RELATION AND BEHAVIOR OF SKIN TO COSMETICS

1.0 Role of Skin

The Skin is the interface between the body and the environment. The skin mainly protects us against environmental aggressions. With a weight of about 4 kg and a surface of about 1.8 m2, the skin is the widest organ of the body.

There are three major compartments of the skin, the epidermis, dermis, and hypodermis. In Cosmetic Science we are mainly concerned with Epidermis because it is the first station of contact with skin.

The epidermis is the outermost structure and it is a multi‐layered epithelial tissue divided into several layers. The outermost structure of the epidermis is the stratum corneum (SC) and it forms the epidermal permeability barrier which prevents the loss of water and electrolytes. Other protective/barrier roles for the epidermis include immune defense, UV protection, and protection from oxidative damage. Changes in the epidermal barrier caused by environmental factors, age or other conditions can alter the appearance as well as the functions of the skin. Understanding the structure and function of the stratum corneum and the epidermal barrier is vital because it is the key to healthy skin and its associated social ramification.

In addition to its main protective part, the skin also ensures numerous other essential functions such as permeation, metabolism, and thermoregulation and actively contributes to the sensorial function. This structural and functional diversity is influenced by intrinsic factors related to subjects, their ethnic group, their age, and their physiological, psychological, and pathological state and by extrinsic factors related to the immediate environment such as the dryness level, sun exposure, temperature, and wind.

2.0 Types of Skin

Numerous skin classifications have been proposed; they are all privilege-specific criteria. So, from a cosmetic point of view, the reference criteria are the users’ feelings and therefore skin types may be normal, oily, dry and mixed.

2.1 Normal Skin

Contrary to all expectations, it is worth noting that there is no definition of normal skin, the latter being qualified in comparison with the other skin types: a normal skin is not a dry skin, not an oily skin, not a mixed skin, and no more a pathological skin. A brief analysis of its structure and of its functions enables to draw a more positive definition of the normal skin. At the more external level, there is a very thin protective epithelium that constitutes the epidermis. It plays the main part in protecting the organism against external aggressions, notably ensured through the cohesion of epithelial cells and the keratinocytes that undergo a specific process of differentiation as they migrate from the dermoepidermal junction to the skin surface. This cohesion results from intercellular ties caused by the desmosomes, which are mainly responsible for the very great mechanical resistance of the epidermis. Therefore, considering its structure and its functions, a normal skin should be a smooth skin, pleasant to touch, because of the cohesion of the cells of its more superficial layers; a firm and supple skin because of the existence of a dense supportive tissue and of the presence of numerous elastic fibers of good quality; a mat skin through its balanced seborrheic production; a clear and pinkish skin because of the perfect functionality of its microcirculatory network. In reality, a skin complying with all these characteristics would only exist in the healthy child before his/her puberty.

At the cosmetological level, we must be content with a less strong definition and consider normal skin as a young skin, structurally and functionally balanced, and requiring no care apart from those necessary for its cleaning.

2.2 Dry Skin

The concept of dry skin has also never been clearly defined. The term “dry skin” conceals several complementary or opposite points of view. It remains completely different from the way it is approached. People connect this notion to the effects observed and to their sensorial dimension. Therefore, for them, it is, first of all, a feeling of drying along with loss of skin suppleness and elasticity, characterized by a rough appearance often associated with important desquamation, and leading to a certain discomfort they intend to correct by using moisturizing products. For the biologist, the xerosis would be first the consequence of a change in the coherence and functionality of corneocytes, the water deficiency of the superficial layers of the stratum corneum, when it exists, only resulting from it. As a matter of fact, the physiopathology of most xerosis is still badly known, and it remains difficult to distinguish the causes from the consequences of these skin abnormalities.

Previous studies have shown the importance of four factors predisposing to dry skin: 1. the lack of water of corneocytes, directly depending on the presence of NMF; 2. the epidermal hyper-proliferation, resulting from a deficiency in the renewal process of the keratinocytes; 3. the change of lipidic synthesis at the cell level; and 4. the deterioration of the functionality of skin barrier, following degradation of intercellular cohesion. All these factors are interdependent. Consequently, dry skin should be characterized by its rough appearance, without referring to its hydration level. Recent research has actually questioned some established ideas notably the influence of the inflammatory process or of the content of calcium ions of the epithelial cells in skin drying.

Skin drying does not correspond to an irreversible state but results from a dysfunction involving the traditional “balance moisture theory” and the “protease regulation theory,” resulting in these new research.  

As already seen, dry skin depends on numerous biological factors; its reparation implies the restoration of the epidermal barrier, actually damaged by the loss of fat and dehydration of the superficial layers of the stratum corneum. Such changes are more easily objectively in African-American subjects in whom the skin takes a perfectly visible ashy appearance.

2.3 Oily Skin

Whereas dry skin reflects a functional change of different skin components, the oily skin results from an overactivity of the sebaceous glands, leading to an overproduction of sebum overflowing on the skin, giving it a characteristic oily and shiny appearance. In fact, sebum results from the disintegration of specific cells, the sebocytes, a short time before they are secreted from the sebaceous gland. Once again it results from a cell differentiation. Originally, sebum contains squalene, waxes, triglycerides, and sterols. Under the effect of resident bacteria, one part of the triglycerides is immediately hydrolyzed, and the main part of the cholesterol is esterified, the sebum excreted containing a significant quantity of free fatty acids contributing to the acidity of the pH of the skin surface. Then this sebum blends with epidermal lipids produced from the destruction of the desquamated horny cells that also contain triglycerides and cholesterol to form the surface lipidic film covering the stratum corneum. Human beings have the particularity to have at their disposal sebaceous glands almost all over the body, but their activity is not the same on all the anatomical sites. The production of sebum is more important on the head, face, neck, shoulders, and thorax, areas where a hyperseborrhea can be the conjunction of high production of the glands and of a greater number of glands. Sebum is a natural skin detergent that gives the skin amphiphilic wettability through the presence of free fatty acids and wax. It also plays a part in the maintenance of the functional qualities of hairs, a fungistatic activity, while having a nutritional function for bacterial species useful for the organism, and finally, a protective function against excessive dehydration in a dry environment through its effect on the epidermal barrier function, even if the sebum is not known to have a dampening activity and has no influence on the skin’s hydration level. The change of its rate of production depends on genetic, endocrine, and environmental factors.

The opposite of oily skin would not be dry skin since they can coexist, for example, on face. Such a statement is currently supported by many workers.

Actually, young children fairly never have a seborrheic outbreak before the age of seven years, when the first secretion of androgenic precursors starts to form. This production will progress to reach its maximum at adolescence and then decrease with age. It is also worth noting the racial differences related to sex, with men globally having an oilier skin than women.

Finally, at the cosmetological level, it must be retained that oily skin is sometimes erythrose, easily irritable, and particularly fragile.

2.4 Mixed Type

It corresponds to a complex skin where the different types previously described coexist on different areas of body or face. The characteristic example is the face, where solid and oily skin with well-dilated pores on the medio-facial area can coexist with a fragile skin with fine grains on cheeks. Such a skin requires conjugating the particularities and sensitivities peculiar to normal, dry, and oily skins.

3.0 Skin V/S Age

Because of the continuous aging of the skin and its incidence on its structure and functionality, the age of the subjects included in a study is often the main element to obtain relevant results.

Age has a direct impact on the evolution of most of the biophysical parameters of the skin. Many publications have shown the incidence of aging on the increase in its roughness, the evolution of the microdepressionary network of the skin, and the development of wrinkles whatever the ethnic group considered.

To simplify, roughness can be considered as submitted to external and internal influences such as the climatic environment, the sun exposure, and the effect of cosmetic products but also the water content of the skin’s superficial layers.

As an example, it has been demonstrated that a significant increase in the sound level between children and adult skins is indicative of their different smoothness. The evolution of the microdepressionnary network is particularly sensitive beyond the age of 40 years as the main lines start to grow hollow progressively. The lines of secondary orientation progressively disappear between the age of 50 and 80 years, and we observe monodirectional lines orientated in the direction of the skin deformation and the multiplication of great spaces whose folds are not visible microscopically.

For all the races, there is a decrease in the hyperpigmentation spots (colour) related to the age of the subjects. We can deduce from the bibliographical data from studies that there is a decrease in the brightness of the skin with aging but also that this variation depends on the site where the measurement is performed.

The conductance generally increases with the age in all the ethnic groups. The capacitance measured comparatively in young and old subjects appears significantly lower in old subjects.

In the adult person, epidermal proliferation rate decreases with age. It can be 10 times higher in younger (second decade) than in older (seventh decade) individuals, and for a given age, the decrease is demonstrated to be 10 times faster in sun-exposed areas than in unexposed ones. These constant reductions seem to be independent of the ethnic origin and season.

4.0 Skin V/S Race

It is useless to talk about the interracial morphological differences. They are obvious and never give rise to confusion at the very risk to complicate the problem of ethnical integrations. At macroscopical level, Caucasian, Hispanic, Asian, and African skins are very different at first sight as their color is enough to give them a well-distinct appearance. So, whereas the mean thickness of the horny layer is similar between the different races, the number of cell layers in the stratum corneum of the black skin is higher than that noted in Caucasian or Asian skins. Black skins therefore have a more compact stratum corneum with a greater cohesion between cells.

The color of black skin is mainly related to the particular migration of the melanosomes that invade all the epidermal layers and reach the horny layer without undergoing degradation, a process that is completely different from what happens in the skin of Caucasians. Pigmentation favors a better protection against sun radiations and therefore actinic aging. This can explain why, from this point of view, aging is quicker for the Asian skin. The racial differences in constitutive pigmentation are also directly related to the incidence of pigmentation disorders, the black skin being much more exposed to hyper-chromatic spots that appear under the effect of external aggressors, or to hypo-chromatic spots for lack of sun exposure.

The measurement of electrical conductance on the skin superficial layers enabled to show that it is the highest for the black skin, lesser for the Hispanic and Asian skin, and the lowest for the Caucasian skin.

5.0 Skin V/S Sex

Although the influence of sex on the results of biophysical measurements is often quoted in bibliography, little precise information is supplied, maybe because this criterion actually has little real influence on the results. However, there are morphological differences in the skin according to the sexes. In fact, the skin thickness is greater in men on most of the sites usually used for biophysical measurements, whereas for women, the skin is thicker at dermal level. Other authors reported no significant differences for the forearms. Observations made on male and female Asian subjects enabled to show no difference between sexes concerning the number of layers of coenocytes. The skin thickness would reduce more quickly with aging in women than in men.

6.0 Skin V/S Site

As previously seen, the racial criteria, age, and sex are not enough to define the skin’s response to an aggression or to a possible restructuring effect. In fact, important variations exist in the subject considered separately according to the sites on which the measurements are performed, these variations being sufficiently important to invalidate the experimental results. The spontaneous changes of the skin’s state over time according to intercurrent factors that depend on physiological and hormonal variations and on its proper aging therefore imply that their incidence is systematically taken into account, such an approach can only be performed case by case. The skin’s thickness is not the same between anatomical sites as established in the publications of many authors through numbered data and different instrumental measurements. So, for example, the skin’s thickness measured in the subject of the Caucasian race is less on the forearm than on the forehead, of the order of 0.9 and 1.7 mm, respectively.

There are important natural variations in the skin color between anatomical sites in absence of the additional effects on melanogenesis induced by sun exposure. The location of the site of measurement is therefore very important during a repeated colorimetric analysis of the skin. The interference that results from the variation in pigmentation according to its exposure to the sun’s UV radiations is very important and can also induce higher deviations than those existing between anatomical locations.

A very great number of research undertaken to have a better knowledge of the state of the skin hydration, notably through the study of its electrical conduction, quickly enabled to establish that it is not homogeneous on the whole human body.

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