Regeneration Of Lizards Tails Provides Lessons For The Regeneration Of Human Limbs

Most lizards have a weakened area in their tail which enables them to break

, or detach, the vertebrae (bone) and muscle easily. Hence lizards themselves can control when they sever their tail. This voluntary self-amputation is called caudal autotomy. Losing a tail is therefore a survival mechanism. The dropped appendage continues to wriggle which attracts the attention of the predator, thus enabling the lizard to escape. Researchers at the University of Michigan, A venomous tale: how lizards can shed their tail when predators attack (Science Daily, 27 March 2010) maintain that a detached tail can cause three main problems for lizards: reduced mobility, loss of energy because the tail stores fat for energy; and reduced social status and ability to maintain their territory. Two things happen when a lizard loses a tail: the muscles around the severed body part contract which constricts the blood vessels so that it does not bleed excessively; and another tail regrows through epimorphic regeneration in which nerves, blood vessels, muscles, tissues and skin are replaced. The larger lizards, such as the Komodo dragons, have tails that are not designed to detach, although the tail can be separated with force, but the tail will not grow back.

A specialized bud, called a blastema, at the site of the amputation, provides the tissue that will eventually form the regenerated parts. If a tail blastema is transplanted elsewhere on the lizards body, it will continue to become a tail. Therefore the blastema holds the destiny of the regenerative tissues. But if a tail blastema is transplanted onto the severed stump of a lizards leg, the structure that regenerates will be a composite of the two appendages. For lizard tail regeneration, scientists know that there must be certain prerequisites, such as a wound, a source of blastema, and the spinal cord.

A lizard can take a few months to almost two years to regenerate another tail. However, the segmented vertebrae are not replaced. Instead, it replaces the bone with a cartilaginous tube with a spinal cord inside and muscles on the outside. These muscles are different from the original ones because they have longer fibers. In addition, the new tail does not possess the exact color and pattern of the original tail. Therefore, a regenerated tail is superficially similar to the original, but not a perfect replica. It is also not uncommon for the regeneration to be incomplete.

In humans, some organs can partially repair themselves, but they cannot regenerate totally. For example, if part of a human liver is removed, the remaining part enlarges to a mass equivalent to the original, according to Brittanicas The range of regenerative capability. This is called compensatory hypertrophy. The missing liver is not actually replaced; it is enlarged.

Scientists from the University of Arizona College, reported in New Scientist (15 September 2012), raise the question whether it will ever be possible to fully regenerate injured human limbs due to the fact that lizards tail regeneration is not perfect. They found that lizards tails have vertebrae with regular gaps that allow blood vessels and nerves to pass through. However, instead of bone, a regenerated tail has cartilage which is full of small holes. These pores only let blood vessels through, and not nerves. New nerves remain trapped within the cartilage tube and dont seem to reach the newly regenerated muscles. Therefore the appearance, flexibility, and functionality of the original tail are not replicated, leaving the regenerated tail inferior to its original.

Although not perfect and not functioning as well as the original, in the case of a lizards limb, studying animals highlights the need for further research into other regenerative organisms, and the challenges of regenerating complex human tissues. Regenerative medicine remains hopeful of human limb regrowth in the future.

by: Martina Nicolls

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