Skin Structure

The skin consists of several layers that are interconnected and complement each other in their functions. Therefore, the skin is much more than the visible surface of the human body. Beautiful and healthy skin is always based on the interplay of the various skin layers.

Our skin can be divided into 3 main layers:

1. The epidermis

2. The dermis / cutis

3. The subcutis

These three skin layers, however, can be further divided into a total of up to eight layers. However, we first want to focus on the three main layers and begin (from the bottom up) with the subcutis.

Subcutis

The subcutis is a flexible connective tissue layer that consists on the one hand of loose connective tissue – forming a kind of connective tissue chambers – and on the other hand of the fat cells embedded within it, which are more or less pronounced depending on gender, age, diet, activity, and constitution. It borders the body below with a fascia of firm collagen fibers and is connected above to the overlying skin layer – the dermis – by firm connective tissue bands. In addition, connective tissue strands pass through the subcutaneous fat tissue, connecting the dermis and the fascia lying beneath the subcutis. These give the skin firmness and elasticity, limit its mobility, and enable it to return repeatedly to its original position. 

But the subcutis not only forms the link between the body and the overlying skin layers, but also functions as a supply station.

The fat tissue stored in the connective tissue chambers of the subcutis fulfills several vital functions for our body. As so-called storage or depot fat, it keeps energy reserves ready for us and is thus the most important energy store of the organism. In its function as "structural fat", the fat tissue serves as a protective cushion for the mechanical cushioning of, for example, impacts or other external influences, as well as for heat insulation and thus protection from cold.

In the event of an oversupply of nutrients, more depot fat is stored and the fat tissue increases in volume. This fat increase is encouraged by the excessive intake of carbohydrates and fats. However, fat can be formed directly from carbohydrates in the cell or taken up from the blood into the cell. Thus, the subcutaneous fat tissue is always located near blood vessels and each fat lobule has its own blood supply. In this way, stored fat can be quickly released into the blood in the event of malnutrition and, conversely, stored in the fat cells in the event of an oversupply in the blood. It is often forgotten that the consumption of sugar also plays a significant role in the volume of the fat tissue, because sugar, especially in combination with dietary fats, drastically slows down the natural breakdown of fat.

In addition to connective tissue fibers, fat cells, blood and lymphatic vessels, the subcutis is permeated by a dense network of nerves whose endings pass through all skin layers and transmit sensory perceptions such as pressure, warmth, cold, and touch. In the uppermost area of the subcutis and the lowest layer of the dermis, there are also sweat glands and hair follicles, which make their way from here up into the epidermis. Furthermore, the subcutis is important for the water balance of our body, as it has the property of storing water in the subcutaneous tissue.

Dermis

The dermis is located between the subcutis (subcutaneous layer) and the epidermis (outer skin layer) and is also called the dermis or cutis.

It consists of elastic and collagen fibers, which have the property of binding water and thus influencing the firmness of the skin. The collagen fibers are cross-linked in an angular fashion and are responsible for the firmness of the skin. The elastic fibers are built into this collagen network and ensure elasticity. This dense connective tissue is permeated by blood and lymphatic vessels, which are arranged in a superficial and a deep network. They supply the epidermis with nutrients, transport metabolic products, and thus enable the removal of harmful substances. 

Furthermore, the dermis consists of many nerve fibers that are responsible for touch, pressure, pain, and temperature perception. All skin appendages are also located in this skin layer. Where the subcutis and dermis meet, hair follicles, sebaceous glands, and sweat glands originate.

The dermis can be divided into two layers: on the one hand, the reticular layer, which is also called the Stratum reticulare and connects to the subcutis, and on the other hand, the papillary layer (Stratum papillare), which is connected to the basal cell layer of the epidermis. 

The Stratum reticulare is primarily responsible for the firmness and elasticity of the skin and thus houses the dense network of collagen fibers and elastic connective tissue.

The Stratum papillare is permeated with fine blood vessels and supplies the basal layer of the overlying epidermis with nutrients.

Other components are connective tissue cells, fibroblasts, nerve endings, touch cells, heat and cold receptors, as well as the defense cells of the immune system.

Epidermis

The epidermis, or outer skin layer, is the uppermost skin layer and forms the contact surface with the outside world.

At the same time, it fulfills the most important protective function for our organism and forms a barrier against external influences. It protects us from the penetration of foreign substances and harmful microorganisms, mechanical stress, as well as UV radiation. 

Essentially, the epidermis consists of intracellular keratin fibers and a cement substance between the cells that keeps the epidermis stable. It is avascular and, unlike the other skin layers, also contains no nerves.

It constantly renews itself from its deepest layer, the basal cell layer. During this so-called cell cycle, which lasts about 30 days, the newly formed cells migrate from the bottom to the top through the layers of the epidermis, gradually losing their cell nucleus along the way, becoming rigid, and forming the stratum corneum upon reaching the top. This fulfills approx. 90% of the skin's protective mechanism.

The epidermis not only determines a significant part of our outer appearance (skin color, complexion, skin texture), but fortunately can also be wonderfully influenced by cosmetic products.

The epidermis can be divided into five layers, starting with the lowest, which connects to the dermis / dermis:

1. Stratum basale – Basal cell layer

2. Stratum spinosum – Spiny cell layer

3. Stratum granulosum – Granular cell layer

4. Stratum lucidum – Clear layer

5. Startum corneum – Stratum corneum

The basal cell layer is interlocked with the dermis (dermis) in a peg-like fashion. This not only brings about better adhesion between the dermis and epidermis, but also provides a larger surface area that is of great importance for the blood supply of the basal layer. In addition, the melanocytes (pigment cells) are located here. They produce the pigment melanin and are thus responsible for our skin color.

In addition to the keratinocytes and melanocytes, the germinal layer contains two other cell types: Langerhans cells and Merkel cells. While the Langerhans cells play a central role in the immune system, the Merkel cells function as the skin's own nerve cells. 

From the basal cell layer, the journey of the keratinocytes continues into the spiny cell layer, which consists of several cell layers and owes its name to the spine-shaped processes of the cells at this stage. These processes connect the cells to each other and thus create a net-like structure that gives the skin elasticity. 

In the overlying granular cell layer , the cells slowly begin to keratinize, become flatter, and the cell nuclei begin to disappear. Changes in fat metabolism lead to the production of cement-like fat substances, the so-called "cell cement". These are stored in the spaces between the cells and are of great importance for the barrier function of the epidermis. 

In the Stratum lucidum, the clear layer , the cells are in the transitional stage from keratinocytes to horn cells (corneocytes). In this layer, there are still remnants of cell organelles, which means that keratinization is not yet completely finished. 

In the uppermost skin layer, the Stratum corneum, or stratum corneum, the keratinization of the cells is completely finished. The keratinocytes have completely lost their cell nucleus and have become rigid, keratin-filled corneocytes, which form the uppermost skin layer in multiple layers in a brick-like arrangement. The fatty substances released from the keratinocytes (cell cement) seal the spaces between the cells watertight. They only separate from each other in the outermost cell layer, to then be shed from the skin as horn scales.

The stratum corneum of the epidermis alone consists of approx. 15 to 20 different layers of constantly renewing horn cells. The functionality of the stratum corneum is important for maintaining the health of the skin. It protects the inside of the body from excessive water loss and protects humans from chemical and physical influences. It contains no living cells, but consists of dead horn scales that are continuously shed from the surface and go unnoticed by us. In addition to this outer "loosened" part, the stratum corneum also consists of an inner, dense cell layer, which fulfills the actual barrier function

The water-fat film of the skin forms together with the cement substance and the water-binding substances contained in the stratum corneum a protective mechanism against dehydration. In addition to a water content of approx. 10 to 20%, the skin surface has a relatively high lipid content of approx. 20%, which consists of squalene-rich sebaceous gland oil and, in the deeper stratum corneum, cholesterol-rich horn fat. They are essential for the barrier function of the skin and counteract transepidermal water loss.

This water-fat film, also called the acid mantle, also offers an unfavorable environment for microorganisms with a pH value of 5 to 6. Good bacteria form a resident skin flora and thus suppress the growth of harmful bacteria, while amino acids are able to neutralize acids and alkalis. To prevent water loss through the epidermis, the barrier mechanism constantly tries to prevent the loss of fatty acids. Care with skin-identical lipids thus not only counteracts transepidermal water loss, but is also crucial for maintaining a healthy skin barrier and preserving epidermal cells.