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The Structure and Function of Skin
At Marie Veronique, we talk so often about how to get and keep your skin healthy that we thought it would be helpful to take a step back and share some fundamental information about the skin. After all, if you want to understand how our products work, it’s helpful to understand how the skin works.
This overview of the structure and function of the skin may take you back to your high school biology days (no passing notes!), but we’ll try to keep it on the interesting side.
The Basics
The major function of skin is to provide a barrier between you and the outside world. As the largest and fastest growing organ in the human body, the skin comprises about 15% of total body weight. It’s also enormously complex. In every inch of skin, there are about 650 sweat glands, 20 blood vessels, 60,000 melanocytes (the cells that make melanin and give your skin its color), and 1,000 or more nerve endings. Overall there are about 19 million skin cells in every inch of your body, and they are constantly regenerating.
Interesting Fact: the skin you can see on your body today will be gone and replaced by new cells in about a month. This is due to the process of skin cell turnover, which we talk about a lot here at Marie Veronique. If you’re starting a new skin care regimen, it’s important to keep in mind that it takes about 30 days for your current skin cells to shed and another 30 days for new cells to grow, which means you need a minimum of 60 days (or longer depending on the state of your skin) to begin to see the results of your efforts.
The skin also serves multiple purposes. We’ll go deeper in a future post, but here are the highlights:
- A protective shield against heat, light, injury, and infection
- First line of defense against pathogens and disease
- Sensation of heat, cold, pain, pressure and vibration
- Regulation of body temperature and moisture
- Storage of water and fat
- Synthesis of Vitamin D from sunlight, which we need for healthy bones
The Skin’s Structure
The three main layers of the skin are the epidermis, the dermis, and the subcutaneous, or fat layer.
Layer 1: The Epidermis
The outermost layer is the epidermis, which provides a waterproof wrapper for your body and is the only layer of the skin that is visible to the eye. It is also the home to the skin’s microbiome, a diverse environment of mostly harmless or beneficial microorganisms that plays a critical role in maintaining skin health. Ongoing research into the microbiome may have an impact on how we regard skin physiology — maybe even promoting the microbiome to equal status with the epidermis, yet separate in terms of function, composition and characteristics. (We’ll keep you informed on that front.)
90% of the cells in the epidermis are keratinocytes, which produce the protein keratin and form the epidermal water barrier by making and secreting lipids. One of the reasons these superstar cells are fascinating is that their function changes as they move up through the layers of the epidermis. Keratinocytes are produced in the bottom layer of the epidermis, and they gradually migrate up through the skin until they are shed at the surface. This process happens continuously, which keeps our epidermis healthy and strong.
In addition to keratinocytes, the epidermis contains Merkel cells, which are essential for light touch sensation; Langenhan cells, which activate the body’s immune response to pathogens; and melanocytes, which produce melanin, which gives the skin its color. Note that there are no blood vessels in the epidermis — it gets its nourishment from the layers below.
The epidermis itself is composed of four main layers. Let’s take a little tour of them, from the outside in.
- The outer layer is called the stratum corneum, which is Latin for ''horny layer.'' “Horny” might be a little overzealous as a descriptor as we’re not exactly covered in scales, but the cells within the stratum corneum are indeed tough. The stratum corneum consists of both living and dead keratinocytes that are continuously sloughed off through a process called desquamation. As the keratinocytes progress toward the upper layer of the skin, they become non-living cells called corneocytes, and these corneocytes layer up in a lipid matrix composed of ceramides, cholesterol, and fatty acids to form the stratum corneum. They’re also remarkably intelligent, performing functions including hydration regulation; protection against toxins, irritants, and allergens; regulation of microbial activity; and the triggering of the immune response (if necessary).
- The next layer only exists on the palms of our hands and the soles of our feet, and it’s called the stratum lucidum. This translucent layer of dead cells provides extra thickness for those high contact areas.
- Next up is the stratum granulosum. The keratinocytes in this layer produce lamellar bodies, a mix of lipids, enzymes and proteins that form the waterproof membrane that is essential to the skin’s barrier function.
- The fourth layer is the stratum spinosum, and it is in this layer that our keratinocyte friends begin producing keratin and secreting the lipids that bond other cells together.
- And finally, the deepest layer of the epidermis is the stratum basale. Stem cells in the basale layer are constantly dividing to make new ones, and those cells make their way up to the stratum corneum to replace the dead ones that have sloughed off.
Layer 2: The Dermis
The dermis is the middle layer of the skin, sandwiched between the epidermis and the subcutaneous. There are also two layers of the dermis, the papillary layer and the reticular layer, although the border between them is a bit less distinct than the layers of the epidermis. Here you will find blood vessels, lymph vessels, nerves, hair follicles, oil glands and sweat glands. You’ll also find tough connective tissue that is composed of a matrix of collagen and elastic fibers, which is what gives the skin its strength and elasticity.
The main types of cells in the dermis are fibroblasts, which make the collagen and elastin; macrophages, a type of white blood cell that fights infection; and mast cells, which are important to the body’s inflammatory response and also support wound healing.
Layer 3: The Subcutaneous
Finally we have the deepest layer of the skin, the subcutaneous, which is made up of fat cells and the connective tissue that holds the skin to underlying structures in the body. It also helps regulate the body’s temperature and protects it from injury by acting as a shock absorber — never forget that all those fat cells we spend so much time worrying about provide some pretty nice benefits.
So that’s your primer on the structure and function of the skin. In future posts, we’ll take a look at some critical components of skin health, and why it’s so important to look after this massive organ.