The Skin II: How Skin Hydration Works

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These articles are meant to inform those who are interested in the formulation of cosmetics at home as hobbyists. They’re not meant for medicine students. Plus, they don’t provide any medical advice. The idea is just to give some basic information on the structure of skin, hair and nails that could be useful for those who are learning how to make cosmetics.

In the previous article we learned the basic structure of the skin. Now we will take a look at the main mechanisms through which the healthy skin is able to maintain hydration. 

One of the functions of the skin is to prevent the water loss from the body. Several factors are involved in maintaining the skin barrier healthy and functional: the Natural Moisturizing Factor, the Stratum Corneum Lipids and their organization, and aquaporins.

Previously, we have seen that the skin is divided into ipodermis/subcutaneous fat, dermis, and epidermis. The epidermis is the most superficial layer and it is divided in multiple layers, too. Of these layers, the external one that we can see is the stratum corneum. In the stratum corneum we find corneocytes, that are the final stadium of the keratinocytes, the cells that are generated deep in the epidermis (in the stratum basale) and undergo several structural modifications as they are pushed up towards the surface. 

The physical arrangement of corneocytes and lipids in the stratum corneum allows the skin to function as a barrier and prevent the transepidermal water loss (TEWL). In this article, I will describe three main components that contribute to the physiological hydration of the skin: the natural moisturizing factor, the stratum corneum lipids and the aquaporins.

Natural Moisturizing Factor

Corneocytes are filled with keratin, and keratin itself can hold a lot of water on its own. However, in the stratum corneum we also find other hydophilic substances that form the natural moisturizing factor (NMF) and are found both intracellularly and extracellularly.

The composition of the NMF is the following:

ComponentPercent (mole, %)
Amino acids40
Sodium pyrrolidone carboxylic acid (PCA)12
Lactates12
Ions18,5
Sugars8,5
Ammonia: uric acid, glucosamine, creatine1,5
Citrate and formate0,5
Urea7

The NMF is derived by a protein named filaggrin, that generates the NMF through its degradation. This degradation and, consequently, the synthesis of the NMF are regulated by the presence of water in the environment (if there is less water, the degradation occurs and NMF is formed). 

NMF components can act as humectants and retain water, or have other functions: for example lactate and potassium have influence on the pH balance of the stratum corneum

Alongside NMF, hyaluronic acid and glycerol are two other important components of the stratum corneum.

Hyaluronic acid (HA) is a glycosaminoglycan and it is a very hygroscopic polymer that we can find not only in the stratum corneum but also in the dermis, at extracellular level. In the stratum corneum, it acts as humectant and regulates its mechanical properties. 

Glycerol interacts with skin lipids and can retain water, acting as humectant. 

Stratum Corneum Lipids

The stratum corneum lipids are mainly ceramides, cholesterol, fatty acids and cholesterol sulfate. . They are intercellular and they are organized in a bilayer which provides protection against TEWL. The major components are ceramides and there are several different ceramide structures that have been identified in the stratum corneum. The bilayer is a barrier to water permeability and avoids water loss through the skin. As a consequence of this, a perturbation of the lamellar (bilayer) structure can strongly affect the condition of the skin and can occur in the dry skin type. 

Aquaporins

Aquaporins are transmembrane proteins that form water channels across the cell membrane. They allow the transport of molecules such as glycerol and urea across the membrane and they are fundamental in maintaining the hydration of the stratum corneum

Factors that can affect the skin hydration

Environmental and behavioral factors that can affect negatively the skin hydration are for example:

  • Excess UV radiation
  • Prolonged exposure to extreme humidity levels (very low or very high)
  • Harsh cleansers
  • Prolonged contact with water
  • Unhealthy diet and in particular poor water supply

Note: The effect of harsh cleansers

Harsh surfactants can damage the skin barrier. Sodium lauryl sulfate (SLS) (nowadays not used much in cosmetic products), one of the most aggressive surfactants, is even able to disrupt the lipid bilayer. One should note, however, that not all surfactants are so aggressive – and in particular not all the “sulphates”, despite the claims in cosmetic products. Nowadays we can find plenty of milder surfactants in cosmetics that we can use to clean gently our skin. In addition, it is not only the intrinsic “aggressiveness” that makes a surfactant milder or harsher, but its combination with counterions or co-surfactants. 

In addition to the use of mild surfactants and their formulation with co-surfactants, many cleansers are formulated in presence of emollients (oils), because it seems that cleansers can remove dirt and deposit emollients on the skin at the same time. 

References

Fowler (2012) Understanding the Role of Natural Moisturizing Factor in Skin Hydration, Practical Dermatology

Lampe et al. (1983), Human stratum corneum lipids: characterization and regional variations, The Journal of Lipid Research, 24, 120-130

Schurer and Elias (1991) The Biochemistry and Function of Stratum Corneum Lipids, Advances in Lipid Research, 24, 27-56

Kilpatrick-Liverman et al. (2009), Mechanisms of Skin Hydration. In Handbook of Cosmetic Science and Technology (eds. Paye, Barel, and Maibach)

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