genuinely pluripotent, meaning they can become any cell type the animal needs. In a landmark experiment, a single transplanted clonogenic neoblast was enough to restore regeneration in a flatworm whose own stem cells had been destroyed by radiation.1PubMed Central. Clonogenic neoblasts are pluripotent adult stem cells that underlie planarian regeneration These stem cells produce neurons, intestinal cells, and every other known cell type in the planarian body.2PubMed Central. The Cellular and Molecular Basis for Planarian Regeneration
Hydra, the tiny freshwater polyp first studied for its regenerative abilities in 1744 by naturalist Abraham Trembley, is another standout.3PubMed. Hydra, a fruitful model system for 270 years A hydra can be dissociated into individual cells and reaggregate into a functioning animal. Its regeneration relies heavily on Wnt signaling, a molecular pathway that also plays key roles during embryonic development in humans. When a hydra is cut, injury triggers MAPK signaling, which activates Wnt genes. The animal’s body maintains a standing gradient of a molecule called β-catenin along its head-to-foot axis, and this gradient determines whether a wound site becomes a new head or a new foot.4PubMed Central. Injury-induced MAPK activation triggers body axis formation in Hydra by default Wnt signaling Researchers can even override this system: by artificially boosting β-catenin signaling at a wound that would normally become a foot, they can force it to form a head instead.5PubMed. From injury to patterning-MAPKs and Wnt signaling in Hydra
Beyond flatworms and hydra, starfish can regenerate lost arms, sea cucumbers can expel and then regrow their internal organs, and some species of ribbon worms can regenerate from tiny body fragments. The sheer diversity of regenerative strategies among invertebrates suggests that the ability to rebuild body parts is deeply rooted in animal evolution.
Salamanders and the Blastema
Among vertebrates, salamanders are the regeneration superstars. The axolotl, a permanently aquatic salamander native to Mexico, can regrow an entire limb, complete with bones, muscles, blood vessels, and nerves, from the stump. It can also regenerate portions of its heart, jaw, spinal cord, and even parts of its brain. What makes this possible is a structure called the blastema, a mound of progenitor cells that forms at the wound site and essentially replays a version of embryonic limb development.6PubMed Central. The axolotl limb blastema: cellular and molecular mechanisms driving blastema formation and limb regeneration in tetrapods
What researchers have found especially striking is how the blastema forms. Mature connective tissue cells in the stump do not simply divide; they revert to a less specialized state that resembles embryonic limb bud cells. Single-cell RNA sequencing has shown that this reversion produces a relatively uniform population of progenitor cells, effectively rewinding the developmental clock so these cells can rebuild all the connective tissues the limb needs.7PubMed Central. Single-cell analysis uncovers convergence of cell identities during axolotl limb regeneration Multiple cell types from the remaining stump contribute to this process at different stages, coordinating to reconstruct the limb’s original architecture.
The axolotl’s spinal cord regeneration is equally remarkable. Unlike mammals, where spinal cord injuries produce permanent paralysis, axolotls can fully regenerate injured spinal cord tissue. Neural stem cells at the injury site regrow and essentially replay spinal cord development, laying down new neurons and support cells to bridge the gap.8PubMed. Salamander spinal cord regeneration: The ultimate positive control in vertebrate spinal cord regeneration The recent development of gene-editing tools like CRISPR in axolotls has allowed researchers to identify specific molecules driving this process, making salamanders one of the most active areas of regeneration research today.
Zebrafish and the Heart That Heals Itself
When a human heart is damaged by a heart attack, the dead muscle is replaced with scar tissue that cannot contract. The heart never fully recovers. Zebrafish take a different path. Remove up to about 20 percent of a zebrafish’s ventricle, and the heart regenerates completely, with new beating muscle replacing the wound. This happens because zebrafish cardiomyocytes, the muscle cells of the heart, re-enter the cell cycle and proliferate to fill the gap. In fish with mutations that disrupt a key cell-cycle regulator, this proliferation fails and a scar forms instead, much as it does in mammalian hearts.9PubMed. Heart regeneration in zebrafish
The signaling behind this recovery involves several pathways, including Notch signaling, which helps regulate how many cardiomyocytes proliferate and when they stop.10PubMed Central. Notch signaling regulates cardiomyocyte proliferation during zebrafish heart regeneration Notch and Wnt signaling, two of the most fundamental pathways guiding animal development, turn up repeatedly across regenerating species, from hydra to fish to salamanders.11PubMed Central. The interaction of Notch and Wnt signaling pathways in vertebrate regeneration The fact that these same pathways exist in mammals but do not trigger the same regenerative response is one of the central puzzles of the field.
Mammals Are Not as Hopeless as You Think
The common narrative is that mammals simply cannot regenerate. This is mostly true for limbs and organs, but it glosses over several notable exceptions. The most dramatic example is the human liver. After surgical removal of up to two-thirds of the organ, the remaining tissue rapidly proliferates to restore the liver’s original mass. In mice, liver size and DNA content recover within about ten days of a major resection, with roughly half the remaining hepatocytes completing at least two rounds of cell division.12PubMed Central. Hyperplasia vs hypertrophy in tissue regeneration after extensive liver resection The process begins remarkably fast: the proliferative signals start before the liver has actually lost mass, suggesting the body detects changes in blood flow and chemical signals even before tissue is gone.13PubMed. The compensatory hyperplasia (liver regeneration) following ligation of a portal branch is initiated before the atrophy of the deprived lobes Strictly speaking, this is compensatory growth rather than true regeneration, since the liver regains its mass without recreating the exact original shape, but the functional recovery is still impressive.
Male deer produce an even more eye-catching feat. Antlers are the only mammalian organs that are completely shed and regrown on a yearly cycle. An adult red deer can grow a pair weighing roughly 30 kilograms in just three months, making antler growth one of the fastest rates of organ growth in any mammal.14PubMed. Deer antlers as a model of Mammalian regeneration The process involves both types of bone formation seen during embryonic development and is driven by a reserve of stem cells in the pedicle, the permanent bony stump from which antlers sprout. The annual bone-loss-and-regrowth cycle even mimics and naturally reverses a condition resembling osteoporosis, making deer a subject of interest for bone-disease researchers.15PubMed Central. Bone metabolism associated with annual antler regeneration: a deer insight into osteoporosis reversal
Then there are African spiny mice, which challenged assumptions about mammalian limits when researchers reported that these rodents can shed large patches of skin under predator attack and then regenerate not just skin, but hair follicles, sebaceous glands, dermis, and even cartilage. Ear-hole punch wounds in spiny mice closed completely, with full tissue restoration rather than the scar tissue a lab mouse would produce.16PubMed Central. Skin shedding and tissue regeneration in African spiny mice (Acomys) The discovery suggested that at least some mammals retain latent regenerative programs that could, in principle, be activated more broadly.
Why the Immune System Matters More Than You’d Expect
One of the less intuitive findings in regeneration biology is the central role of the immune system. In salamanders, macrophages, a type of immune cell, are critical to the earliest stages of limb regeneration. When researchers depleted macrophages during the period right after amputation, the wound still healed over, but the limb permanently failed to regenerate. Instead of forming a blastema, the stump produced dense scar tissue, much like a mammalian wound.17PubMed Central. Macrophages are required for adult salamander limb regeneration
This finding reframed scar formation not as the inevitable default response to injury but as what happens when the regenerative program fails to launch. The macrophages appear to manage the local environment around the wound, regulating which genes are turned on in the extracellular matrix and keeping fibrosis (the scar-forming process) in check. In mammals, the immune response to injury is fast, aggressive, and geared toward rapid wound closure and infection control. The trade-off may be that this inflammatory response actively blocks the slower, more complex sequence of events that regeneration requires.
The Evolutionary Trade-Off With Cancer
A question that haunts regeneration research is why mammals, which carry many of the same genes and signaling pathways used by regenerating animals, have such limited regenerative abilities. One influential idea is that mammals traded regenerative capacity for better cancer suppression. Regeneration requires rapid cell proliferation and the reactivation of developmental genes, both of which, if poorly controlled, could lead to tumors. Mammals may have evolved tighter brakes on cell division, including shorter telomeres and stronger tumor-suppressor systems, that make cancer less likely at the cost of making regeneration nearly impossible.18PubMed. The reserve-capacity hypothesis: evolutionary origins and modern implications of the trade-off between tumor-suppression and tissue-repair
Salamanders and other regenerating animals appear to have solved this problem differently. Axolotls engage in massive cell proliferation during limb regeneration without developing tumors, suggesting they possess mechanisms that allow controlled growth without the runaway division that characterizes cancer. Some researchers have proposed that understanding how regeneration-competent animals suppress tumors while allowing rapid tissue growth could reveal entirely new approaches to cancer treatment in humans.19PubMed. Amphibian regeneration and mammalian cancer: Similarities and contrasts from an evolutionary biology perspective This perspective treats cancer not just as a disease of uncontrolled growth but as a consequence of losing the regenerative framework that once kept growth orderly.
Whole-body regeneration, the kind seen in planarians and hydra, is thought to be an ancestral ability, present in early animal lineages and subsequently lost by most groups as they evolved greater body complexity. Appendage regeneration, like the limb regrowth in salamanders, is more likely a trait that evolved independently multiple times in different lineages.20PubMed Central. Animal regeneration: ancestral character or evolutionary novelty? This distinction matters because it means there is no single genetic switch that was “turned off” in mammals. The story is more complicated than a simple loss.
The Real Costs of Regrowing a Body Part
Regeneration sounds like an unqualified advantage, but for the animals that do it, the process comes at a genuine cost. Lizard tail autotomy (the ability to drop a tail to escape a predator) followed by regeneration has been studied in detail, and the trade-offs are substantial. Across 56 species from 14 family groups, regeneration rates varied enormously, from zero growth to about 4.3 millimeters per day.21PubMed Central. At What Cost? Trade-Offs and Influences on Energetic Investment in Tail Regeneration in Lizards Following Autotomy The speed at which a lizard regrows its tail depends on age, sex, reproductive season, temperature, nutrition, and stress levels.
The energetic investment in regrowth can affect growth rate, reproductive output, and behavior. In Italian wall lizards, water loss increased significantly right after tail loss, then dropped back below pre-autotomy levels once regeneration was underway, suggesting the body reallocates resources heavily during the process.22Journal of Herpetology. Physiological Effects of Tail Regeneration following Autotomy in Italian Wall Lizards, Podarcis siculus In delicate skinks, complete tail loss made lizards less active, less exploratory, and slower sprinters, changes to personality and performance that could affect their survival and foraging ability during the vulnerable period of regrowth.23Animal Behaviour. Impacts of caudal autotomy on personality
These costs help explain why not every animal benefits equally from regenerative ability. An animal that rarely faces predation may have less evolutionary pressure to maintain an expensive regenerative program, while one that routinely loses body parts may invest heavily in the ability to regrow them, even at the expense of growth or reproduction.
Bioelectric Signals and the Blueprint for Shape
Beyond the well-known chemical signaling pathways, a growing body of research points to bioelectricity as a key coordinator of regeneration. Cells maintain voltage gradients across their membranes, and the pattern of these voltages across a tissue can carry information about what shape the tissue should take. In regenerating animals, these bioelectric patterns help guide the restoration of normal anatomy after injury.24PubMed Central. Bioelectric signaling in regeneration: Mechanisms of ionic controls of growth and form
Experiments manipulating these patterns have produced extraordinary results. By altering the electrical state of cells, researchers have changed where organs form, redirected the body axes of developing embryos, and even induced the growth of structures in locations where they do not normally appear.25Cell. The Software of Life: Bioelectric Controls as a Unique, Reprogrammable Medium The voltage patterns function almost like a spatial map, telling cells not just what to become but where they are relative to the rest of the body. This layer of information sits on top of the genetic and chemical signals and may help explain how animals rebuild complex structures in the right proportions and positions rather than producing disorganized growths.
Aging and the Decline of Regenerative Ability
Even within species that regenerate well, the ability tends to fade with age. As organisms get older, their stem cells progressively lose the capacity to maintain tissues and support repair. A 2025 review identified five core features of aged stem cells that underlie this functional decline across multiple tissue types.26Cell Stem Cell. Hallmarks of stem cell aging A large single-cell analysis in mice found that regenerative capacity varied substantially across different tissues and declined progressively with age, with aged tissues showing altered cellular and molecular responses to injury compared to young tissues.27PubMed. Decoding aging-dependent regenerative decline across tissues at single-cell resolution
This age-related decline is visible even in regeneration-competent species. Young axolotls regenerate faster and more completely than older ones. Lizard tail regeneration slows with age. And in mammals, the liver’s regenerative response becomes less efficient as the animal ages. The connection between aging and regeneration is not merely parallel: they share underlying biology. The same stem cell populations that drive regeneration are the ones that deteriorate with age, and the same molecular brakes that suppress cancer may also accelerate stem cell exhaustion over a lifetime.
What Researchers Hope to Learn for Human Medicine
The practical question behind all of this research is whether humans could ever be coaxed into regenerating tissues they currently cannot repair. The field has progressed significantly thanks to comparative genomics. When researchers compared the genes activated during limb regeneration in the axolotl to those activated during fin regeneration in a primitive bony fish called Polypterus, they found a shared genetic program of appendage regeneration, suggesting that the molecular toolkit for rebuilding limbs has deep evolutionary roots predating the split between fish and land vertebrates.28PubMed Central. Deep evolutionary origin of limb and fin regeneration
Comparative transcriptomic studies across three species of salamanders have identified hundreds of genes whose expression changes are conserved during limb regeneration. These shared genes are enriched for functions related to the cell surface, the extracellular space, and immune-cell-associated proteins, painting a picture of regeneration as a coordinated dialogue between the wound environment, the immune system, and the progenitor cells that will build new tissue.29PubMed Central. Comparative transcriptomics of limb regeneration: Identification of conserved expression changes among three species of Ambystoma Databases now allow researchers to compare regeneration data across species, for instance searching how the same gene behaves in zebrafish heart regeneration versus neonatal mouse heart regeneration, to pinpoint where the pathways diverge between animals that heal and animals that scar.30npj regenerative medicine. RegenDbase: a comparative database of noncoding RNA regulation of tissue regeneration circuits across multiple taxa
None of this has produced a therapy for regrowing a human finger or repairing a severed spinal cord, but the direction of progress is clear. Researchers are no longer asking whether mammals have the genetic ingredients for regeneration; they mostly do. The question has shifted to why those programs stay silent after injury in adult mammals and whether the right combination of signals, perhaps immune, bioelectric, and epigenetic, could wake them up. African spiny mice show that at least some mammals have already evolved ways to do this for skin and cartilage, and every new species studied adds another data point to the map of what is biologically possible.