How is Skin Colour Formed?
The skin consists of two layers, an outer Epidermis and an inner Dermis. The epidermis consists of four layers of cells, being from surface inwards the corneal cell layer, the granular cell layer, the spinal cell layer and the basal cell layer. The basal cell layer is a single layer of cells attached to the basement membrane that separates the epidermis from the dermis. Situated amidst the basal cells are the Melanocyte cells, which produce the pigment, melanin. The skin gets its colour, white, black, brown, yellow etc, from the colour of the pigment produced. The inner layer Dermis consists of connective tissue and contains the nerves, blood vessels, lymphatic vessels and secretory glands.
The production of melanin is a complex process. The melanocytes produce the elliptic shaped organelles called the melanosomes, in which the melanin synthesis takes place. The melanin content in the melanocytes is heterogeneous for different skin types. This heterogeneity is regulated by gene expression. The melanocytes are also regulated by the Melanocyte stimulating hormone produced by the pituitary gland of the brain. The variation in the skin colour among various ethnic groups is determined by the number, melanin content and distribution of melanosomes produced. The number of melanocytes in the human skin remains constant but the size and pattern of distribution varies. Highly pigmented skin (Black) contains numerous single large melanosomes which are intensely melanotic. Light coloured skin is associated with smaller melanosomes which are clustered in groups. The unique pattern of the distribution and type of melanosomes is inherited.
There are many internal and external factors which influence skin pigmentation in humans. Tanning or darkening of the skin is popular among whites due to the smooth texture and appearance that it induces. Tanning involves physiological stimulation of the melanocytes and melanin synthesis by the ultra-violet radiation of the sun. Melanin protects the human skin by absorbing, scattering and minimizing the toxic effects of UV radiation. There is a decline in the number of functional melanocytes with age. However there is no loss of pigmentation. With age circumscribed, round, hyper pigmented areas called Age-spots may form on UV exposed areas like the face, hand, upper back, etc. Melasma is symmetrical facial hyper pigmentation seen in pregnancy, which is hormone induced and fades away after pregnancy. Discrete hyper pigmentation may be seen post inflammatory after dry burns, healed injuries, acne etc and disappears slowly over a period of time. Commonly used drugs such as antibiotics, diuretics, pain relievers and oral contraceptive pills can cause hyper pigmentation.
Melanin produced by the melanocytes is the major determinant of human skin color. It offers protection against UV radiation emanating from the sun, preventing skin damage as well as skin cancer development. Human skin color is a multifactorial trait with a number of determinants such as genes, gender and environmental influences such as exposure to UV radiation. The people living in the equatorial belt across the world like in central Africa, south India etc are dark skinned because, the black melanin pigment of the skin gives protection against the harmful effects of the sun. The afro-americans originally dark skinned like the native africans have become much fairer over a few generations due to living in temperate climates! Skin colour is after all a function of the melanocyte cells of the epidermis to protect us from the sun!
To know more about What Controls Variation in Human Skin Color Visit www.ncbi.nlm.nih.gov
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Author: Dr. Pilli Samuel Rajkumar, MD (Hyderabad, India)
Edited by: Rajesh Bihani ( Find me on Google+ )