Why can a lizard’s tail grow back, but a dog’s leg can’t?

Some creatures, such as lizards, salamanders, and planaria, can regenerate after losing a body part, whereas mammals are nearly impossible to regenerate. This is because each organism has developed the ability to regenerate its body in a way that favors its survival during evolution.

 

When a lizard is in danger of being eaten by a predator, it will cut off its own tail and use the severed tail as bait to escape. But does a lizard that loses its tail stay tailless ever after? Not really. A lizard’s tail grows back to its original shape even if it’s cut off. On the other hand, consider the case of a dog. If a puppy’s leg is amputated in an accident, it doesn’t grow back. Once a leg or other body part, such as a tail, is amputated, the wound heals, but it doesn’t return to its original shape. These two examples show that organisms react differently to the loss of body parts. There are many different ways that organisms respond to changes in their bodies. Let’s take a look at them.
First, some creatures respond to the loss of a body part by regenerating it due to trauma. Take a closer look at the regeneration of the lizard’s tail we mentioned earlier. The lizard cuts off its own tail in times of danger. As soon as the tail is cut off, nerve cells send a signal to the brain to recognize that the tail has been severed. When the brain recognizes this, it concentrates a signaling substance called fibroblast growth factor (FGF) near the severed tail. FGF helps the cells in the area to reverse differentiate, that is, to become cells or tissues that play a different role than the one they were playing before the tail was cut off. The cells that undergo this reverse differentiation become gemma. Gemma has the ability to differentiate similarly to stem cells, re-differentiating into tail cells, which then form a new tail. To summarize, cells near the tail reverse differentiation under the influence of FGF and turn into gemmae, and these gemmae differentiate into tail cells to form a new tail. However, there are limits to the lizard’s ability to repair. The new tail is composed only of cartilage, not bone. Although it looks the same, its composition isn’t exactly the same as the old tail. Also, lizards can only repair their tails, not other body parts.
Salamanders, which are amphibians, are more capable of repairing damaged body parts than lizards, which are reptiles. Like lizards, salamanders secrete FGF to form cartilage and regenerate, but unlike lizards, salamanders can regenerate components of their tail, including bone and muscle. In addition, they can regenerate their legs and even their eyes. The secret to this regeneration is hidden in the expression of the ERK gene. The ERK gene is responsible for telling cells at the site of damage to proliferate. In salamanders, this gene is constantly expressed, which is why regeneration is possible.
On the other hand, in some creatures, amputated body parts don’t grow back and remain amputated. This reaction is mainly observed in mammals. The aforementioned ERK gene is also present in puppies and humans, but they don’t experience the same regeneration as salamanders. The reason for this has to do with the persistence of the gene’s expression. In salamanders, the gene is continuously expressed until the body is repaired. However, in humans and other mammals, this only lasts for about four hours, which is not enough time for a body part to regrow.
Finally, there are cases where a severed body part from one individual turns into another, resulting in the creation of two individuals. This is a form of asexual reproduction, in which an organism reproduces by replicating itself, and we can observe this reaction in starfish and planaria. In the case of starfish, this asexual reproduction occurs when they are cut to include a central area called the pyloric stomach. In planaria, stem cells make up 15% to 20% of the body. This is 45 times that of humans. In both species, there are body parts that can differentiate into various body tissues. The high percentage of body parts that can differentiate also allows for the creation of new individuals beyond the regeneration of body parts.
Why do different organisms have different abilities to regenerate lost body parts? Evolutionary theorists interpret this from an evolutionary perspective. They argue that the way each species responds to amputation determines its chances of survival. As a result, evolution has evolved toward responses that increase the chances of survival. Therefore, it is speculated that the duration of expression of the aforementioned ERK gene would have varied depending on the response. In other words, when a salamander is amputated, it is more likely to survive if its body is fully reattached than if it is missing a part, even if it takes longer. That’s why salamanders have evolved to have ERK expressed for longer periods of time. In mammals, surviving with a severed body part is more survivable than spending energy to repair it, so ERK has evolved to be expressed for a shorter period of time. Also, in organisms that are higher up on the evolutionary ladder, like mammals, there are controls in place to keep cells from deviating from their roles. As the body grew in size and the roles of each organ developed over the course of evolution, each cell was designed to perform a variety of bodily functions with precision. Because each cell or tissue must stay in its proper place to ensure that the sum of its parts, life, runs smoothly, it has evolved to inhibit signals that it is about to leave its position. All cells have the same genes, but the expression of a particular gene depends on where it is located and what role it plays in the cell, such as in the case of the ERK gene mentioned earlier. This regulation of expression is tighter in more complex organisms with more complex systems of life activity, which can be interpreted as differences in regenerative capacity.
When an organism suffers an amputation, it has three types of responses. They regenerate the missing part and regain their original appearance, they live with the missing part, or the missing part becomes another individual, thus maintaining the individual or species. As we move up the evolutionary ladder to higher organisms, the principles that sustain life become more detailed and complex, and it is this evolution that drives the differences in response. In order to regenerate, the cells around the lost part must be able to replace the lost part. In higher organisms, however, the system is more organized and it’s harder for already differentiated cells to change in a different direction. This is why starfish and planaria, which are relatively lowly organisms, are able to completely regenerate and give birth to new individuals when they are cut. Lizards and salamanders, which are higher organisms than starfish and planaria, can regenerate body parts. Mammals, which are higher than lizards and salamanders, are almost impossible to regenerate.
The principles and mechanisms of regeneration in various organisms are currently being studied. In recent years, there has been a growing expectation that this research will not only expand our biological knowledge, but also be applied as a treatment to overcome terminal or incurable diseases. By utilizing the principles of body regeneration in lizards, salamanders, and planaria, we may be able to find ways to treat people who have difficulty recovering from surgery or people with disabilities.