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PROTECTING, SENSING, REPAIRING |
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A production site
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While it can cause irreversible damage, the sun is however essential for life and the skin uses its rays for the synthesis of melanin and vitamin D.
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It is in the epidermis and more precisely in the basal layer that the melanocytes
, are found, an army of cells responsible for the synthesis of melanins. This occurs all the time but is strongly stimulated by the effects of UVB rays, 1000 times more effective than UVA radiation. Melanins have the power to absorb ultraviolet rays, and thus they protect the skin from attack by solar radiation. This is translated into a more or less intense tanning of the skin.
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There are two types of melanin: pheomelanin which is yellow coloured, and eumelanin which is black. It is the proportion and size of these pigments, without forgetting the carotenoids and the blood micro-circulation which gives the wide range of skin colour. There is a real continuity of colour from the darkest to the lightest skins.
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Key steps in melanin synthesis.
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Staining revealing melanin synthesis.
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The two types of melanin are synthesised from the same amino-acid
, tyrosine. An enzyme
, tyrosinase, transforms tyrosine in DOPA, then into DOPA-quinone. From DOPA-quinone two routes are possible, one leading to pheomelanin, the other to eumelanin.
In the presence of cysteine
, a sulphur-rich amino-acid,DOPA-quinone is transformed into cysteinyl-DOPA, an intermediary in the synthesis of pheomelanin. In the absence of cysteine, indole 5-6 quinone is formed and it is eumelanin that will be synthesised. These two pigments will then polymerise to a greater or lesser extent depending on the nature of the pigment to form the protective molecules.
Any genetic mutation making tyrosinase less active or absent causes albinism
, a condition characterised by the absence of melanin and therefore white hair and a very light skin which is ultra-sensitive to the sun.
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Melanosome caps protect keratinocyte nuclei
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The synthesis of melanins takes place in the melanocytes, inside the melanosomes. During its long journey, melanin will be synthtized, stored and degraded. At the beginning of synthesis they appear as round colourless vesicles. At the end of maturation of the melanins, they are ovoid and opaque since they are full of pigment. They are then transferred via the dendrites of the melanocytes to the keratinocytes of the mitotic layer of the epidermis which ingest them by phagocytosis. The melanosomes then collect above the nucleus of the keratinocytes
where they form a cap protecting the DNA
which is very sensitive to ultraviolet radiation.
During migration of the keratinocytes to the superficial layers of the epidermis, the membrane of the melanosomes is progressively digested by enzymes, more or less easily depending on the morphology of the melanosomes, which in part explains the colour of the skin. In fact, while in a person with black skin the melanosomes are not more numerous, but they are bigger, more resistant to digestion and arrive at the surface without difficulty and colour it more. The classification of men according to their skin colour has therefore no scientific basis and it is only the type of pigment and its size which are the origin of the remarkable palette of colour shown by the skin.
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Vitamin D synthesis is sun dependant.
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Vitamin D is essential for fixing calcium, the lack of which is responsible for rickets. The organism has two sources of vitamin D: food (oily fish, eggs, butter, cod liver oil etc.) or synthesis, which requires exposure to the sun.
The sun's ultraviolet B rays transform cholesterol into vitamin D3, which is modified in the liver and kidneys into active vitamin D. This vitamin, the anti-rickets effects of which has been known for a long time, improves the final differentiation of the epidermis and tones up the skin. A short exposure to the sun (ten minutes a day during the summer for a child in shorts and T-shirt is enough) can thus play a role in maintaining a good quality epidermis and strengthen the bones by its beneficial action on calcification.
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