Skincare science: Skin anatomy

This is the age of cosmeceuticals. Cosmetic companies advertise new breakthroughs that promise to magically reverse aging. Skincare packaging speaks in vague science-y words like “DNA repair,” “rebuilding collagen,” or “cell regeneration”.

Definitely sounds impressive.

But I’m a skeptic. So I usually walk away wondering about the science of it all. How does that new product work? Talk to me about active ingredients and biological mechanisms. Was there a peer-reviewed blinded randomized controlled study? If so, I want to read it.

There is an enormous amount of confusion and incorrect “common knowledge” when it comes to the science of aging. What happens when skin ages? What is the cause of common skin changes like age spots or moles? Which skin treatments are actually effective and how do they work? In what directions is the science headed?

I will tackle these questions in a series of blog posts over the next several weeks. I thought we should start with a basic introduction to the anatomy and function of skin.

Skin anatomy

Cross section of human skin showing layers and structuresCross section of skin

Epidermis

The epidermis is the outermost layer of skin. Most of the skin cells in the epidermis are not alive. So mostly, the epidermis is like an armor – it passively protects the tissues below. It has a small number of living melanin cells, which produce the pigment that gives skin its color. Mild chemical peels and laser treatments often target the epidermis.

Dermis

Deeper peels or lasers will affect the next layer, the dermis. The dermis gives skin its strength and elasticity. Dermis also contains blood vessels, hair roots, sweat and oil glands, cells of the immune system, and many nerve endings. This is the layer where some fillers like Juvederm or Restylane are injected.

Skin anatomy - the epidermis and dermis junction is pockmarked and the layers fit together like two corrugated sheets of metal.Roof tiles fitting together like dermis and epidermis

Dermal-epidermal junction

The border between the dermis and epidermis is pockmarked, so that the dermis and epidermis fit into each other like two corrugated sheets. 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.

Hypodermis

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 muscles underneath — the way a quilt is divided by lines of stitching. This is the fat that is targeted by procedures like liposuction or Kybella.

Variations in anatomy

You’ve probably heard that the skin is the largest organ in the human body. But it’s not one homogenous organ. For example, its intuitively clear that the skin on your eyelids is very different from the skin on the soles of your feet. Skin needs to do different things in different places, and as a result is structured differently in different locations. Excluding the palms and soles (which are highly specialized) and calluses (which are highly variable), epidermis thickness ranges from about 0.05 mm to 0.15 mm (or 2 to 6 thousandths of an inch). The dermis varies a bit more in thickness, depending on location: Eyelid skin has very thin dermis (0.5 mm, 2/100 inch), while the lower back can have dermis up to 3 mm (1/10 inch) thick.

The skin is the first line of defenseSkin is the first line of defense.

The defense mechanisms of skin

Skin protects your body from the outside world, with different responses to different kinds of threats:

  • Direct force or pressure will result in thicker epidermis (e.g. the calluses on the soles of your feet)
  • Chemical threats are repelled by a layer of oil, secreted by glands in the skin
  • Skin cells are connected by a system of watertight rivets, providing a secondary layer of chemical protection
  • The skin has a tremendous number of immune system cells that will attack and kill germs trying to enter the body.
  • Radiation is dealt with in two ways. Melanin absorbs and neutralizes UV light. Various DNA repair systems correct any genetic damage caused by radiation.
  • Finally, skin cells are constantly being replenished. Each epidermis cell lasts about a month.

Conclusion

The skin is actually a very complicated organ. Each layer of skin, and each type of cell in it has an important function. Skin has to deal with a lot of different threats on a constant basis. Over time, this takes a toll, resulting in the signs of aging. In the next post, we’ll go over the science of skin aging. How does time and sun exposure change the skin, and how can those changes be reversed?