Regeneration occurs across a surprisingly wide range of life, from simple freshwater polyps that can rebuild their entire body from a small fragment to mammals that regrow liver tissue after surgery. The ability is not evenly distributed, though. Aquatic and simpler animals tend to be regeneration champions, while most terrestrial vertebrates, including humans, have only limited versions of this ability.
Hydra: Regeneration Without Growth
Hydra, a tiny freshwater relative of jellyfish, is one of the most impressive regenerators in nature. Cut one in half, and the head end regrows a new base while the base end regrows a new head. Cut it into several pieces, and each middle section regenerates both a head and a base at the correct ends. The remarkable part is that this happens through repatterning of existing cells, not by growing new ones. The result is simply a smaller hydra. Biologists call this process morphallaxis.
Hydra’s regenerative power is tied to relentless cellular turnover. Three populations of stem cells in the body column divide continuously, with outer and inner tissue cells dividing roughly every three to four days and a third stem cell type dividing every day and a half. New cells are constantly pushed toward the tentacle tips or the base and sloughed off. This perpetual renewal is also why certain hydra strains show no signs of aging, effectively making them biologically immortal as long as conditions are favorable.
Planarians: Whole Bodies From Tiny Fragments
Flatworms called planarians can regenerate new heads, tails, sides, or entire organisms from small body fragments, a process that takes days to weeks. A planarian sliced into dozens of pieces can produce dozens of new worms.
This ability depends on a population of adult stem cells called neoblasts, which are the only dividing cells in a planarian’s body. Every new somatic cell, whether during normal tissue maintenance or after an injury, comes from neoblasts. Some of these cells, known as cNeoblasts, are individually pluripotent, meaning a single transplanted cNeoblast can restore full regenerative capacity to an animal whose own stem cells have been destroyed. In transplant experiments, one donor neoblast slowly converted an entire host animal into a genetic clone of the donor.
After an injury, neoblasts flood the wound site as part of a missing-tissue response. Their sheer numbers at the wound, combined with positional signals that identify what’s absent, allow them to specialize into progenitors for whatever body parts are missing, without needing to detect the absence of each specific tissue type.
Starfish and Other Echinoderms
Sea stars are famous for regrowing lost arms, and some species can regenerate an entire body from a single arm if part of the central disc is attached. The process begins within hours of an injury, when clotting cells rush to the wound site and a ring of clotted tissue seals the opening. Within 48 to 72 hours, existing skin cells at the wound edge stretch and migrate to cover the surface, forming a new layer of epidermis without any initial cell division.
Over the first week, the wound area fills with a temporary swollen tissue similar to the granulation tissue seen in mammalian wound healing. Immune cells clear debris and break down damaged muscle. Only after this cleanup is complete does the animal begin depositing new structural proteins. Importantly, no fibrotic scar forms, which is a key difference from how most mammals heal. The full regrowth of a complex arm with its skeleton, tube feet, and nerve cord can take months to over a year depending on the species.
Other echinoderms share this talent. Sea cucumbers can expel their internal organs as a defense mechanism and regenerate them entirely. Brittle stars routinely shed and regrow arms.
Salamanders: The Vertebrate Gold Standard
Among animals with backbones, salamanders, particularly axolotls, are the most capable regenerators. They can regrow entire limbs complete with bones, muscles, nerves, and blood vessels. The process works through epimorphosis: mature cells at the wound site lose their specialized identity and revert to an embryonic-like state, forming a mound of undifferentiated tissue called a blastema.
Limb regeneration begins when a wound heals over and a thin layer of skin cells forms a specialized signaling center at the tip. This cap recruits nerves and sends chemical signals into the underlying stump tissue, triggering cells to dedifferentiate and migrate toward the wound. The blastema grows outward, with cells closest to the stump beginning to re-specialize into limb tissues first while cells at the tip remain undifferentiated and proliferating. Over weeks, the blastema progressively matures from base to tip, rebuilding the limb in the correct pattern.
What makes this especially relevant to human medicine is that the signaling pathways driving axolotl blastema formation are highly conserved. Researchers have induced blastema-like structures using purified mammalian growth factors, suggesting the genetic toolkit for regeneration exists in mammals but is not normally activated.
Zebrafish: Regrowing Heart Tissue
Zebrafish can regenerate up to 20% of their heart muscle, a feat that adult mammals cannot accomplish. When heart tissue is damaged, existing heart muscle cells lose some of their specialized features, essentially reverting to a less mature state. These dedifferentiated cells then divide, migrate into the injured area, and re-specialize into functional heart muscle. The result is complete restoration of cardiac function with only temporary scarring that eventually resolves.
Newts share this cardiac regeneration ability through a similar mechanism. Adult mammals, by contrast, respond to heart damage by forming permanent scar tissue that cannot contract, which is why heart attacks cause lasting damage in humans.
What Mammals Can Still Regenerate
Mammalian regeneration is limited compared to aquatic animals, but it is not absent. The liver is the standout example. Surgeons routinely remove up to 50% of the liver to treat cancer, and in many patients, the remaining tissue grows back to nearly its original size within a month. Unlike salamander limb regrowth, liver regeneration does not involve a blastema or dedifferentiation. Instead, all five types of liver cells simply divide to produce more of themselves while continuing to perform their normal jobs, including glucose regulation, toxin processing, and bile production. This is sometimes called compensatory regeneration.
Humans can also regenerate fingertips, but only under specific conditions. The amputation must be at the level of the last bone of the finger (the distal phalanx) and cannot extend past the last joint. The nail bed must be intact, because the specialized cells there are required to drive the regenerative response. When these criteria are met, a fingertip allowed to heal on its own, without surgical closure, can restore its original length, bone, skin, and even some sensation. Children regenerate fingertips more reliably than adults, but the capacity persists to some degree throughout life.
African Spiny Mice: A Mammalian Surprise
African spiny mice in the genus Acomys are the most regenerative mammals discovered so far. Their skin is extremely fragile, tearing easily to help them escape predators, but they can regrow large patches of skin complete with hair follicles, sebaceous glands, and cartilage rather than forming scar tissue. In ear-punch experiments, spiny mice closed holes that would leave permanent gaps in ordinary lab mice.
Several biological differences explain this. The wound environment in spiny mice is dominated by a looser, more porous form of structural protein (type III collagen) that appears to favor regeneration over scarring. Cells called myofibroblasts, which drive scar formation in most mammals, are almost completely absent from healing spiny mouse wounds. And key signaling molecules involved in hair follicle development during embryonic life are reactivated in adult spiny mouse wounds, allowing new follicles to form from scratch, something ordinary mice cannot do.
Why Some Animals Regenerate and Others Don’t
The broad pattern is that aquatic animals and those with simpler body plans tend to regenerate more effectively. Most arthropods (insects, crustaceans, spiders) and nematodes (roundworms) have very limited regenerative ability, generally restricted to wound healing and scarring. A few arthropods, like crabs regrowing claws, are exceptions. Among vertebrates, regenerative capacity generally declines with complexity and distance from aquatic life: fish and aquatic amphibians regenerate best, terrestrial amphibians and reptiles (like lizards regrowing tails) show intermediate ability, and mammals are the most restricted.
The underlying mechanisms, stem cell activity, growth factor signaling, and the ability to form a blastema, are broadly conserved across species. The difference is not that mammals lack the genes for regeneration but that these programs are typically suppressed in favor of rapid wound closure and scar formation, a trade-off that may have evolved to reduce infection risk in terrestrial environments.