Color of the skin

The color of the human skin , still called dyed or complexing , presents a continuous gradation of the white to almost black dark chestnut, with sometimes of let us tons rosy or coppered.

The quantity and the nature of the Mélanine S contained in the skin, like their distribution, are the principal criteria determining its color. Producing on average less mélanine that the men, the women have a dye slightly more clearly. In addition, the food Carotène can, in significant amount, to give a slightly yellowish component to the skin. The Hémoglobine of the blood capillary gives a coloring dew to the clear skins, except at the people having a layer of thick subcutaneous grease, frequent case in the Natives of the Far East.

The Génétique plays a key role in the determination of the dye, but also the exposure to the sun, the majority of the human bronzing under the effect of ultraviolet radiations . The absorption of certain drugs, as well as pathologies like the Disease of Addison, can also cause hyperpigmentations, or contrary to the dépigmentations (example of the Vitiligo). The female Hormone S in great quantity cause among certain women a hyperpigmentation partial of the face (mask of pregnancy).

Social aspect

The color of the skin is a striking individual characteristic, often considered as an ethnic marker (importance of heredity) or socio-economic (living conditions determining the duration to the sun). It can thus become pretext with the discriminations based on the Racisme or the social hierarchy. In the physically homogeneous populations, the individuals being characterized by an unusual skin color, often caused by a genetic mutation, suffer in general from rejection.

Among the populations likely to bronze alive in little industrialized companies, the people with the clear dye often enjoy a social advantage. Indeed, the dark skins are associated with a socio-economic level low, imposing work in the open air. This situation was gradually reversed starting from the beginning of the XXe century in Europe. Bronzing became the prerogative of those which had the means of leaving on vacation, distinguishing them from the underprivileged layers which more and more often worked since the XIXe century in places closed with the shelter of the sun. Thereafter, the medical campaigns informing of the misdeeds of sun excesses modified mentalities, and in XXIe century, in the developed countries, one does not judge any more of the socio-economic level strictly according to the color of the skin; this criterion can nevertheless play as a component of a physique of “alien” population of origin to which it general opinion allots a given socio-economic level. In addition, the negative social influence of the dye related to the true racism (which considers that certain hereditary physical features announce negative characteristics independent of education and environment) did not disappear.

Subject of research

Research is undertaken to identify the factors influencing the color of the skin, with for goal to improve the prevention of the damage due to the sun, most serious being the cutaneous Cancer S like the Mélanome. They are based on novel methods using the Spectrophotométrie, make it possible to evaluate the dye more objectively than the nuanciers. This research can also help to recall the history of the first men (origin, migrations) and with better including/understanding the relations between the Génotype and the physical aspect (Phénotype).

Physiology

The mélanine, pigment determining the dye, is produced in cells called Mélanocyte S. Their median number by surface of skin is the same one at all the human ones, whatever the color. It depends on the other hand on the part of the body considered (it is weak, for example, on the palms of the hand).

The differences between the clear and dark skins are primarily due to the characteristics of the Mélanosome S, kinds of granules charged with mélanine manufactured by the mélanocytes which distribute them thanks to their arms (Dendrite S) to the other cells of the basal layer of the skin, called Kératinocyte S; those distribute them above their core, thus decreasing the risks of damage caused by UV with the genetic material. A mélanocyte serves on average forty kératinocytes. More the mélanosomes are large, numerous and charged in mélanine, more the skin is dark. At the people with dark skin of African origin, the mélanosomes measure in general more than 0,8 microns, whereas at the Asian ones and Europeans, even sheepskins, they are typically smaller.

In addition, in the black skins of African origin, the mélanosomes are independent from/to each other, which allows a better spreading out. Cultures of cells In vitro showed besides that the kératinocytes also showed different characteristics, those of the African black skins distributing the mélanosomes in a more regular way. At the people of European or Asian origin, whatever their dye, the mélanosomes independent from/to each other, but are not gathered in kinds of small bags; they are not able thus to be distributed very regularly above the core.

A last factor, which plays especially at the populations of European origin, Mediterranean or center-Asian, is the nature of the pigment contained in the mélanosomes (Eumélanine or Phaéomélanine) and their respective proportions. The phaéomélanine, orange-red in strong concentration, is naturally less dark than the eumélanine brown-black.

Genetics

Since 1964, a research carried out in Great Britain on the color of the skin in afro-European mixed families had highlighted a clear influence of heredity, and the setting concerned of more than one gene. In the studied population, the dye of the children seems to result from the addition of the dyes of the parents (ex: white + black = brown clearly). In fact, except in the case of pathological changes , the genes determining the color of the skin are still badly known. They could be numerous: on the hundred genes controlling the color of the hair in the domestic rodents, fifteen or twenties have a counterpart at the human ones.

The genes influencing the color of the skin already identified are the MC 1R, whose certain alternatives (Allèle S) determine the “skin of russet-red” and the presence of Freckles, and more recently the slc24a5. MC 1R especially plays a big role at the European populations or of European origin. Many of its changes, which involve an explanation of the skin, exist only out of Africa.

The role of the slc24a5 was discovered at the time of research concerning delivered Poisson streaks, cypriniforme of Fresh water. The researchers showed that the explanation of delivered at the variety Golden is determined by the change of a Protéine coded by this gene. As it has its equivalent in the mankind, research concerning its role was extended to a sample of people of afro-European ascent. In this type of population, the slc24a5 would be responsible for almost 38% of variability. The explanation of the skin would be thus due in a certain number of cases to only one change of this gene, appeared either in North Africa, or with the the Middle East, and which would have been then spread in Eurasia where it would have been stabilized.

Apart from normal variability, certain changes considered as pathological can influence the dye. The Albinisme is an example.

Dye and latitude

The gradation of the dye according to the Latitude was observed for a long time. In 1997, Relethford showed that the index of reflection of the skin, less at the equator, grew according to the latitude. Concretely, that means that the more one moves towards the poles (the less the skin receives UV on average in the year), the more the skin color of the autochtones is cleared up. This phenomenon seems confirmed by a search for 2000 the assumption most generally selected to explain this fact is that a too dark skin under high latitudes increases the risk to suffer from Rachitisme for lack of Vitamine D, this one being synthesized only if one sufficient quantity of UV penetrates until the Derme. In fact, before the appearance of the vitaminized supplements, the rickets were particularly frequent in winter in the moderated zones, even at Europeans with clear skin. The Inuit S (Esquimaus) with the coppered dye draw their vitamin D from a particular food, rich in grease of marine animals. The opposite phenomenon, the sinking of the skin progressively of the increase in UV, is in general explained by the risks of burn and cutaneous cancers incurred by the clear skins. Nevertheless, other assumptions were proposed, like the increased frequency of the miscarriages in the naked Hominides with pale skin, UV destroying the reserves in Vitamine C.

The color of the skin is thus a character adapted to a factor of the environment, the sunlight. The variations, according to any probability, must with the Natural selection.

References

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