We live in an age of cosmeceuticals, but the folks selling skincare frequently speak in anecdotes (burn victims, stem cells, plastic surgeons, proprietary molecules). I’m a skeptic, and I often walk away from these conversations wondering about the science of it: How does that product work? Talk to me about active ingredients and biological mechanisms. If there was a study, I want to see it.
The lack of clear answers in the anti-aging field (and the discrepancies even in “common knowledge” on the subject) inspired me to search out “real,” conclusive answers to the most common skin claims. I plan to tackle these questions in a series of posts, but I thought we should probably start with an introduction to skin itself.
You’ve probably heard that skin is the largest organ in the human body. But it’s not one homogenous organ: You intuitively know that the skin on your eyelids is very different from the skin on the soles of your feet (because skin needs to do different things in different places).
Skin protects your body from the outside world, with different responses to different kinds of threats:
- in response to mechanical threats, skin builds up a thick outermost layer (the calluses on the soles of your feet);
- chemical threats are repelled by a layer of oil, secreted by glands in the skin;
- radiation is dealt with in two ways. Melanin absorbs and neutralizes UV; various DNA repair systems correct any mistakes caused by radiation.
- skin is a physical barrier: skin cells are connected by a system of watertight rivets, preventing microbes from entering the body;
- and finally, but most importantly — because no protective mechanism is perfect — skin cells are constantly being replenished. Each one only lasts about a month before it is sloughed off.
These defense mechanisms are mostly passive; your skin is not changing or reacting to the world on a second-to-second basis. (Though skin does have other, more active functions — like temperature regulation.)
The epidermis (the skin’s outer layer) is responsible for most passive protective functions. It has a small number of melanin cells, which produce the pigment that gives skin its color.
An inner layer (dermis) gives skin its strength and elasticity. The dermis also contains blood vessels, hair roots, sweat and oil glands, cells of the immune system, and many nerve endings.
Interestingly, the border between the dermis and epidermis is pockmarked, so that the dermis fits into the epidermis like a peg into a socket. This increases the area of contact between the two layers — since the blood and nutrient supply is in the dermis, this allows more oxygen and energy to get to the epidermis.
There is also a layer below the dermis (the hypodermis) which is mostly a layer of fat (for insulation and shock absorption). The hypodermis accounts for about half of your body fat. This fat is divided into compartments by bands of fibrous tissue that run from the dermis to the muscle underneath — the way a quilt is divided by lines of stitching.
Excluding the palms and soles (which are specialized) and calluses (which are created in response to your unique lifestyle), epidermis thickness ranges from about 0.05 mm to 0.15 mm. The dermis varies a bit more in thickness, depending on location: Eyelid skin has very thin dermis (0.5 mm), while the lower back can have dermis up to 3 mm thick.
This matters because the thickness of skin affects its physical behavior. There’s much more to say on the subject of aging, but thinner skin shows signs of damage (either from aging or sun exposure) more quickly. You’ve probably noticed that wrinkles on your face develop earlier than wrinkles on your back or thighs. Also, since the epidermis is so thin, it’s semi-transparent, meaning many characteristics of the skin are actually features of the dermis being seen through the epidermis.
Next week: The physiology of skin aging and photo-damage
1) Acta Derm Venereol. 2003;83(6). Epidermal thickness at different body sites: relationship to age, gender, pigmentation, blood content, skin type and smoking habits. Sandby-Møller J, Poulsen T, Wulf HC.
2) Surg Radiol Anat. 2002 Aug-Sep;24(3-4). Skin thickness of Korean adults. Lee Y, Hwang K.