Melanocyte

In addition to their role as UV radical scavengers, melanocytes are also part of the immune system, and are considered to be immune cells.[18] Although the full role of melanocytes in immune response is not fully understood, melanocytes share many characteristics with dendritic cells: branched morphology; phagocytic capabilities; presentation of antigens to T-cells; and production and release of cytokines.[18][19][20] Although melanocytes are dendritic in form and share many characteristics with dendritic cells, they are derived from two different cell lineages. Dendritic cells, such as Langerhans cells, are derived from hematopoietic stem cells in the bone marrow. Melanocytes on the other hand originate from neural crest cells. As such, although morphologically and functionally similar, melanocytes and dendritic cells are not the same.

Melanocytes are capable of expressing MHC Class II,[19] a type of MHC expressed only by certain antigen presenting cells of the immune system, when stimulated by interactions with antigen or cytokines. All cells in any given vertebrate express MHC, but most cells only express MHC class I. The other class of MHC, Class II, is found only on "professional" antigen presenting cells such as dendritic cells, macrophages, B cells, and melanocytes. Importantly, melanocytes stimulated by cytokines express surface proteins such as CD40 and ICAM1 in addition to MHC class II, allowing for co-stimulation of T cells.[18]

In addition to presenting antigen, one of the roles of melanocytes in the immune response is cytokine production.[21] Melanocytes express many proinflammatory cytokines including IL-1, IL-3, IL-6, IL-8, TNF-α, and TGF-β.[18][19] Like other immune cells, melanocytes secrete these cytokines in response to activation of Pattern Recognition Receptors (PRRs) such as Toll Like Receptor 4 (TLR4) which recognize MAMPs. MAMPs, also known as PAMPs, are microbial associated molecular patterns, small molecular elements such as proteins, carbohydrates, and lipids present on or in a given pathogen. In addition, cytokine production by melanocytes can be triggered by cytokines secreted by other nearby immune cells.[18]

Melanocytes are ideally positioned in the epidermis to be sentinels against harmful pathogens. Melanocytes reside in the stratum basale,[21] the lowest layer of the epidermis, but they use their dendrites to interact with cells in other layers,[22] and to capture pathogens that enter the epidermis.[19] Melanocytes likely work in concert with both keratinocytes and Langerhans cells,[18][19] both of which are also actively phagocytic,[21] to contribute to the immune response.

Numerous stimuli are able to alter melanogenesis, or the production of melanin by cultured melanocytes, although the method by which it works is not fully understood. Certain melanocortins have been shown in laboratory testing to have effect on appetite and sexual activity in mice.[23] Eicosanoids, retinoids, oestrogens, melanocyte-stimulating hormone, endothelins, psoralens, hydantoin, forskolin, cholera toxin, isobutylmethylxanthine, diacylglycerol analogues, and UV irradiation all trigger melanogenesis and, in turn, pigmentation.[24] Increased melanin production is seen in conditions where adrenocorticotropic hormone (ACTH) is elevated, such as Addison's and Cushing's disease. This is mainly a consequence of alpha-MSH being secreted along with the hormone associated with reproductive tendencies in primates. Alpha-MSH is a cleavage product of ACTH that has an equal affinity for the MC1 receptor on melanocytes as ACTH.[25]

Melanosomes are vesicles that package the chemical inside a plasma membrane. The melanosomes are organized as a cap protecting the nucleus of the keratinocyte. When ultraviolet rays penetrate the skin and damage DNA, thymidine dinucleotide (pTpT) fragments from damaged DNA will trigger melanogenesis[26] and cause the melanocyte to produce melanosomes, which are then transferred by dendrites to the top layer of keratinocytes.

The precursor of the melanocyte is the melanoblast. In adults, stem cells are contained in the bulge area of the outer root sheath of hair follicles. When a hair is lost and the hair follicle regenerates, the stem cells are activated. These stem cells develop into both keratinocyte precursors and melanoblasts - and these melanoblasts supply both hair and skin (moving into the basal layer of the epidermis). There is additionally evidence that melanocyte stem cells are present in cutaneous nerves, with nerve signals causing these cells to differentiate into melanocytes for the skin.[27]