While not all lizards can regrow their tails if they lose them, there are several, including the green anole lizard, most salamander species, geckoes and iguanas, who possess this amazing ability. Common in invertebrates such as worms and starfish, no other vertebrates besides these few lizard species can regenerate body parts.
Why Tails Drop
Lizards have tails especially designed to drop in an effort to save their lives. When attacked by a predator, a lizard can lose his tail and run away while the predator is distracted with the still-moving appendage. Some lizards can drop their tails without being touched by a predator, with the tails falling off as soon as the lizards sense danger. Others require a predator to pull on the tail, which causes it to release along a breaking point between the vertebrae along the tail or at the tail's base.
Although scientists don't completely understand how lizards regenerate their tails, they know lizards are capable of dedifferentiating cells. This means existing cells at the base of the tail, or the base of the break along the tail, revert back to a more primitive cell, similar to a stem cell. These cells can be "programmed" to become the cells necessary to build a new tail, such as a muscle or skin cell. When a break occurs, a cell that might have once been a muscle cell dedifferentiates, waiting for instructions on what kind of cell to become. It then begins to divide into more cells of the necessary type.
Nerves seem to play a critical role in the regrowth of lizard tails. Lizards who have nerve damage at the base of the tail as their tails drop aren't able to regenerate the tails. Scientists also note that the new tails don't usually include new nerves. Some lizards don't have nerves in the new tails, while some have existing nerves that grow longer to reach into the regenerated area.
No two tails are alike, it seems. When a lizard regrows his tail, it can look significantly different than his original tail, inside and out. The skin and scales can be colored differently than the first tail or sport a different pattern. Also, there aren't new bones in the regenerated tail. Instead, the lizard grows thick cartilage to support the tail structure. The muscles are different as well; instead of short, fast-twitch fibers, the lizard ends up with longer, slow-twitch muscles. This makes the tail less useful than the original, with a main function of distracting predators and saving the lizard's life by dropping off again.
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