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PROTECTING, SENSING, REPAIRING |
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A barrier put to the test
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To carry out its protective task, the skin must be a resistant and impermeable barrier. This role falls in large part on the epidermis, directly exposed as it is to the external environment.
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While the dermis is mechanically strong, it is hydrophilic, i.e. it avidly takes up water. It is the superficial layer of the skin or horny layer which makes the skin impermeable and hydrophobic, protecting the dermis from massive water entry. This layer also resists chemical attacks, thanks to the corneocytes filled with the hard insoluble protein
keratin and the lipid cement which ensures cohesion between the corneocytes, and therefore impermeability.
However, the epidermis is not an impenetrable barrier. Free water from the dermis continually crosses the epidermis by capillarity and evaporates from the surface of the skin, known as the Trans Epidermal Water Loss, or TEWL (on average 9g/m2 of skin per hour). In the reverse direction, the skin lets water in, and active molecules too.
This property of cutaneous absorption is used to hydrate the skin and promote the action of cosmetics or medical products.
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Zoom
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Scanning electron microscopy of skin bacteria.
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The superficial layer of the skin is covered by the acidic mantle (pH between 4 and 5.5). This is the hydrolipidic film which has all the properties required to prevent non-resident bacteria from developing and maintain the skin's barrier. It is composed of a mixture of sweat, sebum and lipids to which antibiotic peptides
antibiotiques appelés défensines et dermcidines are added called defensins and dermcidins synthesised by the sweat glands. The horny layer is not a sterile place and numerous resident bacteria (1012 bacteria/m2) are sheltered there and prosper in the inter-corneocytic spaces. Three species of bacteria are particularly well adapted to withstand the acid environment and antibiotic peptides: Staphylococcus, Propionibacterium and Corynebacterium. They feed on corneocyte debris or on sebum
cells and they prevent other undesirable bacteria from developing. They therefore play a fundamental role as "cleaners".
Maintaining the physiological pH
of the skin is essential to avoid the proliferation of pathogens and an excess or lack of hygiene upsets the equilibrium of this cutaneous flora.
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Confocal Microscopy of a Langerhans cells.
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The skin has a very elaborate defence system, where different types of cells act together or successively. It is therefore not surprising to find that the skin has an immunological function. This function is in the first instance carried out by the Langerhans cells, which are dendritic cells originating from the bone marrow, then by T-lymphocytes and macrophages.
Langerhans cells which make up 2 % of the epidermal cells are located in the Malphigian layer and are responsible for immune response against an antigen
applied locally to the skin. If an antigen penetrates and crosses the horny layer, these cells take charge of it and leave the epidermis. Within less than 6 hours they may be found in dermal lymphatic vessels where they pass the antigen to T-lymphocytes Real killer cells, the T-lymphocytes
release the immunological cascade reaction to eliminate the antigen and keep it in the "immunological memory". If the same antigen again penetrates the epidermis, even years later, it will be immediately recognised and the immunological reaction will be more rapid and effective.
If the antigen however manages to cross this first line of defence, the macrophages of the dermis detect it and eliminate it. This duo is complemented by the keratinocytes which, in the case of an infection, produce more cytokine
IL1, which stimulates T-lymphocyte activity.
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Like all mammals, man is homeothermic: he maintains his internal body temperature constant, near 37°C, even if the external temperature varies. When subjected to temperature variations, no matter how small, the organism reacts by starting a series of processes to restore its optimum temperature. This is known as thermoregulation
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The organism is capable of eliminating excess heat by perspiration. To perform this task, a thermoregulatory centre is needed the hypothalamus
, as well as radiators (the blood vessels) and finally a cooling system (the sweat glands). Thermolysis is said to be the phenomenon which aims to lower a raised body temperature and the phenomenon aiming to raise body temperature is known as thermogenesis.
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