Structure and function of the three skin layers

A few facts about skin

It's a well-known fact that our skin is the largest organ in our body. It contains countless receptors for sensory perception and is therefore also our largest sensory organ. Given its size and importance as an organ, it makes perfect sense to care for it well and regularly. While the skin may cover a large surface area, it's only a few millimeters thick. However, these few millimeters are packed with complex systems.

The skin is divided into three layers: the upper epidermis (also called the outer layer of skin), the middle dermis (middle layer of skin), and finally the subcutis (inner layer of skin). These three main layers can themselves be further subdivided into smaller layers.

Our skin performs an incredible number of functions. It is our first line of defense against pathogens. It also protects us from UV radiation through the production of melanin. However, since the skin's natural protection usually doesn't last very long, you should always apply sunscreen with a sun protection factor of at least 50 for extended periods in sunlight. The skin also supports our immune system. Furthermore, it regulates its own moisture balance and body temperature. The body can be cooled through sweat, while fatty tissue provides insulation. The fatty tissue of the skin has yet another function: it cushions and protects deeper organs, muscles, bones, and the like from impacts.

Vitamin D is produced in the skin. In general, the skin metabolizes many vitamins. It can store fat-soluble vitamins, as well as moisture, even in deeper layers. Water-soluble vitamins, however, must be continuously supplied. You can find an overview of vitamins and their effects on our skin in our vitamin guide .

The protective function of the skin

Let's now take a closer look at the different ways in which the skin protects us. As mentioned earlier, it protects us from pathogens such as viruses and bacteria through the acid mantle . This is a very thin layer above the outermost layer of the epidermis, which we will examine more closely in the next section.

Furthermore, the skin protects us from extreme temperatures, water loss, and UV radiation. It has various mechanisms for doing this. It protects against UV radiation by producing melanin when exposed to sunlight. This darkens the skin and makes it more resistant to UVB radiation. It prevents water loss through the skin barrier , which seals the skin and prevents moisture from diffusing outwards. To regulate its temperature, the skin can either cool itself through sweating in hot weather or conserve energy and stay warm by cleverly adjusting blood circulation.

The skin also forms an important barrier against other external influences. It protects deeper tissue from damage caused by impacts and the like, with the lower layers of skin cushioning the impact. If damage still occurs in the form of abrasions or burns, the outermost layer of skin absorbs it. Fortunately, our skin also has an amazing capacity for regeneration. It can heal very quickly. You can further accelerate this healing process with the wound-healing ingredient panthenol. Most wounds are so superficial that they heal seamlessly. Scars only form when the lowest layer of the epidermis, the basal layer, or the underlying dermis is affected.

An often underestimated protective function of our skin is the formation of calluses. The skin registers when it is subjected to increased friction in certain areas. It reacts by depositing more corneocytes there to provide additional protection against friction. Even though the appearance of calluses often bothers us, it is essentially a helpful gesture from our skin.

The outermost layer of skin (epidermis)

The epidermis is the uppermost of the three skin layers. It is therefore also called the outer layer of skin. Its main function is protection. Incidentally, the epidermis is only 0.1 mm thick. In the area around the eyes, its thickness is only half that, while in areas subject to heavy wear and tear, such as the soles of the feet, it can be between one and five millimeters thick.

The epidermis is further divided into five layers. From bottom to top, these are:

Basal layer (stratum basale)

Spine cell layer (stratum spinosum)

Granular cell layer (stratum granulosum)

Stratum lucidium (gloss layer)

Stratum corneum (layer of the stratum corneum).

The corneocytes, or keratinocytes, are integral to the epidermis. New cells are formed in the basal layer. Over a period of approximately 28 days, these cells migrate through the individual layers of the epidermis, keratinizing as they go, until they finally reach the outermost layer, the stratum corneum. These older corneocytes perform important functions and also serve to protect the skin. Nevertheless, they should be regularly exfoliated and washed away with a cleanser to support the skin's natural process and make room for a new generation of corneocytes. It is also beneficial to stimulate cell regeneration in the basal layer with suitable skincare products. This is how the skin regenerates.

Our skin's own lipids are deposited between the corneocytes in the stratum corneum. Together, these form the skin barrier. Further up, there is an emulsion of sweat and sebum – the acid mantle, or hydrolipid film.

Apart from that, the epidermis contains immune cells and cells for sensory perception. The basal layer is home to melanocytes, which produce the skin-coloring pigment melanin. Therefore, pigment production also takes place in the epidermis.

The middle layer of skin (dermis)

The dermis is also known as the corium. It can be further divided into two layers: the upper papillary layer (stratum papillare) and the lower reticular layer (stratum reticulare). The former connects to the epidermis. The papillary layer forms pegs that extend into the lower layer of the epidermis. A lively exchange of nutrients takes place between the epidermis and dermis above this layer. The reticular layer, meanwhile, forms a smooth transition to the third layer of skin, the subcutis.

In general, many substances are transported through the dermis; sebum, sweat, nutrients and waste products are each brought either to the surface or deeper into the skin.

The dermis is exceptionally versatile, as evidenced by the multitude of cells, glands, tissue types, and vessels that run through it. It contains blood and lymph vessels, nerve cells and fibers, sebaceous and sweat glands, immune cells, hair follicles, sensory receptors, and connective tissue. The latter provides the skin's elasticity and cushioning effect. Connective tissue makes up a large part of the dermis. It contains collagen and elastin fibers, with hyaluronic acid deposited between them. Collagen and elastin are two components of the skin that ensure its elasticity. However, they break down with age, which is why skin aging is most noticeable in the dermis. To counteract this, a slow-aging skincare regimen containing retinol , vitamin C, and peptides is beneficial.

The lower layer of skin (subcutis)

The subcutaneous tissue, also known as the hypodermis, is the deepest layer of skin. Structurally, it is far less complex than the dermis. It consists primarily of connective and adipose tissue. The latter acts as thermal insulation and helps the skin store energy. It also stores fats that the body can access when needed. In addition to thermal insulation, the subcutaneous tissue cushions bones, muscles, joints, and internal organs against impacts and shocks. The thickness of the subcutaneous tissue varies depending on body weight and its fat content. However, its thickness also depends on sex and the location on the body where it is measured.

In addition to fat cells, the subcutaneous tissue also contains blood and lymphatic vessels, collagen, and a dense network of nerves. Sweat glands and hair follicles can also extend into the subcutaneous tissue.

Skin appendages

Finally, we want to mention one more category that shouldn't be overlooked: skin appendages. We've already mentioned some of them in the text, such as the sebaceous and sweat glands, which supply the components of the acid mantle. This, along with the skin barrier, plays a crucial role in preventing our skin from drying out. Especially when it gets warmer, the sweat glands are, of course, particularly important for regulating body temperature. A special type of sweat gland are the scent glands, which are located, for example, under the armpits. Nails and hair are also skin appendages. They are formed from keratin scales.