Do you know how hard your belly works to avoid bursting?

We all know that our skin (epidermis) is stretchable, but the science behind this is surprisingly simple.

There are two reasons why the epidermis can stretch: the increase in cell volume and the increase in cell number. However, until recently, scientists have uncovered the physiological mechanism by which the epidermis can stretch under long-term external forces, and stem cells once again played a key role .

When we make faces, we stretch our eyelids and the skin on our faces, but they return to their original shape as soon as we let go.

This is a classic example of the skin's ability to stretch.

(Image source: flickr/lincolnblues)

On May 30 this year, a research paper jointly published by the Institute of Stem Cell and Regenerative Medicine of the Chinese Academy of Sciences and other units further revealed the specific mechanism by which stem cells play a role in skin repair.

The relevant results were published online by the journal Frontiers in Cell and Developmental Biology, and all the authors of the paper are from research institutions in China.

Do you know how hard your skin works to repair itself?

During pregnancy, the abdominal circumference of a pregnant woman can increase by 20-30 cm compared to normal, and the belly bulges into a ball shape. We have never heard of a pregnant woman bursting her belly, and even mothers who are pregnant with multiple babies can gradually increase their belly as the babies grow.

Copyright image, no permission to reprint

The same principle applies to the smooth implementation of breast augmentation, nose augmentation, and face lifts in plastic surgery without causing skin damage, which is also related to the skin's tensile and stretching properties. Although the medical community has been enjoying the benefits of this characteristic of the skin, no one can explain the specific mechanism.

It was not until 2020 that a research team from Belgium and the United Kingdom found the answer to the question, and the relevant research was published in the journal Nature. It turned out that the ubiquitous stem cells once again "struggled" to help the skin carry the burden of self-repair . After that, scientists continued to conduct in-depth research on the role of stem cells in the skin repair process. The research results of Chinese scientists this time revealed several key regulatory factors that play a decisive role in this process .

The epidermis is the bottleneck in the expansion of a pregnant woman's belly

If we carefully consider the structure of the belly, we can find that its bottom layer is muscle, the middle layer is subcutaneous tissue mainly composed of fat, and the top layer is skin, including dermis and epidermis, etc. Among the above structures, except for the epidermis, all have strong stretch and deformation capabilities, which is more than enough to cope with the deformation caused by pregnancy.

Diagram of the abdomen and fetus during pregnancy (copyright image of the library, unauthorized reproduction)

But the epidermis is different. Although the epidermis also has a certain degree of stretchability, its stretchability is gradually reflected in the slow stretching process .

In other words, if a woman's belly suddenly becomes as big as a 10-month pregnant woman, the epidermis may burst. (In fact, in the late pregnancy, the dermis may not be able to hold up first, resulting in the formation of stretch marks.)

Stretch marks (copyright image from the gallery, no permission to reprint)

The epidermis' ability to stretch is also reflected in surgical and cosmetic procedures. For example, after breast augmentation surgery, the skin on the chest may feel tight at first, but it will soon return to a smooth and natural appearance. But what is the mechanism behind this? To understand this question, we need to talk about the structure of the skin.

Skin structure and regeneration

As the first barrier of the human body against external pathogens, the contribution of the skin is often overlooked. In fact, without the barrier of the skin, bacteria can easily invade human tissues on a large scale.

The skin is the largest organ in the human body and has a certain elasticity. Moreover, when external stress is applied to the skin, the skin will also expand, that is, grow slowly.

The skin is mainly composed of three parts: epidermis, basement membrane and dermis . The epidermis is the outermost layer, and from the outside to the inside, they are stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum and stratum basale. The stratum corneum is the thickest part and is well-deserved as the first line of defense of the human body.

The basement membrane is the thinnest layer, sandwiched between the basal layer of the epidermis and the dermis. The dermis is dozens of times thicker than the epidermis and is composed of connective tissue, which can buffer the pressure and strain on the body. The dermis has good elasticity and has blood vessels, nerves and lymph running through it.

Skin and underlying muscle tissue (copyright image from the gallery, no permission to reprint)

Just like the withering and growth of leaves, the skin is constantly shedding and regenerating, and this process is closely related to skin stem cells.

Skin stem cells are a type of cell that exists in the skin and has a strong ability to proliferate and differentiate. They are closely related to the expansion and repair of the skin. Skin stem cells can participate in regeneration by dividing to produce two different types of cells, both of which initially exist in the basement membrane.

After the epidermal stratum corneum falls off, these cells start from the basement membrane layer and move from the inside to the outside through the epidermis, like a process of outward diffusion. They go through multiple differentiation stages until they move to the epidermal stratum corneum to become a new stratum corneum, forming a complete process of skin metabolism.

This process repeats approximately every 48 days, so it is completely invisible to the naked eye.

What happens to the skin when it is stretched?

In addition to the regular skin regeneration process led by skin stem cells, when subjected to sudden mechanical stress, the skin resists such changes by adjusting its size and shape to maintain its function as the body's first barrier.

In clinical practice, doctors bury an inflatable " skin expander " under the skin and expand it slightly, so that the covering skin feels a tiny stress, thereby promoting faster regeneration and stretching of the skin.

Different Types of Skin Expanders

(Image source: Reference 5)

This "skin expander" provided inspiration for research by scientists from Belgium and the United Kingdom, who injected a similar inflatable hydrogel under the skin of model mice to give the skin appropriate expansion stress and made detailed observations on the responses of epidermal cells.

Initially, the volume of mouse epidermal cells increased while the density per unit area of ​​skin decreased, which seemed to be a "stopgap measure" for the skin to deal with emergencies. After all, it is not difficult to increase the volume of cells, but it takes a certain amount of time to increase the number of cells.

Since then, scientists have discovered that stem cells sense external stress and their own abnormalities during differentiation and upward movement from the basement membrane. These stem cells then transmit this perception to neighboring stem cells and promote their temporary self-renewal and division to cope with possible cell number loss.

Proliferation and differentiation of stem cells in mouse skin subjected to stretch

(Image source: Reference 3)

It is like each stem cell has a switch and a key to turn on the proliferation mode. When they sense stress, the stem cells that were originally in normal working mode will use the key to turn on the proliferation switch of the adjacent stem cells, allowing them to briefly change to proliferation mode to increase the number of cells, thereby accelerating the regeneration of skin cells.

It is worth mentioning that the researchers also found that not all stem cells will respond to external stretching, only some specific stem cells will respond .

These fascinating findings reveal why the skin is able to stretch when subjected to external forces:

When subjected to external forces, skin cells can sense stress and respond - the epidermal cells will adjust their own shape, and then some specific skin stem cells will briefly divide and proliferate to expand the cell number, thereby maintaining the original integrity of human skin.

It can be imagined that the reason why the skin on a pregnant woman's belly does not rupture due to expansion as it gradually grows is that the deformation and expansion of her belly skin cells and the increase in the number of cells caused by the proliferation of stem cells make the skin as tough as a rubber membrane .

The latest research results confirm the omnipotence of skin stem cells and reveal the reason behind the skin's superb ability to repair itself after severe deformation or surgery. For the plastic surgery industry, this research also objectively provides a certain theoretical basis for various plastic surgeries. However, there are individual differences in physique, and there are differences in level and experience of beauty medical institutions. Before choosing to undergo plastic surgery, you should still do sufficient research and preparation.

References:

[1] Nature: The facelift technique has been used in plastic surgery for more than 60 years. The principle is as follows: https://xw.qq.com/cmsid/20200809A0G1S800

[2]Stretch exercises for stem cells expand the skin Stretch exercises for stem cells expand the skinhttps://www.nature.com/articles/d41586-020-02158-y

[3]Mechanisms of mechanical-stretch-induced skin expansion revealedhttps://medicalxpress.com/news/2020-07-mechanisms-mechanical-stretch-induced-skin-expansion-revealed.htm

[4]Mechanisms of stretch-mediated skin expansion at single-cell resolutionhttps://www.nature.com/articles/s41586-020-2555-7

[5]Trebuňová, Marianna, et al. "NEW TECHNOLOGIES OF TISSUE EXPANSION-REVIEW ARTICLE."

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