Why don’t severed limbs regrow?

legs

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In the biological system, each kingdom exhibits REGENERATION. Regeneration is the renewal or growth of a body part to support the morphological integrity of an organism. Regeneration ensues following a series of gene regulatory processes and thus are of two types.

  1. The exact repair/regrowth of the lost tissue refers to complete regeneration.
  2. When the lost tissue is not generated in its original form, it is referred to as incomplete regeneration

Regeneration of tissue – complete or incomplete – is found in nearly all living organisms, from bacteria to humans, in varying degrees. An animal Hydra exhibits complete regeneration of organs and uses it for reproduction as well. While Hydra is an animal, its relation to humans is distant and hence this type of regeneration is not observed in humans. On the other hand, the most closely linked animal to humans which can show complete regeneration is the lizard. Lizards can regrow tails to salvage an injury.

 

How does regeneration occur?

Hydra can regrow its entire body from a severed body part like a head or a tail. This happens due to a cascade of mechanisms like cellular repair. The tissues at the site of the injury recognize the severed part and remodel their cellular repair systems to regrow the lost tissue and form a whole new organism exactly like the previous one. Since this process forms Hydra offspring, it is a reproductive system as well.

hydra regeneration

In lizards, regeneration of tail is most observed. Renewal of a tail is the product of various genes being switched on – genes that are commonly detected in embryonic development and wound healing. A similar set of genes has been found in humans as well, but an injury resulting in loss of a limb does not switch these genes on.

 

Does regeneration happen in humans?

The answer to this fall in the grey area – the answer is yes, and no. Renewal of injured tissues in humans is in a capacity, which depends upon the site of injury and the type of tissue associated with it. The most primitive instance of regeneration in humans is that of repair of wounds and cuts, which form a SCAR.

tissues

With practices such as regenerative therapy, more people have been able to regain their appendages in full capacity, but these technologies are not available to all. For comparison, we will restrict our discussion to the native capacity of appendages on the human body to renew themselves. An excellent example of such an appendage is the LIVER.

human body

The liver is an organ in the human body actively involved in metabolizes and secretes substances necessary for digestion. It also acts as a warehouse of biomolecules – in particular, glucose (as GLYCOGEN). While being such an important organ, it also exhibits the rare property of regeneration. The liver cells are HEPATOCYTES, which UNIPOTENCY. Unipotency is a property of a STEM CELL (an undifferentiated cell; one which has not been designated any function yet) to differentiate into more hepatocytes. The liver tissue can regenerate from one-quarter of its mass to the entire mass and function optimally. It has also been observed that fingertips can regrow given that there is a part of the nail retained – another instance of regeneration in humans.

However, the comparison of regeneration of the liver, repair of wounds, and regeneration of severed limbs fall on different planes. While the liver utilizes stem cells to regenerate itself, wounds heal via simple tissue repair systems. The procedure for limbs to be regenerated becomes much more complex than that of the liver or the wounds. The regrowth of limbs would need more pre-requisites – such as bones, blood vessels, nerves, and muscles. These regenerate more intricately than tissues. It would need a MULTIPOTENT STEM CELL (a stem cell that can differentiate into any cell) for this – the limitation being that limbs do not have stem cells capable of performing this function.

cells

In theory, even if limbs did have such a stem cell, the complete regrowth of limbs would have been a reach. While blood vessels and nerves regenerate, the renewal of bones and muscles poses a complicated circumstance since these would be inactive. Since the muscle and the bone forms the basic framework of a structure, the hypothetically regenerated limb would be rendered useless. In such cases, a robotic prosthetic will show better performance than such a limb.

 

Is there scope for limbs to regenerate in humans?

It is a fact that humans share a large part of their genome with the fellow members of the kingdom Animalia. Genes for regeneration in lizards and salamander have been found in humans as well but in a ‘turned off’ condition. If these genes were even turned on (which will have ethical and technological implications), no scientist can guarantee the status of the regenerated appendage. But such a set of genes fuels the research in this field.

While a solid hope for natural regeneration of limbs in humans may not be present, technology has come far from the times where a severed limb would limit an individual. The development of newer and improved prosthetics acts as a ray of sunshine for the amputees.

legs

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