Human skin and hair pigmentation not only have a crucial role in social behavior but also helps in photoprotection from the harmful effects of ultraviolet light. The main pigment in mammalian skin is melanin, synthesized within specialized organelles called melanosomes in melanocytes, which sit at the basal layer of the epidermis and the hair bulb. These melanins get transferred from melanocytes to keratinocytes, where they accumulate perinuclearly in membrane-bound organelles as a "cap" above the nucleus. 

The transfer mechanism, the nature of the pigmented organelles within keratinocytes, and the mechanism governing their intracellular positioning are debatable and poorly understood. Still, they play a crucial role in the photoprotective properties of melanin in the skin. 

A better understanding of the cell biological processes describing the entry, transport, biogenesis, positioning and maintenance of the pigment organelles in keratinocytes will help to explain whether these processes influence skin photoprotection. Also, defining the role of melanin in driving some of these fundamental processes and the role of the genetic background of the keratinocyte in altering them will give an insight into new experimental approaches to dissect the links between MC+ organelle structure and function. 

Researchers are trying to establish robust, flexible and easy-to-use experimental cell model systems to accurately reflect keratinocyte pigmentation in situ. If successful, this will be a valuable tool for future research, much as robust cultured melanocyte systems have allowed new advances in understanding melanosome biogenesis in the pigment-producing cells.

Source: Benito-Martínez S, Salavessa L, Raposo G, et al. Melanin transfer and fate within keratinocytes in human skin pigmentation. Integr Comp Biol. 2021;61(4):1546-55.