20 Interesting Facts About Skin » The Unclouded Path

You’ve probably heard of the expression “skin-deep”. It means something is superficial. In fact, many expressions regarding skin have something to do with superficiality or outward appearance. It turns out though, skin is much more than superficial. From fulfilling important health functions to enabling our complex sense of touch, these facts about skin show that it’s anything but.

1. Skin is the second (or third) largest organ in the human body in terms of surface area…

The largest is the interior lining of the small intestine (roughly 30-40 m² or 320-430 ft²). If you count the surface area of all the little air sacs in both the lungs, then the lungs are even bigger (about 50-75 m² or 540-810 ft²). But that doesn’t mean the skin isn’t impressive on its own. In an average adult, it covers a total of about 2 m² (22 ft²) and consists of roughly 16% of the body’s weight. In terms of weight though, it actually is the largest organ.

2. …but it’s quite thin.

Excluding fat (because it’s highly variable), it ranges from 0.5 mm at the eyelids to 4 mm at the heel. The thickness can also change from things like age, external stresses (ex. manual labor), and from various health conditions.

3. Skin consists of 3 main layers.

They are the epidermis, dermis, and hypodermis.

The epidermis actually consists of 4-5 layers (palms and soles have an extra layer). They are (from innermost to outermost):

Stratum Basale: This section borders the dermis and consists of a single layer of cells. This is where the cells of the rest of the epidermis come from. Here, basal cells divide and move up the layers of the epidermis until they reach the top layer and eventually flake off. There are also other kinds of cells in this layer. They include cells that produce melanin (responsible for skin color), immune cells, and touch receptor cells.
Stratum Spinosum: This section consist of the next 8-10 layers of cells. Here the cells start the process that turns them into the flat cells that make up the final layer of the epidermis. Since the cells are tightly connected to neighboring cells at distinct points, when they shrink slightly due to that process, they appear spiky, giving this section its name.
Stratum Granulosum: In the next 3-5 layers of cells, the cells start to lose their nuclei and organelles and flatten out. In the process, they leak special organelles containing lipids and proteins that help maintain the skin’s barrier properties.
Stratum Lucidum: This is the relatively clear (under a microscope) extra layer 3-5 cells thick on the palms and soles. The cells here aren’t much different from the previous layer, just a bit further along the process of becoming the cells of the final layer.
Stratum Corneum: The final layer of the epidermis is 15-30 cells thick and consists of flat cells that have no nucleus or organelles in a “brick-and-mortar” formation. Though the cells themselves are essentially dead and filled with keratin, the layer itself isn’t. It protects against impacts and other external stresses and has numerous feedback mechanisms that allow it to reduce water loss, react to infections, and maintain antioxidant levels.

Next is the dermis. Structurally, it’s mostly collagen and elastin (fibers that give skin its strength and elasticity), a gel-like extracellular matrix made of proteins and sugars, fibroblasts (cells that produce collagen and the extracellular matrix), and macrophages and mast cells (two types of immune cell). But this layer also contains different nerves that sense touch and temperature, hair follicles, sweat and oil glands, lymphatic vessels, and blood vessels. Both the epidermis and dermis depend on the blood vessels here for sustenance and waste removal.

The deepest layer is the hypodermis. It’s mainly for connecting the rest of the skin to muscles and for fat storage. It contains fat cells, fibroblasts, and macrophages. Fat serves as both an energy storage and as padding that reduces damage to muscles, bones, and internal organs.

4. The most touch-sensitive parts are the fingertips, tongue, and lips…

Each fingertip has over 3000 touch receptors with many being sensitive to pressure. The tongue and lips have even more densely packed receptors. Research using small gratings with only horizontal or vertical bars shows that people can discern the difference using their fingertips when the bars are only 0.94 mm apart. For the tongue, that threshold is only 0.58 mm, and for the lips, it’s 0.51 mm.

Some research seems to suggest that our fingertips can tell the difference between surfaces that differ only in their topmost layer of molecules. How? According to the researchers, by dragging your finger across the surfaces, you create different enough vibrations that you can tell if they are different or not.

5. …and the least is the trunk.

In fact, the entire trunk has about as many touch receptors as a single hand. A crude test you can do to see this difference is the two-point acuity test. Take two pencils, or similarly pointed objects, and lightly touch a part of your skin with the points. Measure at what distance you feel the two objects as two separate points. For example, for the inner wrist, most people start to feel the two points when they are about 25 mm (1 in) apart. That distance is way more for the lower back.

6. Fingertips can “see” with touch similar to the way eyes see.

Ever wonder why Braille works so well? It turns out that the nerves in your fingertips are organized so well that when you move your finger over something, the signals stay together as one coherent “image” as they travel to the brain. It’s very similar to the way signals from the retina in the eye travel to the brain, allowing you to see.

7. The skin contains at least 12 different types of nerves…

There are receptors for hot and cold, pressure, pain, and even vibrations. Interestingly, each of those nerves is specialized, meaning it only reports one kind of sensation. For example, if you stimulated a cold receptor with a hot point, you’d still only feel a cold sensation.

The temperature receptors, in particular, can detect changes as small as a fraction of a degree. However, they stop firing when tissue damage occurs, and pain receptors take over.

8. …but you lose them slowly as you age.

It’s the reason why the elderly have less sensitive skin than younger people. The receptors peak in number at around 16-18 years old and then decrease very slowly over time.

9. Skin also becomes thinner, more fragile, and less elastic as you get older.

This is mainly due to a decrease in producing collagen and elastin. In addition, the rate at which skin cells divide is slower, which means it takes more time for damaged skin cells to be replaced. Other factors include cumulative damage from environmental factors (ex. sunlight, pollution) and from smoking and other lifestyle habits.

10. Too much ultraviolet light (UV) exposure can cause skin damage and make it look older.

UV light damages the collagen fibers in the skin making it more susceptible to wrinkles. In addition, it also destroys vitamin A in skin, which the skin needs to maintain itself. Remember your sunscreen!

11. The amount of melanin in your skin determines how easily you get sunburned.

Melanin serves as a shield against UV, which causes sunburns. People with pale skin (least amount of melanin) always get sunburned and never tan, while people with very dark skin never get sunburned. The spectrum of skin colors and their susceptibility to sunburns is detailed in the Fitzpatrick scale.

12. Skin renews itself about once every month and a half…

Cells in the epidermis take about 2 weeks to travel from the stratum basale to the end of the stratum granulosum. Then they take another 4 weeks to go through the stratum corneum. The entire epidermis refreshes itself with new cell growth in about 48 days.

13. …but things like scars and tattoos remain in place far longer because of certain skin cells.

In the case of scars, fibroblasts lay down a thick layer of collagen that differs from collagen in normal tissue. The collagen fibers in normal tissue are in a random “basket-weave” pattern, while in scars, the collagen fibers are mostly arranged in a single direction allowing them to be densely packed. New cell growth doesn’t replace this patch of stiff collagen, so it remains.

For tattoos, another cell is responsible: macrophages. Macrophages are essentially garbage collectors and eat the ink particles from tattoos. But due to the particles’ size, they can’t really break them down. When the macrophages die, the ink particles get taken up by new macrophages. Over time, this causes the ink particles to move deeper into the dermis, which is why tattoos slowly lose their detail. Laser tattoo removal works by using lasers to break the particles into smaller bits that macrophages have an easier time dealing with.

14. About a thousand different species of bacteria live on your skin.

Most of them live in the upper parts of the epidermis and hair follicles. But they aren’t distributed equally. That’s because skin isn’t the same everywhere. There are places where skin is more oily or moist and places where there’s more hair growth. As you can see in the following image, the distribution of various species of bacteria varies widely across different body parts.

The distribution of bacterial species at different regions of skin.

The vast majority of those species are harmless. Some even help in various ways, such as by crowding out harmful species of bacteria, making chemicals against them, and stimulating the immune system. Of course, there are also opportunistic bacteria (usually harmless but can cause infection if given the chance) and harmful ones as well. That’s where washing your hands comes in.

15. Regular soap is better than antibacterial soap for keeping your skin clean.

It’s better for a number of reasons. First, regular soap and water are already pretty effective at removing bacteria from your skin. Second, using antibacterial soap promotes bacterial resistance to those antibacterial chemicals. Third, antibacterial chemicals are indiscriminate, meaning they kill all bacterial, even the beneficial ones. Lastly, needlessly exposing yourself and the environment to antibacterial chemicals can lead to unexpected health effects and ecosystem disruption, respectively. Just stick with regular soap and water.

16. Water temperature doesn’t open or close pores.

You may have heard that hot water opens pores and cold water closes pores. Well, both are wrong since pores are technically always open. They’re the openings of hair follicles. The confusion probably comes from the fact that pores can get blocked by dead skin cells. When that happens, they can dilate. While it’s possible hot water can help dislodge a blockage, there are far more effective treatments, such as salicylic acid.

17. The health of your gut influences the health of your skin.

Surprisingly, the health of the bacterial ecosystem in your gut influences the health of your skin. When you start to lose the beneficial bacteria in your gut, it can lead to skin irritation, inflammation, and rashes. Research has shown that this is in part due to the beneficial bacteria reducing the release of cortisol, a stress hormone. Cortisol degrades collagen and reduces collagen production. It also reduces certain aspects of immune function.

18. You have an “emotional” touch system.

Why do hugs feel nice and pain, well, not so nice? It turns out there are different neural pathways for the physical aspect of touch and the emotional aspect of it. Special receptors called C-tactile fibers that connect to a part of the brain called the insula are responsible for the “pleasantness” of touch. Another set of receptors connected to the insula makes pain feel bad. The insula takes the signals from those nerves and translates them into corresponding emotions.

One example that shows this distinction is the case of pain asymbolics. These are people who have had their insula disrupted somehow, either through actual damage or through certain drugs like morphine. They can feel pain just fine. They know where it is and how strong it is, but it doesn’t seem to bother them.

19. Your emotional state can affect your sense of touch.

Emotions can positively or negatively reinforce sensations. You’ve probably experienced this yourself at some time. If you’re feeling crummy, any pain suddenly seems a whole lot worse. Conversely, if you’re happy, a similar pain doesn’t bother you as much. It’s the same for pleasant touch, too. Pleasant touch feels nice when you’re in a positive mood but won’t feel as good when you’re angry.

20. Touch seems to be important during infancy for proper development.

The reason why is unclear but touch seems to be very important for infants’ development, particularly newborns. Newborns cry less, sleep better, and learn to recognize their parents earlier when they have regular skin contact. These aspects might affect their hormone levels (they’re very different from newborns without much contact), which can in turn affect their development.

A lack of touch and interaction as very young children is also associated with an increased risk of emotional and behavioral problems later on in life. Studies on children in orphanages have suggested that kids benefited greatly from being transferred to an actual home, but the longer they stayed at an orphanage, the more likely they were to have long-term problems.