The layers of the skin

How is the skin built up? What functions do the layers of skin perform? What active substances do the skin layers need? Our article will help you find out.

The structure of the skin is now known not only at the histological level, but also at the molecular level - a fact that hasskin treatment to a new level.

The purpose of facial care cosmetics is to replenish the hydro-lipidic layer and strengthen the skin's defenses. Experience has shown that the best cosmetics to do this are those containing skin-zoning active ingredients and those that are as compatible as possible with the skin's natural hydro-lipidic components. The skin's hydro-lipidic layer is a mixture of sebum (sebum produced by the sebaceous glands) and sweat that is released through the sweat ducts to the skin's surface, bound together by the skin emulsifier ceramide. The most important components of sebum are various unsaturated fatty acids (stearic acid, gamma-linoleic acid, palmitoleic acid), wax and wax esters, squalane, cholesterol and cholesterol esters. In the skin, moisture is provided by hyaluronic acid, amino acids and urea in sweat.

The following ingredients are found on the surface of the skin:

1. Sweat
2. Tallow
3. Corneocytes, hair cells during exfoliation
4. Materials of the cornealization process
5. Surface water from the deepest layers (transepidermal water)

The chemical composition of human sebum (skin fat):

mono- di- triglycerides - neutral fats, esters of glycerol with fatty acids
fatty acids (saturated and unsaturated)
squalene - a waxy substance
cholesterol - a steranase compound, also formed when fatty acid esters are broken down, but also synthesised by the body - a component of the cell membrane
cholesterol ester - compound of cholesterol with fatty acids - soluble soluble form of cholesterol
ceramides - sphingolipids, membrane-forming lipids in which the alcohol component is not glycerol but sphingosine

The protective acid mantle

Due to the weak acidic components, the aqueous part of the hydrolipidic film layer forms the protective acid mantle, which performs three important skin functions: 1. Supports the formation and maturation of epidermal lipids, which helps to maintain the barrier function 2. Provides indirect protection against invasion by microbial pathogens 3. Provides direct protection against alkaline noxae

Components of the protective acid mantle:

Lactic acid and various amino acids from sweat
Free fatty acids from tallow
Amino acids and pyrrolidine carboxylic acid from the cornification process

The average physiological pH of healthy skin ranges between 5.4 and 5.9. (In the axillary grooves, anal fissure and genital area, the pH is around 6.5.) In this pH range, the skin is covered by normal skin flora. These inhibit the growth of pathogenic microorganisms. The acidic pH of the stratum corneum plays an essential role in the formation and structure of epidermal lipids and with them the permeability barrier.

The acidic environment is important because:

Activation of enzymes responsible for the synthesis of important epidermal lipids
Creating the bilayer lipid membrane
Restoring the horn layer after mechanical or chemical damage

Natural moisturising factors and surface lipids

The most important substance for skin elasticity is water. In young skin, the water content of the upper stratum corneum accounts for 10-20% of the total body water content. The skin derives its moisture from water in the deeper layers (transepidermal water) and normal sweat secretion. Various factors, such as lack of water retention or extremely low humidity, can lead to increased moisture loss. More exposed areas of the body, such as the face and hands, are more prone to this. Natural moisturising factors, which are the body's own substances extracted from sweat and sebum (e.g. urea), as well as substances derived from the cornification process (e.g. pyrrolidine carboxylic acid) help the skin to retain water in the stratum corneum. Without the natural moisturising factors, water would evaporate, leaving the skin dry and cracked.

Epidermal lipids

The composition of the fatty acid content produced by our skin:
59% unsaturated, 37% saturated

46 % oleic acid (unsaturated fatty acid)
25% palmitic acid (saturated fatty acid)
10% linolenic acid (unsaturated fatty acid)
8% stearic acid (saturated fatty acid)
3% myristic acid (saturated fatty acid)
1% lauric acid (saturated fatty acid)
3% other (unsaturated fatty acid)

In addition to corneocytes, epidermal lipids are also central to the skin's barrier function. They form the "malter", the cement of the cells of the stratum corneum, which runs between the "bricks", the corneocytes.

The skin's own lipids are synthesised in the cells of the epidermis from intermediate metabolic products and essential fatty acids taken up from the diet. In addition to mono-, di- and triglycerides, they contain cholesterol, ceramides and phospholipids.

The composition of epidermal lipids

The membranes in living epidermis contain mainly phospholipids, which are needed by the body in the form of phosphate. During differentiation (cornification) of keratinocytes, the membranes are degraded. This results in high levels of ceramide, cholesterol and free fatty acids in the stratum corneum.
The epidermal lipid precursors are synthesised in the Golgi 'apparatus' of the keratinocytes of the stratum corneum. They are then stored in the form of bilayer lipid membranes in microscopic granules, the Odland corpuscles, which are membrane-confined cell cavities. In the upper part of the stratum granulosum (granular cell layer), the Odland bodies expel the bilayered lipid membranes into the intercellular space by exocytosis.

During the ripening process, polar glycolipids, phospholipids and sterol esters are enzymatically converted into apolar lipids such as ceramides or free fatty acids. The semi-permeable corneocyte lipid barrier, which functions as a permeability barrier, is formed.

The consequences of permeability barrier damage

If the top corneocyte layers are removed, for example with a sticking plaster, the epidermal lipids are also lost from the skin. Water, chemicals and pathogenic micro-organisms then enter the deeper layers of the skin. In the meantime, more water from the deeper layers of the skin reaches the surface, i.e. transepidermal water loss is increased.

Phospholipids are important components of the cell membrane. Phosphorus is essential to the body in the form of phosphate ions, so phospholipids are broken down during epithelial differentiation (cornification).

Epidermal lipids are synthesized in the Golgi "apparatus" of keratinocytes.

Lipids are stored in Odland bodies and are transported to the intercellular space by exocytosis in the upper granular layer.

Energy management in skin cells

Like other cells in the body, skin cells need energy to maintain their vital functions, regenerate, repair and grow. Energy is generated through intracellular metabolism.
The dermal blood vessels supply the cells of the basal layer with fats, carbohydrates, proteins and oxygen. Free fatty acids, when present in sufficient amounts, play the largest role in energy management. The cells of the stratum granulosum, which can take up little glucose by diffusion, use the fatty acids of the disintegrating cell membranes for energy production.
Skin ageing and a specific electron transporter Decreased levels of coenzyme Q10 in the respiratory chain play an important role in the reduced cellular regenerative capacity caused by ageing or oxidative stress. Patients with cardiovascular disease are recommended to take oral supplementation of KoQ10 to compensate for the deficiency. Topically applied Coenzyme Q10 reduces the signs of ageing.
Effects on regeneration The activation of the skin barrier regeneration system can occur in response to several stimuli: increased cholesterol, fatty acid and sphingolipid synthesis leads to restoration of barrier function. In the case of more extensive damage to the stratum corneum, e.g. by 'tearing' or acetone application, the natural barrier regeneration of the stratum corneum can only occur in a sufficiently acidic environment. A neutral or alkaline medium will noticeably slow down regeneration.

What are dermal active ingredients?

The components of tallow can be replaced with vegetable oils that have the most similar unsaturated fatty acids to those of tallow, or those with the highest proportion of these fatty acids. Thus, the stearic acid content of tallow is best replaced by shea butter or macadamia oil, the gamma-linoleic acid content by linseed oil or pomegranate seed oil, and the palmitoleic acid content by macadamia oil. The best sources of missing wax are jojoba oil and olive oil for squalane. For moisture replenishment, we have high purity, highly skin-friendly, naturally derived moisturisers (hyaluronic acid, urea, various amino acids).

In addition to the hydro-lipidic layer, the stratum corneum (epithelium) plays a crucial role in the skin's intact protective function. The stratum corneum is composed of metabolically inactive cells and an intercellular matrix of multiple layers of double-layered lipids.

The stratum corneum barrier layer is composed mainly of ceramide, cholesterol and palmitic acid. Dermal barrier skin protection means similar substances and similar structure. Such systems are called DMS (Derma Membrane Structure) systems. They usually contain ceramide, phytosterols (very similar to cholesterol) and hydrogenated phosphatidylcholine (chemically bonded with palmitic and stearic acids).

The advantage of such systems is that they do not contain emulsifiers, which is very important for atopic skin, for example.

The regeneration of the damaged skin layer is mainly achieved with ceramides, which are produced by the skin in the epidermis and released into the dermis on about day 28 of the skin cell's life. One of the most important of these ceramides is ceramide I, which contains linoleic acid. The decrease in its quantity in the skin is in direct proportion to the increase in abnormal skin lesions.

In order to produce sufficient amounts of ceramide, the birth of skin cells and their migration towards the stratum corneum must be protected and supported, as this is the pathway by which cells can be damaged by adverse environmental effects (free radicals, UV radiation). This is why liposomal and nanosomal delivery systems are of great importance, as they contain high concentrations of linoleic acid in their shells.

Three vitamins are essential for healthy skin care: vitamins A, E and C.