In fact, dermatologists and plastic surgeons have long noticed that after the wounds of the elderly heal, the scars formed are often thinner than those of young people, and it is more difficult to leave scars. This phenomenon, which is contrary to our intuition, has been puzzling scientists, because it may indicate that the skin regeneration ability of the elderly is stronger than that of the young.
This phenomenon aroused the curiosity of Dr. Thomas Leung, and he decided to study the problem in depth to eliminate people's confusion. The results were published in the latest academic journal Cell Report.
How was the wound repaired?
Let's first understand the process of biological wound repair, which involves the synergy of two biological processes: scar formation and tissue regeneration.
Scar is the fibrous tissue formed at the wound, which acts as a barrier and can protect the wound. Scar has no function of normal tissue, no pores, sebaceous glands and so on. Lack of elasticity affects the appearance. Tissue regeneration is to re-form normal tissue without leaving scars.
After the small wound is healed, the fibrous tissue will be replaced by the regenerated normal tissue, and the scar will disappear. However, if the injury is large or the tissue is repeatedly damaged, the wound healing process may be incomplete, leading to scar deposition. The scars left by our injuries or operations, as well as some pathological scars, are all caused by excessive deposition of fibrous tissue.
Why don't old people leave scars? The key is blood.
Then, why do the wounds of the elderly leave less scars and form a complete skin? The research team first confirmed in mice that compared with young mice, the tissues of skin wounds in old mice regenerate faster and leave almost no scars, while young mice leave obvious scars.
Previous studies have pointed out that the function of stem cells in some organs of the body will be improved after the old mice are exposed to the blood of young mice, which indicates that there may be some factors in the blood that can regulate the function of stem cells. Leung suspects that there may be such factors in the blood of mice, which regulate the regeneration of skin tissue.
Following this idea, they exchanged blood between old mice and young mice, and then conducted skin puncture experiments. Sure enough, there are obvious scars on the wound of the old mouse, which shows that the answer lies in the blood!
Cytokine SDF 1
The research team compared the difference of gene expression profiles between the two groups and the difference of gene expression products in blood, and locked the cytokine SDF 1 secreted by skin keratinocytes.
It has been found that SDF 1 can regulate tissue regeneration, and the expression of SDF 1 in young mice is higher than that in old mice, which may suggest that old mice can inhibit the expression of SDF 1 and scar formation. In young mice, SDF 1 may play a role in promoting scar formation.
Subsequent experiments also proved this point. Knocking out SDF 1 in young mice can obviously improve the tissue regeneration ability of mouse wounds, leaving almost no scars, just like old mice. It seems to be SDF 1, which promotes scar formation and inhibits skin tissue regeneration in young rats.
The researchers also found the reason why the aged mice inhibit SDF 1: With the increase of age, mice will express another protein EZH2, and this protein will shut down the expression of SDF 1 gene and prevent SDF 1 from playing its role.
Does this principle apply to the human body? The answer is yes. The research team has proved that skin injury in young people can trigger the expression of SDF 1. In the elderly, the expression of EZH2 will increase, and then the expression of SDF 1 will be inhibited.
The conclusion of "against common sense"
Why is the ability of tissue regeneration stronger in the elderly and weaker in the young? Obviously, the wound repaired by tissue regeneration is more complete in function and better than scar formation. Young people's resilience is not as good as that of the elderly, which is a bit contrary to common sense.
The key to the answer lies in the speed of repair! Because when tissue regeneration is completed, it takes about one month to differentiate into various types of cells and rebuild various tissue structures. Injury is very dangerous in nature. If you can't stop the wound in time, it is likely to cause infection, affect your actions and be detrimental to your survival. However, scar formation only takes a few days, which is beneficial from the evolutionary point of view.
However, for modern humans, leaving scars is not a good thing. Pathological scars and scars left by burns, trauma and other ways will affect the appearance, cause dysfunction and bring inconvenience to people's lives.
Therefore, this study can be of great help to prevent the formation of scars and solve the scars left by plastic surgery. Dr. Liang said that the research results may be used in clinic soon. There are already inhibitors of SDF 1 on the market, which were originally used to activate stem cells in vivo for treatment. This drug is expected to be used to prevent human scar formation in the future.