Stephane Roy, an assistant professor of dentistry at the University of Montreal who studies regeneration, explains.
Chameleons are very interesting animals that are well known for their unique ability to blend in with the surrounding environment by changing their color, but they are unable to reproduce or regenerate their body parts. That said, there are many animals that can regenerate perfectly throughout their lives. Among invertebrates the capacity to regenerate parts like legs or entire sections of the body is fairly common. The planarians (flat worms) can be cut into 50 pieces and you will end up with 50 smaller worms. If an arm of a starfish is cut off, you will have two starfish after regeneration. Crickets can regenerate their legs. These are just a few examples of invertebrates capable of regenerating complex structures.
When we consider vertebrates, however, the situation is reversed. There is only one group of vertebrate that has the capacity to regenerate highly complex structures such as limbs, jaws, tail, spinal cord, or eyes throughout their lives: the urodele amphibians. Urodeles are salamanders like the ones found in ponds across North America and Europe. Those that are most often used for research are the newts (Notophthalmus viridescens) and the axolotl (Ambystoma mexicanum). Although people often say that lizards can regenerate, it is not quite the case. Lizards will often shed their tails to escape a predator. The tail will regrow, but it will not be a perfect replacement of the original, and if a lizard loses a leg it will never regrow. Frogs, meanwhile, can regenerate their tails as tadpoles, but they lose all their regenerative capacity after metamorphosis.
Urodele amphibians, however, have the outstanding ability to regenerate perfectly complex tissues over and over again, and each time the regenerated tissue is identical to the original in appearance and function. The exact mechanism of how these animals can regenerate is not currently well understood. One of the most unique aspects of regeneration in urodeles that has been well documented is that they regenerate by dedifferentiating their cells at the amputation site. Dedifferentiation is a process by which specialized cells regress to a more basic, less specific form. So far researchers have not been able to isolate or identify stem cells in these organisms. This may be one of the most important facets of studying regeneration in these animals because they provide a completely different approach to tissue regeneration than that offered by stem cells. This is not to say that stem cells are not necessary to the process, however, because the cellular dedifferentiation process in urodeles is often presented as a means to locally generate pluripotent cells (perhaps stem cells or cells very similar to them). The presence of a number of nerves is also important for the regeneration process, as well as the formation of a wound epidermis over the wounded site. If a limb is denervated at the onset of the process, it will not regenerate. Nerves may be responsible for the release of growth factors or chemotactic agents necessary for the regenerative process. These salamanders are very intriguing and offer a unique opportunity to study the mechanisms leading to the restoration of complex structures in vertebrates. Answer originally published on July 26, 2004.