Axolotl

[abdulhakeem]

How do chameleons regenerate lost body parts?

C. Shmee
Orland Park, Ill.
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 posted on July 26, 2004

http://www.sciam.com/askexpert_quest...3&chanID=sa005

[abdulhakeem]

The Axolotl (pronounced Axe-oh-lot-ul), Ambystoma mexicanum, is an amphibian, a salamander, part of the order Caudata/Urodela. Because it's a salamander, it's part of one of the three branches of class Amphibia, which also includes the frogs and toads (the Anurans), and the mainly eel-like order Gymnophiona which are also known as the Caecilians. Have a look at the Axolotl Physiology Page for a short guide to the Axolotl's body and characteristics. One common misconception is that axolotls and other salamanders are lizards or reptiles - in actual fact, amphibians are a completely separate group of animals, as are mammals like us.

This page is a brief introduction for those new to the Axolotl and salamanders. If you require specific information, you can use the form at the top right of this page to search the site, and the site map button above right contains a list of all of the pages here. I hope that you find the site useful, but most of all I hope you enjoy what you read and find here. If you're looking for information about metamorphosed axolotls, click on the link.

Axolotls come in many colour varieties. Grey, tan, brown, white, golden albino, white albino, as well as other varieties, such as the black melanoid. The normally coloured or "wild type" can come in anything from near-black like the one in the group photo to the left, to chocolate brown like the one at the top left of the page, to almost cream in colour and anything in between. There are even "piebald" axolotls known as "harlequins". You can learn more about how colour comes about and is passed on by taking a look at the Genetics Page.

The name "Axolotl" comes from the Aztec language "Nahuatl". One of the most popular translations of the name connects the Axolotl to the god of deformations and death, Xolotl, while the most commonly accepted one is "water-dog" (from "atl" for water and "xolotl" which can also mean dog). Prior to the growth of Mexico city in the basin of Mexico, the Axolotl was native to both Lake Xochimilco, and Lake Chalco. Of these two high altitude freshwater lakes, only the remnants of Xochimilco as canals can be seen today.

The Axolotl is now on the CITES endangered species list, although there have been recent efforts to breed and release the animal in order to re-establish its numbers. Fortunately, due to the importance of the Axolotl in scientific research, it is unheard of for them to be taken from the wild because of the huge numbers bred in captivity each year.

The fact that the animal is on the endangered species list may, surprisingly, go some way to offsetting its use as a laboratory animal. It has long been known that the Axolotl is a worthy study due to its amazing healing abilities. Normal wound-healing in animals occurs through the growth of scar tissue, and this also means that most animals won't re-grow a lost limb. However, the axolotl is fully capable of complete limb re-growth. The animal has the added attraction of having especially large embryos, making it easier to deal with under laboratory conditions. Its embryo is also very robust, and can be spliced and combined with different parts of other embryos with a high degree of success.

The Axolotl is a fascinating creature for a number of reasons, including its grotesque appearance, its ability to regenerate, and primarily the fact that it exhibits the phenomenon known as neoteny. Ordinarily, amphibians undergo metamorphosis from egg to larva, and finally to adult form. The axolotl, along with a number of other amphibians, remains in its larval form, meaning that it retains its gills and fins, and doesn't develop the protruding eyes, eyelids and characteristics of other adult salamanders. It grows much larger than a normal larval salamander, and reaches sexual maturity in this larval stage. Another term to describe this state is "perennibranchiate". It is completely aquatic, and although it does possess rudimentary lungs, it breathes primarily through its gills and to a lesser extent, its skin.

It is generally accepted that neoteny is a "backward" step in evolution, because the axolotl is descended from what were once terrestrial salamanders like the closely related species, the Tiger salamander, Ambystoma tigrinum/Ambystoma mavortium spp. (in fact, one theory suggests that the Axolotl is in fact a Tiger salamander off-shoot, as it can interbreed with that species quite successfully). Through some quirk of nature, a neotenous form developed and, probably due to environmental conditions, prospered. Neoteny is sometimes found in other amphibians, but tends to be caused by low levels of iodine (an essential element in the making of thyroxine hormones, essential to growth and development), or random genetic mutation. In the axolotl the condition is totally genetic (click for more information on the Axolotl's genetics). When treated with hormones, the axolotl will usually begin to metamorphose, but in very rare cases it will metamorphose spontaneously, such as the metamorphosed wildtype axolotl pictured here, which bears a close resemblance to the Mexican subspecies of the Tiger Salamander, Ambystoma mavortium valasci.

http://www.caudata.org/axolotl/