This blog post explores the difference in regenerative abilities between lizards and dogs from a scientific and evolutionary perspective in an easy and interesting way.
When threatened by a predator, a lizard will voluntarily sever its own tail and flee, leaving the severed tail behind as bait. So, does a lizard live without a tail forever after losing it? No. A lizard’s tail grows back to its original shape after being severed. Now, let’s look at the case of a dog. A dog that undergoes surgery to amputate a leg due to an accident will not regrow that leg. Not only legs, but other body parts like tails also do not regenerate once severed; the wound at the amputation site simply heals, but the original form is not restored. These two examples show that organisms exhibit different responses when they lose a body part. Thus, organisms respond to bodily changes in various ways. Let’s examine these methods.
First, some organisms exhibit a regenerative response when a body part is lost due to trauma. Let’s examine the lizard’s tail regeneration process mentioned earlier in detail. Lizards sever their tails in dangerous situations. When the tail is severed, nerve cells immediately send a signal to the brain, alerting it that the tail has been cut off. Once the brain recognizes this, it concentrates a signaling molecule called Fibroblast Growth Factor (FGF) near the severed tail. FGF helps nearby cells undergo dedifferentiation, meaning they transform into cells or tissues performing different functions than before the tail was severed. These dedifferentiated cells transform into gemma. Gemmae possess a differentiation capacity similar to stem cells, allowing them to redifferentiate into tail cells, thereby forming a new tail. To summarize: cells near the tail, influenced by FGF, undergo dedifferentiation to become gemmae, and these gemmae differentiate into tail cells to create a new tail. However, lizards’ regenerative capacity has limitations. This is because the new tail consists only of cartilage, lacking bone. While the shape is the same, its composition is not entirely identical to the original tail. Furthermore, lizards can only regenerate their tails; they lack the ability to regenerate other body parts.
Amphibians like newts possess superior regenerative capabilities for damaged body parts compared to reptiles like lizards. In salamanders, like lizards, FGF is secreted to form cartilage and enable regeneration. However, unlike lizards, salamanders can regenerate the tail’s components, including bone and muscle. Not only that, they can regenerate limbs and even eyes. The secret to salamanders’ regeneration lies in the expression of the ERK gene. The ERK gene signals cells in the damaged area to proliferate. In salamanders, this gene is continuously expressed, enabling regeneration.
In contrast, some organisms, primarily mammals, cannot regrow severed body parts, which remain permanently lost. The ERK gene mentioned earlier exists in dogs and humans too, but phenomena like salamander body regeneration do not occur in them. The reason lies in the persistence of gene expression. In salamanders, this gene is continuously expressed until the body is restored. However, in humans and other mammals, this persistence lasts only about 4 hours—a timeframe woefully insufficient for body parts to regrow.
Finally, there are cases where a severed body part from one individual transforms into another individual, resulting in the creation of two organisms. This is a form of asexual reproduction where the organism replicates itself exactly. Such responses can be observed in starfish and planaria. In starfish, this asexual reproduction occurs when the body is severed in a way that includes the central region called the pyloric stomach. In planarians, stem cells constitute 15% to 20% of the body, which is 45 times higher than in humans. Both species possess body segments capable of differentiating into various tissues. Furthermore, the high proportion of differentiable segments allows for the generation of new individuals, going beyond mere regeneration of body parts.
Why do organisms differ in their ability to regenerate lost body parts? Evolutionary biologists interpret this from an evolutionary perspective. They argue that each species’ survival probability varies based on how it responds to amputation injuries. Consequently, they suggest that evolutionary direction likely favored responses that increased survival probability. Therefore, they hypothesize that the duration of ERK gene expression, mentioned earlier, likely varied depending on the response strategy. That is, for salamanders, even if it takes time, having the body fully regenerate offers a higher survival rate than living with a missing body part. Consequently, salamanders evolved to have a longer ERK expression time. In mammals, however, living with a severed body part offers a higher survival rate than expending energy to restore the body, leading to the evolution of shorter ERK expression times. Furthermore, in organisms at a higher evolutionary stage like mammals, control mechanisms operate to prevent cells from deviating from their designated roles. As bodies grew larger and the functions of individual organs became more specialized during evolution, each cell was designed to precisely execute diverse bodily functions. Since the sum of these roles—the life activities—can only function smoothly when each cell or tissue remains faithful to its role, organisms have evolved to suppress signals attempting to deviate from their designated positions. Similar to the suppression of ERK gene expression mentioned earlier, while all cells possess the same genes, whether a specific gene is expressed depends on the cell’s location and function. This expression regulation is applied more strictly in organisms with more complex life systems, which can explain differences in regenerative capacity.
When subjected to amputation, organisms exhibit three types of responses. They either regenerate the lost part to restore their original form, live with the defect, or allow the lost part to become another individual, thereby maintaining the organism or species. As organisms evolve into higher life forms, the principles sustaining life become more intricate and complex. This evolution drives the differences in the aforementioned responses. For regeneration to occur, cells surrounding the lost part must be capable of transforming into the missing tissue. However, in more advanced organisms, systems are highly specialized, making it difficult for already differentiated cells to switch functions. Therefore, relatively lower organisms like starfish and planaria can completely regenerate from amputation wounds, giving rise to new individuals. Higher organisms than starfish and planaria, such as lizards and salamanders, can regenerate parts of their bodies. Mammals, higher organisms than lizards and salamanders, are nearly incapable of regeneration.
Research into the principles and mechanisms of regeneration in various organisms is currently active. Recently, expectations are growing that this research could lead not only to expanding biological knowledge but also to developing treatments for incurable or intractable diseases. Utilizing the principles of body regeneration in lizards, newts, and planaria could help discover methods to treat people who have difficulty recovering after surgery or those with disabilities.
