What Is Regeneration?
Regeneration is the biological ability to restore or regrow body parts after injury or loss. Across the animal kingdom, this ability varies widely—from complete body regeneration in flatworms to partial tissue repair in mammals. Scientists have been fascinated by regeneration for centuries, but we are still uncovering how it works.
A Brief History of Regeneration
Interest in regeneration goes back to ancient times. Aristotle noted that lizards could regrow their tails around 350 BC. In 1744, Abraham Trembley famously demonstrated that a hydra cut in half could grow into two separate animals. By the 18th century, scientists began to question early ideas like preformation—the belief that animals were fully formed in miniature inside eggs or sperm—and started exploring regeneration through experiments. If the animal is preformed, how can it be that an appendage regenerates?
The first detailed study of salamander limb regeneration was published in 1777 by Charles Bonnet. In 1901, Thomas Hunt Morgan published the book Regeneration, which defined key terms like morphallaxis and epimorphosis, concepts still used today.
Types of Regeneration
Regeneration is categorized in several ways:
Physiological regeneration
This is routine renewal that happens naturally, such as skin shedding, gut lining turnover, or deer antler regrowth.Reparative regeneration
This occurs after injury. It includes:Epimorphic: New structures form from a mass of dividing cells called a blastema or neoblast. Examples include salamander limb regrowth and flatworm regeneration.
Tissue: Tissue is rebuilt without a blastema, such as in zebrafish heart regeneration.
Cellular: Individual cells repair themselves. For example, axon regrowth after vincristine-induced nerve damage.
Hypertrophy
This is an increase in organ size to compensate for damage. For example, if one kidney is removed, the remaining kidney grows larger.Or cell proliferation to restore the damaged mass of internal organs. For example, liver and pancreas.
Morphallaxis
Seen mostly in invertebrates like hydra and planarians, this is regeneration through tissue reorganization. New structures form by reshaping existing ones, often without much cell division.
When Did Regeneration Evolve?
Fossil evidence shows that regeneration is ancient. Some extinct sea animals already had the ability to regrow limbs. In early vertebrates like primitive amphibians, fossilized tails show signs of regrowth, suggesting that appendage regeneration may be an ancestral trait in tetrapods.
(Source: Fröbisch et al. 2015)
Nowadays, regeneration appears in many animal groups but is not universal. Some species, like salamanders and flatworms, have extensive regenerative abilities. Others, including most mammals, have limited capacity.
Why Do Animals Regenerate?
Ecology plays a role. In some species, regeneration is part of survival strategies:
Predator evasion: Lizards can drop their tails (autotomy) and later regrow them.
Injury recovery: Crabs and insects can regrow lost limbs after molting or predation.
Seasonal regrowth: Deer regrow antlers each year as part of their reproductive cycle.
Big Questions in Regeneration Biology
Even with centuries of research, key questions remain:
Where do new cells come from during regeneration? Cell lineage tracing
How do cells "decide" to regenerate instead of just healing?
What reactivates developmental genes in adults?
Why do nerves play such a key role in triggering regeneration?
What makes some species better at regenerating than others?
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Can humans unlock dormant regenerative abilities?
