Ask Smithsonian: How Does Skin Heal?

The skin is an organ system that is unique to each individual, so not everyone heals the same way

Skin serves many purposes. It protects our internal workings, it can reflect our mood and our health, and it provides an exterior protective wrapping. When skin is damaged, the repair process is not just a biological curiosity, it can be a matter of life and death.

The skin is an organ system, weighing 8 pounds on average, that regulates body temperature, senses pleasant and painful stimuli and temperature, secretes sweat and oils, and helps protect us from the harmful effects of ultraviolet radiation.

Skin is as unique as each one of us, so it’s not surprising that even though there’s a general healing process, not everyone heals the same way.

“Wound healing is extraordinarily complex, and has lots of external and internal factors,” says Adam Friedman, an associate professor of dermatology at George Washington University School of Medicine and Health Sciences. “There’s no one-size-fits-all approach. You have to take the wound and the person into account when figuring out an algorithm of healing,” says Friedman.

Typically, healing is initially dictated by the depth of the wound. Superficial wounds tend to reach only into the epidermis. That’s the top-most layer of skin, which is very thin. The upper part of the epidermis contains keratin, a substance made from dead cells that helps protect the skin from harmful substances. The bottom part contains melanocytes, which are cells that produce the dark-colored pigments known as melanin.

Skin scratch
“Wound healing is extraordinarily complex, and has lots of external and internal factors,” says Adam Friedman, an associate professor of dermatology at George Washington University School of Medicine and Health Sciences. RusN/iStock

A scrape of the epidermis won’t draw blood and heals easily and quickly, says Friedman. Some very deep wounds—such as pressure sores or burns—don’t bleed either. The deeper a cut or puncture wound or burn, the more has to happen for healing. Deeper wounds extend into the dermis—which contains blood vessels, nerves, hair follicles, sweat and oil glands and the support structures, including collagen and elastin—or even deeper, into the body’s fat layer.

Wounds always heal from the inside out and from the edges inward. In a healthy person it works this way: Within seconds to minutes of an injury, blood vessels will constrict to reduce bleeding. Platelets—sticky blood cells—flood the area and aggregate into clumps. Clotting factors soon come on the scene, joining with the platelets to form a clot. Meanwhile, white blood cells (macrophages) come through, scavenging for infectious invaders. Over the next few days, macrophages also make growth factors to help repair the wound.

The clots turn into scabs, and, underneath, fibroblast cells produce collagen, a protein that connects tissues together. In a weeks-long process, the collagen creates new capillaries and the skin on the edges of the wound gets thicker and starts stretching under the scab. The skin might look reddish and start to itch—a normal part of healing, says Friedman. The nerve cells send itch signals in response to the perceived movement of the new skin coming in, he says.

The scab usually falls off by itself—that is, unless it is picked off. That shouldn’t be done, but the wound shouldn’t be allowed to dry out, either, says Friedman. Dry, crusty scabs are like brick walls preventing new cells from migrating to the wound and can also be a food source for bacteria, he says.

To prevent the area from drying out, he recommends a coat of petroleum jelly with a bandage on top. First, a wound should be cleaned with soap and water. Alcohol and peroxide are okay initially, but unnecessary, and repeated use actually is toxic to the new skin and retards healing. Use of antibacterial ointments is also totally unnecessary, unless the wound is already infected. Otherwise, they could cause bacterial resistance or allergic reactions.

It’s especially important to keep a new wound protected from sun exposure. The radiation can promote inflammation and interfere with the rebuilding process, says Friedman.

Almost every deeper wound forms a scar, which is basically skin that isn’t as strong or flexible as original skin. Even if the scar does disappear, it might take a few years.  Some people are more prone to form very thick scars, known as keloids. It’s still not clear why these occur, but it’s likely due to genetic mutation, Friedman says. And keloids aren’t just exaggerated scars; they can keep growing, become itchy and painful, and can be disfiguring.

Lots of people will have delayed or difficult skin healing—smokers, heavy drinkers, diabetics, and individuals who are malnourished, have poor blood flow, compromised immune systems or pre-existing infections. Some medications can interfere with healing also, including corticosteroids and non-steroidal anti-inflammatories like ibuprofen and naproxen.

And babies have an advantage over their grandparents. As people age, the skin loses elasticity, blood flow and the ability to secrete protective oils, which slows down healing.

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