Melanocytes and melanin
Melanocytes are dendritic cells derived from the neural crest that are able to produce melanin. Mammalian melanocytes are categorized in two groups: cutaneous melanocytes, which are involved in hair and skin pigmentation, and extracutaneous melanocytes, which represent the melanogenic cells of the eye, inner ear, adipose tissue, brain, heart and bone (Kawakami and Fisher, 2011). The life cycle of melanocytes is split in several steps: lineage specification from neural crest cells, migration and proliferation, differentiation into melanocytes, maturation of melanocytes, transport of mature melanosomes to keratinocytes in the case of cutaneous melanocytes and cell death (Cichorek et. al, 2013).
Melanogenesis happens during the phase of melanocyte maturation in specialized organelles, called melanosomes. Melanosomes are large organelles (~500 nm), which are the cellular sites of the synthesis, storage and transport of melanin (Wasmeier et. al, 2008). Two types of melanin pigment are synthesised during melanogenesis: the brown/black colored eumelanin and the yellow/ red colored pheomelanin. The color of the skin, eye and hair depend on the ammount of pigment and on the balance between the synthesis of eumelanin and pheomelanin. It is important to note that only eumelanin can efficiently protect against UV radiation (Borovanský and Riley, 2011).
Melanosomes are transfered to keratinocytes only in the case of cutaneous melanocytes and it has important roles in the immune response to various stimuli and in the protection against UV caused DNA damage and oxidative stress. The melanocytes of hair follicles are involved not only in hair pigmentation, but also in elimination of toxic byproducts of melanin synthesis (Borovanský and Riley, 2011). Melanocytes of the hair are restricted below keratinocytes, to the bulb of the hair, and from here the minimally digested melanosomes are transported to keratinocytes, which ultimately form the outer pigmented shaft of the hair. Instead of this, follicular and epidermal melanocytes both originate from neural crest cells; during development, the follicular melanocytes become larger, more dendritic and produce larger melanosomes. In addition melanogenesis in the hair bulb melanocytes is coupled with the growth cycle of the hair (Slominski et. al, 2005).
The melanocytes of the skin are found not only in the basal layer of the epidermis but also in the dermis. In contrast to epidermal melanocytes of the skin, which are surrounded by keratinocytes and are able to transport completely degradated melanosomes to the surrounding cells, the melanocytes of the dermis are surrounded mostly by fibroblasts and are not able to transport melanosomes. Melanosomes of the epidermis have a protective role against the destructive effect of UV radiation by reducing the penetration of UV rays and also by scavanging reactive oxygen species generated in response to UV exposure. The reactive intermediates generated...