Why Do We Scar, But Lizards Don’t?

Table of Contents (click to expand)

Scars are the result of our cells’ actions to heal broken tissue. Different animals, such as lizards, reindeer, and the spiny mouse, can heal without scarring because they perform something call regenerative healing.

We all have scars, whether they’re from a serious accident or just from being clumsy. Our skin miraculously heals from the damage, but the scar will always remind us of that childhood tumble from the swing set.

How does such a scar form? And, more interestingly, what have we learned about the healing process from animals that don’t develop scars?

Big wounds leave scars after they heal (Credits: Photohobo/Envato Elements)

How Wounds Heal

When you injure your knee, for example, your body activates multiple biochemical pathways almost immediately. Various molecules alert various different types of cells to jump into action.

The first step is to stop the body from losing any more blood through the wound. To do this, the body forms a blood clot. This serves as a scaffold or skeleton for cells that arrive at the wound later. These cells release pro-inflammatory chemicals, which recruit specialized immune cells to fight any pathogens that may have entered the body through the wound.

After this, this skin can begin to repair the cut. Skin cells proliferate and migrate to the site of injury, and then new blood vessels form to restore blood flow to the area.

Later, cells called fibroblasts produce extracellular matrix (ECM – the material between cells) proteins, mainly collagen, which form the bulk of the scar tissue.

The wound healing process (Credits: Designua/Shutterstock)

Collagen is present in all skin, whether it is scarred or unscarred. Collagen helps maintain the structural framework of the skin tissue. However, collagen in scar tissue is disorganized.

Eventually, upon healing, all the biochemical processes that were activated after the injury get switched off. The cells that came to the site of injury either die (a unique type of cell death called apoptosis) or exit from the wound.

This leaves behind scar tissue consisting mainly of collagen and some other ECM proteins. Rats and mice undergo a similar wound-healing process, while salamander and lizard skin wounds heal without forming scars.

Scar tissue replaces normal skin after an injury (Credits: Rattiya Thongdumhyu/Shutterstock)

Also Read: Do Different Parts Of The Body Heal At Different Rates?

Not All Animals Scar

Remarkably, the fetus version of you didn’t scar. Human fetuses can heal wounds that leave no trace behind, but we lose this ability once we’re born. In fact, all mammalian embryos that have been studied thus far, including marsupials, mice, rats, sheep, pigs and rabbits, all heal wounds without scar formation.

For some invertebrates, fish and amphibians, birth changes little. They can still heal their wounds without any scars. This type of wound healing process is called regenerative healing. Planaria, zebrafish, and salamanders can all heal regeneratively. Among mammals, spiny mice show regenerative healing. Male reindeer antler skin (velvet) regenerates after wounding, while wounds on the skin of their backs leave scars behind as they heal.

Also Read: Organ Regeneration: Why Can’t Humans Regenerate Organs?

How Do Some Animals Manage To Remain Scarless?

We’re mammals, just like the spiny mouse and reindeer. Why can they heal without scarring in some scenarios, while most other mammals can’t?

The answer, unfortunately, involves various factors. Though scars appear to be collagen that’s not aligned well, the reasons for that structural formation are various, and involve different processes of healing. Inflammatory and immune responses, growth factors, ECM protein deposition, and mechanical stress all govern whether wound healing will leave scars or not.

Inflammatory And Immune-Cell Responses

A higher inflammatory response is correlated with scar formation. The skin of embryos that heal without scars has a minimal inflammatory response, but increased activity of anti-inflammatory chemicals.

This relates to how fibroblasts help rebuild tissue. When the cell builds by suppressing the immune system, then wounds heal without any scars forming, like in fetuses and reindeer antler skin.

The fibroblasts in our skin (after birth( express inflammatory chemicals, so the wounds proceed to heal with scars.

Fetal skin and reindeer antler skin heal without scars, while human adult skin and reindeer back-skin show scarring upon injury (Credits: Sinha, S., Sparks, H. D., Labit, E., Robbins, H. N., Gowing, K., Jaffer, A., … Biernaskie, J. (2022, December). Fibroblast inflammatory priming determines regenerative versus fibrotic skin repair in reindeer. Cell. Elsevier BV. http://doi.org/10.1016/j.cell.2022.11.004 )

Enzymes And Growth Factors

A particular type of mouse called spiny mice naturally heal without scarring. Scientists have been studying how these mice heal on a molecular level.

A paper published in 2023 found that an enzyme called the ERK (extracellular signal-related kinase) is the difference between scars and no scars in spiny mice. Spiny mice show sustained ERK activity, which promotes skin cell proliferation and induces a regenerative healing response.

Indeed, all wounding leads to an immediate activation of ERK, even our own, but when ERK levels eventually decline in activity within the mouse skin, the mouse skin scars.

Lower levels of certain molecules called growth factors—such as Platelet-Derived Growth Factor (PDGF) and Transforming Growth Factor-β (TGF-β)—are also associated with scarless wound healing.

Mechanical Force

The skin consists of tightly packed cells that give rise to a mechanical force in the skin layer. Upon injury, the way in which the skin surrounding the wound pulls and stretches is one of the factors that determines how much scarring will occur. A higher mechanical stretch stimulates certain biochemical pathways, which lead to the accumulation of collagen and ECM. This situation favors the development of scars.

Also Read: Why Do Healing Wounds Itch?

Animal Size

The size of animals also correlates to stress on their cells. As animals get bigger, the force between their cells increases. A study has identified that these forces in larger animals, such as humans, contribute to scarring. Smaller animals, such as zebrafish and salamanders, do not have such strong forces, and thus show scarless healing.

Why Study Scarless Wound Healing?

The process of wound healing is one of the most complex biological processes in nature. The correct wound healing process involves the activation of several biochemical pathways at the appropriate time. However, once the wound is healed, it is equally important that these pathways be switched off. In some people, an impaired wound-healing process may lead to over-scarring, causing skin hardening and eventual loss of function. This incurs a significant burden on the healthcare sector,

Additionally, scar tissue is inflexible and has less strength than undamaged skin. It also doesn’t contain essential skin structures, such as hair follicles and sweat glands. Thus, the ability to avoiding scarring can help in regenerating the skin and restoring original tissue, considerably improving human health overall.

Several factors determine whether wound healing will proceed with or without scar formation (Credits: Pereira, D., & Sequeira, I. (2021, July 26). A Scarless Healing Tale: Comparing Homeostasis and Wound Healing of Oral Mucosa With Skin and Oesophagus. Frontiers in Cell and Developmental Biology. Frontiers Media SA. http://doi.org/10.3389/fcell.2021.682143 )

A Final Word

Wound healing elicits a biochemical response involving multiple cells. Specialized cells called fibroblasts, activated by immune cells called macrophages, secrete ECM proteins like collagen, which forms the bulk of a scar tissue. Some animals can heal wounds without forming a scar. Several factors, including inflammatory and immune responses, enzymes, growth factors, ECM protein deposition and mechanical stress, govern whether wound healing will proceed with or without scars.

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About the Author

Sneha has an MSc in Biochemistry from MS University, Baroda, after which she was a research fellow at inStem, Bengaluru for four years. Eventually, she realised she likes writing about science more than doing science, and switched lanes to science journalism. As a science journalist, she likes to write about science where it intersects with the society. When she’s not writing, she is huddled in a corner with a cup of tea, reading novels, or watching Netflix.