Native to just two lakes in central Mexico, the axolotl, Ambystoma mexicanum, looks like something out of a sci-fi film.
Besides the innocent smiley face and tiny legs, the axolotl’s most eye-catching features are its branch-like gills protruding from each side of its head. Gills on a supposedly adult salamander, you ask? Such are the results of an unusual trait called neotany, or paedomorphosis, in which larval stage characteristics are retained throughout life. This metamorphic failure is caused by a lack of the thyroid-stimulating hormone, which induces the thyroid to produce thyroxine that would make them full-grown salamanders. Quite simply, the axolotl doesn’t really grow up.
As if that wasn’t strange enough, the axolotl is also known for its amazing regenerative powers – the ability to regrow multiple structures like limbs, jaws, tail, spinal cord and parts of the brain (the list goes on) throughout their lives, a process very rarely observed in vertebrates.
When a limb is amputed, the epidermis migrates to cover the stump in less than 12 hours, and changes in the underlying stump tissues soon result in the formation of a blastema (a mass of dedifferentiated proliferating cells). As the blastema forms, pattern formation genes are activated as they were when the limb was formed in the embryonic stage. Motor neurons, muscle, and blood vessels grow with the regenerated limb, quickly reestablishing the connections that were present prior to amputation. To top it off, the timely halt of fibroblast growth (which almost always causes scars in humans) ensures a perfectly scarless healing. Amazingly, axolotls can also readily accept transplants from other individuals, including eyes and parts of the brain—restoring these alien organs to full functionality.
For these reasons, the axolotl is bred and used extensively in laboratories as a model organism. Unfortunately, it is now critically endangered in the wild (where it is usually dark grey in colour, instead of the albino form commonly seen in labs), where human development is pushing it to the brink of extinction. More should be done to ensure this miracle of an animal lives on in nature, shouldn’t it?
Brandon, R., J. Armstrong, G. Malacinski. 1989. Natural history of the axolotl and its relationship to other ambystomid salamanders. Pp. 13-21 in Developmental biology of the axolotl. New York, NY: Oxford University Press, Inc.
Duellman, William E.; Trueb, Linda, “Biology of Amphibians“, 1994, Johns Hopkins University Press.
Endo T, Bryant SV, Gardiner DM. A stepwise model system for limb regeneration. Dev Biol. 2004 Jun 1;270(1):135-45.
N. J. De Both, The Developmental Potencies of the Regeneration Blastema of the Axolotl Limb, 1970, Hubrecht Laboratory, Utrecht, Netherlands