INTRODUCTION

The skin, our biggest organ, is the outer layer of the body and protects us against noxious influences from our environment. It also has very important immunologic and defense functions and is our largest sensory organ. It maintains the body’s external shape and exhibits important direct and indirect social functions. Genetic and hormonal factors, sex, lifestyle and gender characteristics are responsible for determining differing tolerances of exogenous factors and for influencing skin structure and function.

The skin is made up of three layers:

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  the epidermis

  
  
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  the dermis

  
  
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  the subcutis.

The epidermis is the outermost layer of our skin. It is made up of:

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  the basal layer, with cells in continuous division to allow lifelong renewal

  
  
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  the spinous layer, the thickest of the epidermal layers

  
  
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  the granular layer, which prepares the epidermis for cornification (or keratinization)

  
  
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  the outermost stratum corneum or horny layer.

The stratum corneum is functionally the most important layer as it prevents the skin and thus the body from losing its water content. Without this layer, which is just a few microns thick but consists of many layers of horn cells, we would not be able to live outside an aqueous environment. The stratum corneum is also very important in controling the penetration of substances into the skin, including drugs and cosmetics.

Hair and nails are derived from the epidermis, as are sweat and apocrine glands.

Melanocytes populate the basal layer. Female hormones have an influence on the biochemical composition of the horny layer and improve its quality. Skin pigmentation is lighter in women than in men, but is subject to female hormonal influences.

The dermis is the carrier layer of the epidermis and is a tough and resistant layer of connective tissue. It is made up of collagen and elastic fibers embedded in the so-called ground substance of glycosaminoglycans, of which the most important is hyaluronic acid. Hyaluronic acid is able to bind water, and functions both as a lubricant for the collagen and elastic fibers and as a volume-giving substance for the skin.

The skin is less thick in women than in men, and that discrepancy increases noticeably with age. Estrogen therapy over a 12-month period leads to a thickening of the dermis.

The subcutaneous fat is both a mechanical and thermal insulator, as well as an energy storage organ. Its distribution over the body is determined by gender-specific hormonal influences and by genetic factors. Its total amount depends greatly on the calories consumed and used every day. Although most people do not like fat in their bodies, it is important for thermal insulation and for the shape of the face and the rest of the body. However, once the body has accumulated excess adipose tissue it tends not to lose it again. This tendency is what makes weight loss so difficult. Fat is unique and contains approximately 5000 stem cells per gram, and since it can be easily grafted from a donor site of the body to a receptor site, fat transplantation is probably also a ‘stem cell transfer’.

The cutaneous fat layer is thicker in women than in men and it has architectural differences, particularly where it is the thickest – so-called cellulite. The difference in fat distribution is the probable reason that women’s tolerance to cold is superior to men’s.

All three skin layers are equally important for good shape and health. There are many myths about skincare that are supposed to guarantee beautiful skin. Although beautiful skin is healthy skin, the reverse is not always true. When the epidermis is not healthy it may become red, rough, sallow, dry, scaly, spotty or lose its luster and smooth texture. It develops large pores and dilated blood vessels, and a variety of benign and precancerous lesions as well as cancers. When the dermis gets old and/or sun-damaged, it loses its turgor and elasticity, takes on a yellowish color and develops wrinkles and folds.

BASIC SKINCARE

Beautiful skin has a smooth and shiny surface, is evenly pigmented and has a silky touch – it is like babies’ skin. Traditional knowledge tells us that prevention is better than treatment. The best protection from the harmful effects of ultraviolet (UV) rays are UV-proof clothes (hats, shirts, trousers), avoidance of direct sun exposure (applying sunscreen 30 minutes before exposure) and perhaps the use of oral and topical application of vitamins C, E, A, other antioxidants and scavengers of reactive oxygen species.

The stratum corneum, as the outermost layer, has to be kept moist since it becomes scaly and loses its elasticity when dry ( Fig. 3.1 ). Unfortunately, moisture cannot be easily added to the skin; in fact, what moisturizers do is to help the skin to keep its own moisture. Contrary to common belief, oil-in-water emulsions may even impair the barrier function of the stratum corneum. Glycerol may reverse this harmful effect on the horny layer. Glycerol-containing hand creams have proven to be effective, and facial skincare may also benefit from glycerol as an ingredient. Urea is another natural moisturizer with unique properties since it can penetrate the horny layer and keep moisture in the skin. The addition of ammonium lactate may enhance this effect. However, topical preparations that contain higher urea concentrations tend to itch when applied to the dry skin of atopic individuals. Dexpanthenol is another substance that improves stratum corneum hydration ( Fig. 3.2 ).

Effect of a simple emollient on skin roughness. A , Before treatment; B , after 6 weeks of once-daily application.

Dex-panthenol containing cream. A , Before treatment; B , skin-smoothing effect is better than that achieved with an emollient.

The stratum corneum is also able to store fat-soluble substances. Many lipophilic drugs and cosmeceuticals are retained in it and slowly released. This property is used in dermatology for many creams and ointments that only require a daily application. Simple emollients provide a protective film of fat, preventing transcutaneous water loss and therefore keeping the skin moist. It is not a paradox that the higher the content of fat in the skincare product, the better it maintains skin moisture. This applies to all skin areas, from the very thin eyelid skin to the far more oily and thicker forehead skin.

It is not only the active ingredients of skincare products that are important. The art of making products that are well tolerated, easily applied, non-allergenic, non-toxic or otherwise harmful and feel comfortable on the skin must be supplemented by optimizing existing delivery systems and developing new application systems. A number of stable carrier systems have been developed that allow controlled release of the incorporated ingredients. These systems include liposomes, sphyngosomes, niosomes, multiple emulsions, microemulsions, microsponge delivery systems and thalaspheres, as well as solid lipid nanoparticles and nanostructured lipid carriers. In particular, solid lipid nanoparticles have good physical long-term stability, protect incorporated labile substances against degradation, ensure controlled release of incorporated ingredients, prolong the release of perfumes and repellents, enrich active ingredients in certain skin regions, increase skin hydration and protect against UV light.

More than 80% of the visible aging of facial skin is due to UV exposure. Smoking and other exogenous influences are also important. Avoiding UV exposure and not smoking are the most important factors for beautiful skin.

Whereas UVA is relatively low in energy, it penetrates into the dermis where it causes connective tissue damage, a process erroneously called ‘photoaging’, although it is in fact photodamage. It causes solar elastosis, a peculiar type of connective tissue damage, blotchy hypo- and hyperpigmentation, and sun-burst vessels. On either side of the neck, a multicolored aspect, called poikiloderma of Civatte, may appear. It is therefore logical to add potent sun-protective substances to daycare products, particularly during the sunny season. High-energy UVB does not penetrate so deeply and is mainly absorbed by the DNA of the epidermis, where it is responsible for tanning, a defense mechanism of the skin against sun exposure. In the long term, however, UVB is responsible for precancerous lesions and skin carcinomas. It dries the epidermis out, causing scaling, rough hyperkeratotic areas and pigment spots. Sunscreens containing UVB-absorbing substances can be effective in preventing skin cancer development. However, sunscreens protecting against both UVA and UVB have the best spectrum of action, protecting against photoaging and also considerably reducing UV-induced immunosuppression.

Washing and cleansing of the skin are important daily skincare measures. There is no scientific proof that washing the face once daily with a mild soap is harmful and makes the skin dry. It is self-evident that the use of soaps should never be overdone and depends on the degree of soiling. Whether or not common alkaline soaps are more harmful than non-alkaline synthetic detergents and skin cleansers remains unproven since these substances are more aggressive degreasing agents than normal soaps. Many people claim that they do not tolerate perfumed soaps or detergents. It is therefore logical for these people to abandon them and not to try one soap after the other. Tolerability and cleansing properties do not depend on the price of the product.

For people who really cannot use soaps or detergents, a well-tolerated emulsion may be used instead. In fact, it is the ability of the soap or detergent to dissolve or emulsify fat that is responsible for the cleaning action. The oily part of the emulsion dissolves the debris and allows it to be removed from the skin surface.

Skin irritation is more frequent in women than in men, and apparently Asian females complain more readily about skin irritation and intolerance of cosmetics than Causasians. However, skin irritability is very difficult to quantify and remains mostly a subjective impression of the individual.

COSMETIC SKINCARE

Cosmetics are defined as articles without therapeutic function (US Federal Food, Drug and Cosmetic Act, 1938). EU legislation forbids cosmetic preparations from containing active ingredients (EU 76/768). Usually, they are substances with immediate effects on skin physiology to clean, protect, maintain and regenerate the skin. Topicals with active ingredients are considered drugs.

Two major classes of cosmetics are distinguished:

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  decorative cosmetics, which may also have a minor effect on skin physiology

  
  
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  substitutive-regenerative cosmetics, which have a marked effect on the condition of skin, mucous membranes and their appendages, such as the hair, nails, sweat and apocrine glands, teeth and minor salivary glands.

Decorative cosmetics are effective for ‘camouflage’, but have no effect on photodamage. One example is the self-tanning agents that are based on dihydroxyacetone, which reacts with free amino groups of keratin to produce a decorative brown color. Although this is not harmful to the skin it has no UV-protective action.

Regenerative-substitutive cosmetics are applied to regenerate, maintain or improve good skin condition. By law, they must not have a systemic effect.

A third class of substances called cosmeceuticals has recently been introduced . These are cosmetic agents with improved and/or enhanced action, a proven benefit, but a low risk. Fruit acids, α-hydroxy acids, vitamin A and vitamin E are now considered cosmeceuticals when applied topically. They may also exhibit adverse side effects. Nutriceuticals, substances with similar actions, may be useful as absorption from the skin is often limited.

There is a great deal of advertising of products that can prevent cutaneous aging and improve photoaged skin. Many of these products have demonstrated either antioxidant or repairing activities in in-vitro experiments, but most have not been tested according to the stringent rules of good dermatologic practice and science. Some of the most important and effective cosmetics are sunscreens. They prevent not only sunburn, but also more harmful consequences that are not immediately seen, such as immunosuppression, DNA damage and elastotic degeneration of the connective tissue. It is important to keep in mind that even if these products are correctly used, consumers cannot remain in the sun for longer periods. The skin needs to be protected from carcinogenic UVB and the less energetic UVA. Ultraviolet light also induces release of iron into the tissues, which in turn promotes the generation of free radicals and connective tissue damage. Topical iron chelators – substances that are able to bind iron – have been shown to be protective.

Oxygen radical scavengers – α-tocopherol (vitamin E), ascorbic acid (vitamin C), propyl gallate and a number of antiinflammatory agents such as ibuprofene, naproxene or hydrocortisone – prevent UV-mediated DNA damage and squamous cell carcinoma. Just as sunscreens may often contain these products, cosmetic preparations for outdoor use should also contain UV filters.