Keratinocytes have a significant role in regulating adhesion, proliferation, survival, and morphology of melanocytes. Skin color differences can emerge from the quantity and quality (pheo/eumelanin ratio) of melanin produced as well as size, number, composition, mode of transfer, distribution, and degradation of melanosomes inside keratinocytes, whereas melanocytes numbers typically remain relatively constant. Eumelanin has black color, which works mainly to protect the nucleus from UVR. Pheomelanin has a reddish to brownish color, causing reactive oxygen species production under UVR stimulation. Melanin pigment can be divided into two types-pheomelanin and eumelanin. Skin pigmentation is a specific and complex mechanism that occurs due to accumulation of melanosomes in keratinocytes to protect skin from solar irradiation. Melanocytes are specialized cells derived from unpigmented precursor cells called melanoblasts, originating from embryonic neural crest cells which can migrate towards the skin and other tissues during embryogenesis. Synthesis of melanin starts with formation of melanosomes, melanin-containing organelles, in epidermal melanocytes by a process called melanogenesis.
It is crucially involved in the defense mechanism to protect skin from adverse effects of ultraviolet radiation (UVR) and oxidative stress of environmental pollutants. Production, quantity, quality, and distribution of melanin, a group of natural pigments, determines the color of human skin, eyes, and hair.
Skin color is one of the topmost concerns for human beings as it provides uniformity, identity and represents the overall appearance of a person. This review covers the mechanistic aspects of skin pigmentation caused by inflammation. Understanding of mechanisms of skin pigmentation due to inflammation helps to elucidate the relationship between inflammation and skin pigmentation regulation and can guide development of new therapeutic pathways for treating pigmented dermatosis. It is confirmed by some recent studies that several interleukins (ILs) and other inflammatory mediators modulate the proliferation and differentiation of human epidermal melanocytes and also promote or inhibit expression of melanogenesis-related gene expression directly or indirectly, thereby participating in regulation of skin pigmentation. These acute or chronic inflammatory responses cause inflammatory cytokine production from epidermal keratinocytes as well as dermal fibroblasts and other cells, which in turn stimulate melanocytes, often resulting in skin pigmentation. Various stimuli such as eczema, microbial infection, ultraviolet light exposure, mechanical injury, and aging provoke skin inflammation. Melanocytes reside in the basal layer of the interfollicular epidermis and are compensated by melanocyte stem cells in the follicular bulge area. The production of melanin pigments by melanocytes and their quantity, quality, and distribution play a decisive role in determining human skin, eye, and hair color, and protect the skin from adverse effects of ultraviolet radiation (UVR) and oxidative stress from various environmental pollutants.