What is the process in the darkening of the skin during regular sun tanning?
Darkening of the skin, or tanning, following an increased exposure to the sun is the result of a two stage process. Firstly, the pre-existent melanin is darkened by a physicochemical reaction and subsequently the rate of melanin synthesis in melanocytes and transfer of melanin granules to the cells of the epidermis is increased. Melanin is not usually observed in the more superficial cells of the stratum spinosum or in the more superficial strata. In the cells of the mid stratum spinosum the melanin granules fuse with lysosomes and the melanin is broken down. This explains why the skin does not stay permanently tanned, and it will only last 7 to 10 days. Also, the very top layer of the epidermis stratum corneum continually sheds being replaced by new cells.
