Mechanisms of skin pigmentation

Skin pigmentation is caused by melanin, which is a dye produced in melanosomes – melanocyte organelles that help transport melanosomes to keratinocytes. Melanin is composed of the following elements: yellow-red pheomelanin, black DHI-eumelanin and light-brown DHICA-eumelanin. The colour of human skin and hair depends on the proportion of these three components in melanin. In skin chronically exposed to ultraviolet radiation the level of DHI increases significantly. The damage of the DNA of keratinocytes initiates the process of melanogenesis after exposure to light/ultraviolet. As a result, p53 – a transcription factor for proopiomelanocortin (POMC) – is activated. The protein, which is the product of the POMC gene, is then cut into three peptides: ACTH, endorphin and MSH – a hormone stimulating melanocytes. The lack of an appropriate MC1R receptor is the reason why the skin of people of the Celtic race (Fitzpatrick type 1) does not tan. There are differences in the location of melanosomes within keratinocytes, the size of melanin grains and the composition of this dye depending on the human race. In the skin of oriental and negroid race melanosomes are located separately, grains are twice as big as in the case of the white skin, and DHI is the dominant dye.