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Understanding skin

As well as being an envelope that protects our body, the skin is our largest organ: in an adult, it measures between 1.5 and 2 m2 and makes up approximately 16% of body weight. Its structure and the way it works are complex. It defends the body against external elements and safeguards it against temperature changes. It is also the organ that gives us our most important sense: touch.

The structure of skin

Our skin is composed of three main layers: the epidermis, the dermis and the hypodermis, from the outermost layer to the deepest.

1. The epidermis

The epidermis is the external layer of the skin, in contact with the elements. Its surface is pierced with a dozen pores per cm2, through which sweat and sebum are evacuated. These two substances combine to form the hydrolipidic film, which nourishes the skin, protects it and keeps it supple. The epidermis is very fine with an average thickness of 0.1 mm. Keratinocytes make up 90% of it. These are cells filled with keratin and lipids. They form in the deepest layer of the epidermis (basal layer), then gradually migrate towards the surface, flattening out as they go. Four to six weeks after their formation, they arrive at the surface, lose their nucleus and become corneocytes. Despite being biologically dead, corneocytes play an important role: they form a semi-permeable barrier (the corneal layer or stratum corneum) that protects the body against external elements, yet still allows the skin to breathe. Corneocytes are eliminated through desquamation, to make way for a new generation of cells. In this way, the epidermis is constantly regenerated.

The epidermis also contains three other types of cells that all play a fundamental role: • Melanocytes, which synthesise the pigment that colours our skin (melanin) and protect it against UV rays.

Merkel cells, which contribute to the sense of touch.

Langerhans cells, which are connected to the immune system and contribute to defending the body.

2. The dermis

As the skin’s supporting tissue, the dermis is 10 to 40 times thicker than the epidermis. It is mainly composed of fibroblasts – cells that synthesise elastin and collagen. As they are supple and highly resistant, these fibres give the skin its strength and elasticity.

The dermis is irrigated by numerous blood vessels, which regulate skin temperature and bring the epidermis the nutrients it needs. It also contains the sebaceous and sweat glands – which respectively secrete sebum and sweat – and the hair follicles. Lastly, it contains nerve endings for our sense of touch.

3. The hypodermis

The hypodermis is the deepest layer of the skin. It is essentially made up of fat cells (adipocytes). These act as a thermal insulator and protect the body against pressure and blows. They also store energy for the body.

Skin functions
The skin is a complex organ that plays three vital roles in ensuring our body functions properly:

• First and foremost, it acts as a protective barrier against external elements. Thanks to a highly elaborate defence system, it is capable of stopping most undesirable elements from entering the body: chemical substances, fungi, viruses, harmful bacteria, etc. Its suppleness and resistance also enable it to protect the internal organs from traumas. Lastly, to a certain extent, the melanin contained in the epidermis protects the skin against UV rays.

• The skin is also the carrier of our sense of touch, thanks to various sensory receptors which enable us to very keenly feel pressure on the skin, vibrations, heat and pain.

• Finally, the skin is essential for maintaining a constant core body temperature: the dilation or constriction of the blood vessels that irrigate it, combined with the production of sweat in varying amounts, protect the body against outside temperature variations.

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