Definition

The skin is the largest organ in the human body. It weighs on average 4kg and represents an area of 2m². Its thickness is 2 to 3mm on average in an adult; it varies from 1mm at the level of the eyelids to 4mm at the level of the palms and soles of the feet. It is a physical barrier that protects internal tissues and organs from most external, physical, chemical and infectious attacks.

By virtue of its visibility and above all its chromatic characteristics, the skin has for a very long time been used as a criterion for “ranking” men. In our current society, it is given great importance because it is on it that we first look. Its firmness, color and consistency are signs of good health and indicators of socio-economic position.

Anatomically, the skin consists of 2 main parts, namely the outer superficial part which constitutes the epidermis and a thicker internal part which constitutes the dermis and hypodermis.

The epidermis

It is made up of four layers deep towards the surface: the basal layer, the Malpighi mucous body layer, the granular layer, and the stratum corneum. The epidermis is made up of four types of cells.

• - Keratinocytes represent 80% of the cell population of the epidermis and are responsible for the formation of keratin, which is used in the constitution of the horny elements of the skin, hair and nails. These fibrous, water-insoluble proteins provide impermeability and protection against external agents. These keratinocytes last 35 to 45 days and die off as they migrate to the skin surface. Their production accelerates in the places most subject to friction (hands, feet).

• - Melanocytes synthesize under the control of a pituitary hormone, MSH, a black pigment, melanin, responsible for the color of the skin. There is on average, for a subject, 1 melanocyte for 7 keratinocytes. The number of melanocytes is the same whatever the origin, they represent about 5% of the cells. The differences in skin pigmentations are due to the amount of melanin produced by melanocytes and transferred to keratinocytes.

• - Langerhans: cells playing a role in immune reactions, they are located in the mucous body of Malpighi and represent 3 to 8% of the cells of the epidermis.

• - Merkels: they occupy 1% of the cells of the epidermis. They belong to the diffuse endocrine system whose cells are able to capture and store amines and their precursors in order to secrete polypeptic hormones.

The dermis:

This is a richly vascularized and innervated connective tissue on which the epidermis rests. Their cohesione is ensured by an epidermal-dermal junction provided by the hemidesmosomes to allow exchanges between the epidermis and the dermis. It is composed of a set of proteins such as hyaluronic acid on which mineral salts bind to form different connective tissues, collagen fibers, reticulin fibers and elastic fibers, mainly fibroblast cells, macrophages, mast cells, plasma cells and other blood cells in low proportions under normal conditions.

The hypodermis:

The hypodermis is the deepest layer of the skin. This layer constitutes the more or less thick fat according to the individuals. It is contained in lobules separated from each other by fibers identical to those of the dermis. These fibers provide both nutrition and support for the hypodermis. It has an essential role of shock absorber and protection from the cold by insulation.

Functions of the skin

The skin is a barrier between the external and internal environment of the human body. It is a complex organ whose functioning has two purposes: to ensure communication between our own organism and the surrounding environment on the one hand and on the other hand, to protect our organism from external aggressions.

Protective organ:

The skin mainly performs a physical barrier function that protects tissues and organs from external physical, chemical and microbial attacks thanks to the stacking of corneocytes in the stratum corneum and against solar rays and heat. It also avoids the loss of bodily fluids and represents a semi-permeable membrane against the external liquid.

Thermoregulatory organ:

The vasodilation of the arterioles allows the transport of heat from the depths of the body to the skin and the vasoconstriction helps prevent heat loss.

An exchange function:

Through the entry and exit of water thanks to its permeability and the elimination of sweat.

Metabolic functions:

Including the synthesis of vitamin D under the action and influence of ultraviolet rays.

Absorption functions:

Oily substances, drugs and excretion of waste and toxic products through sweat.

Organ modulating thymic:

Keratinocytes produce endorphins under the action of UV rays which are involved in the regulation of the individual's thymic (depressive syndromes more common in winter).

Immune organ:

Skin cells are presenters of antigens which can therefore activate T lymphocytes.

sensory organ:

The skin is the seat of touch and allows adaptation to the surrounding environment. Nerve endings in the skin allow the body to explore its surroundings through touch. It allows the body to have sensitivity to pressure.

Vascularization organs:

True reservoirs, the blood vessels of the dermis represent 10% of blood in adults. During physical exertion, these vessels contract and promote blood supply to the muscles.

Organ of social relationship and communication:

The skin, through its color, texture and smell, transmits social and sexual messages. For example, blushing suddenly.

Skin pigmentation:

Skin color varies from individual to individual, ranging from extremely light shades to dark black depending on ethnicity. It also varies intra-individually depending on different parts of the body, but also over the year depending on the degree of sunshine. It is regulated by genes that control the embryogenesis, proliferation, migration and differentiation of melanocytes, but also by environmental factors, of which solar ultraviolet (UV) rays come first.

The normal color of human skin is determined by pigmentation genes. About 130 pigmentation genes have been identified. Skin color is a result of three components: melanin, hemoglobin and carotene. In the epidermis, the two types of cells involved in skin pigmentation are the melanocytes in which the synthesis of melanin takes place and the keratinocytes where the melanin synthesized by the melanocytes is accumulated.

Melanin

Melanin is the pigment responsible for the color of the skin, hair and eyes and is synthesized in the melanosome which is a cytoplasmic organelle. Melanin pigments can be classified into two groups: eumelanins (are darker: brown and black pigment) found in the epidermis, hair and body hair. Phaecomelanins, a reddish or yellow brown pigment that is more common in redheads or blondes.

After synthesis, melanin is transferred to keratinocytes by melanocytes. In keratinocytes, melanosomes migrate and accumulate in the suprareal region to protect DNA from UV rays. The proportion and size of transferred melanosomes will be responsible for the coloring of the skin. Melanocytes are distributed throughout the epidermis in a non-homogeneous manner, the areas most exposed to the sun contain 2000 / m2 and the rest of the body 1500 / m2. These are large cells with dendritic extensions reaching the 3rd Malpighi layer. Skin color does not affect the number of melanocytes.

Different types of skin

On a daily basis, we can see a differentiation of skin color. Indeed, the skin is naturally rather white in color in Caucasians and Asians, and dark in some African countries. A classification according to the protective capacities of the skin against UV rays makes it possible to distinguish 6 phototypes. Ethnic variations in skin color do not depend on the melanocyte density in the epidermis.

Indeed, the number of melanocytes per square meter of the different regions of the skin is practically identical in the different ethnic groups. It is above all the melanogenic activity of melanocytes and the dispersion of melanosomes in epidermal keratinocytes which are responsible for variations in skin color. Phototypes make it possible to classify the type of skin according to its reaction to ultraviolet radiation. People with darker skin have better tolerance to ultraviolet rays and lower sensitivity. People with fair skin have low tolerance to ultraviolet radiation and greater sensitivity.

Depending on the phototype of the individuals, the size of the melanosomes as well as the mode of uptake by the keratinocytes are different. In keratinocytes, large melanosomes distribute themselves in isolation while small melanosomes cluster in clusters. Melanosomes are found in isolated form in black-type skin and as a complex in Caucasoid-type skin.

The only difference between black and white skin is the melanin content. In fact, the skin is not really black or white, they are variations in the red spectrum.