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In the human skin, melanocytes are present in the epidermis and hair follicles. The basic features of these cells are the ability to melanin production and the origin from neural crest cells. This last element is important because there are other cells able to produce melanin but of different embryonic origin pigmented epithelium of retina, some neurons, adipocytes. Melanocytes of the epidermis and hair are cells sharing some common features but in general they form biologically different populations living in unique niches of the skin.
Melanocytes form a heterogeneous group of cells in the human body. Although all of them have ability to produce melanin and originate from embryonic cells named neural crest cells NCCtheir particular functions in all target places are much wider than the melanin synthesis only [ 1 ].
Melanocytes have been also found in the inner ear, nervous system, heart and probably it is not the end of a list where these cells exist [ 23 ]. It is necessary to stress that not only melanocytes have ability to produce melanin but also other cells e. The life cycle of melanocytes consists of several steps including lineage specification from embryonic neural crest cells melanoblastsmigration and proliferation of melanoblasts, differentiation of melanoblasts into melanocytes, maturation of melanocytes melanin production in special organelles — melanosomes, dendritic morphologytransport of mature melanosomes to keratinocytes and eventual cell death.
Several populations of neural crest cells cranial, dorsal trunk, ventral trunk give melanocytes of the skin. The embryonic origin of epidermal and hair melanocytes is the same but development is different [ 67 ]. The first such place was a hair bulge, but if only…? Experimental data clearly show that the trunk NCC migrating through a ventral pathway could remain in a myelin sheath of the cutaneous nerves and in particular situations give melanoblasts [ 910 ].
The embryonic development of melanocytes give an opportunity to better understand the skin diseases e. The microscopic analysis indicates that mature melanocytes are oval or fusiform, dendritic cells, smaller than keratinocytes. In the cytoplasm there are present special membrane-bound organelles producing melanin, melanosomes [ 11 ]. Melanocytes reside in the basal layer of epidermis where they form the epidermal melanin units as a result of the relationship between one melanocyte and associated keratinocytes [ 12 ].
The ratio of melanocytes to keratinocytes is 1: 10 in the epidermal basal layer Figure 1. This balance is maintained through the human live but the exact mechanism is unknown [ 13 ]. About melanocytes exist per mm 2 of the skin independently of the human race [ 14 ]. Adhesion molecules such as E- and P-cadherins participate in building cell-cell contact structures [ 15 ]. Melanin granules are accumulated above the keratinocyte nucleus and are removed with the shed epidermal cells [ 1 ].
The molecular mechanisms of the melanosomes transfer from melanocyte to keratinocytes is still a subject of investigations. Recently, Ando et al. PG are captured by microvilli of keratinocytes, which incorporate them in a protease-activated receptor-2 PAR-2 -dependent way. Scheme of the epidermis structure. Melanocyte reside between the basal layer Mobile Alabama girl with dark skin and through dendritic processes communicates with about keratinocytes in the epidermal melanin unit.
Melanocyte synthesizes melanins in melanosomes transported into keratinocytes to protect them from UV radiation. The size of the epidermal melanin units is similar independently of the human race, but varies Mobile Alabama girl with dark skin body areas. Racial distinctions are manifested in an arrangement of the melanocytic dendrites and in the melanogenesis intensity [ 18 ]. The precise mechanisms that control the organization and of melanocytes in the epidermis are unknown.
Melanocytes, keratinocytes and dermal fibroblasts communicate with each other by secreted factors and by cell-cell contacts [ 19 ]. Keratinocytes control melanocyte growth and activity through a system of paracrine growth factors and cell adhesion molecules [ 1320 ]. The cross-talking of different aling pathways between keratinocytes and melanocytes is a part of an epidermal complex network involved in the maintenance of skin homeostasis.
Graphical presentation of the basic elements in keratinocytes-melanocytes cooperation. The melanocyte proliferation, differentiation, melanogenesis are under control of factors secreted by surrounding keratinocytes. The paracrine factors secreted by keratinocytes after UV radiation that influence on melanocyte biology.
These cytokines influence not only the growth and pigmentation of melanocytes, but also their shape, dendricity, mobility and adhesive properties [ 2327 ]. In the epidermal melanin unit melanocyte is a very active element that secrets a of al molecules targeting not only keratinocytes but also skin immunological system cells [ 2829 ]. Secreted substances act also as autocrine factors, e. Thus, melanocytes and cooperating keratinocytes form well-organized units in the epidermis. The stable element in each unit is the melanocyte that lives long, keratinocytes die and are shedding. It is an open question how long melanocyte lives in the human skin.
Melanocytes are located in the proximal bulb of each hair follicle and also near hair, e. The bodies of bulbar melanocytes are located at the apex on the dermal papilla. Melanocyte dendrites enter between the cortical and medullar keratinocytes [ 32 ]. The ratio of melanocytes to keratinocytes is 1: 5, it is more dense than in the epidermis Figure 3 [ 33 Mobile Alabama girl with dark skin.
Follicular pigmentation is a result of structural and functional interactions between follicular melanocytes, matrix keratinocytes and dermal papilla fibroblasts. This tripartite system is described as the hair melanin unit or follicular melanin unit. The process of hair pigmentation includes the melanogenic activity of follicular melanocytes, the transfer of melanin granules into keratinocytes and the formation of pigmented hair shafts [ 32 — 34 ]. It is considered that a transport of melanin granules to keratinocytes in the growing hair shaft is similar to the epidermal phagocytosis of melanosomes mediated by receptor PAR2 on keratinocytes.
But differences concern degradation of melanosomes and their quality. Hair melanocytes, in contrast to epidermal ones, include mainly bigger mature Stage IV melanosomes melanogenesis and stages of melanosomes are described further in this work and more expanded Golgi apparatus and rough endoplasmatic reticulum RER. These pigment cells are larger and more dendritic than epidermal ones [ 3133 ].
Moreover, in epidermis almost whole transported melanin is degraded in the differentiating keratinocytes, but in hair cortical keratinocytes pigment granules are digested only slightly [ 33 ]. Diversity of hair color arises mostly from the quantity and ratio of the brown-black eumelanin and the yellow-red pheomelanin [ 35 ]. Melanocyte localization in the hair between cells covering the hair papilla in the hair bulb. Stem cells for melanocytes are located in the region named the hair bulge.
Melanin synthesis in the hair occurs under control of products secreted by neighboring cells as keratinocytes, fibroblasts and endothelial cells, which act through paracrine or autocrine mechanisms and may be modified by hormonal als.
The biochemical pathway of pigment formation and melanosomes biogenesis run likewise in the epidermis, but it is stressed that hair follicle melanocytes are more sensitive to aging influences than epidermal melanocytes, what in hair greying [ 33 ]. It seems that fibroblast of dermal papilla derived factors: insulin growth factor IGF-1keratinocyte growth factor KGFnoggin, SCF have special ificance for control the hair matrix keratinocyte and melanocyte proliferation and activity during the hair growth [ 3236 ].
Epidermal melanocytes are long-living cells, while hair melanocytes die at the end of the hair cycle which lasts years [ 31 ]. The melanogenesis process takes place only during the anagen stage growing phase of the hair growth cycle; pigment formation is turned off in the catagen stage regressing phase and absent in the telogen stage resting phase [ 31 ]. Additionally, it is marked that during catagen completely differentiated bulbar melanocytes die through apoptosis, but new melanocytes develop from melanoblasts residing in the hair bulge [ 832 ].
Summarizing, survival, proliferation and differentiation of melanocytes are regulated by microenvironment of the hair follicles.
Melanogenesis is a biochemical pathway responsible for melanin synthesis [ 37 ]. It takes place in melanocytes, in separate cytoplasmic organelles called melanosomes [ 11 ]. Two major types of melanin are produced — pheomelanin and eumelanin.
They differ in color and the way of synthesis. Melanin has numerous properties which are beneficial to the body: UV light absorption and scattering, free radical scavenging, coupled oxidation-reduction reactions and ion storage [ 233839 ]. The availability of substrates and the function of melanogenesis enzymes decide about the types of melanins produced Figure 4. In the presence of cysteine DOPAquinone react with it, yielding 3- or 5-cysteinylDOPAs, which then oxidize and polymerize, giving rise to yellow-red soluble melanin — pheomelanin [ 3741 ].
In the absence of thiols cysteine, glutathione or thioredoxin brown-black eumelanin is produced. Human skin contains a mixture of all melanin types, and the ratio of those in part determines visible pigmentation [ 19 ]. Diversity of skin pigmentation among different ethnic groups is preserved and depends on eumelanin content. The ratio of eumelanin to total melanin decide about skin color [ 30 ].
Pheomelanin does not correlate with skin pigmentation, a similar amount of this pigment is observed in the dark and light skin. While in hair, the ratio of eumelanin to pheomelanin decides about the color [ 35 ].
Eumelanin comparing to pheomelanin has better photoprotecting properties — higher resistance to degradation and ability to reactive oxygen species ROS neutralization [ 44 ]. Eumelanins are considered to be more effective in terms of photoprotection than the reddish pheomelanin. As a consequence, the risk of skin cancer in lighter skin is fold higher than in the darker one [ 41 ]. Products of genes regulating melanogenesis act at subcellular, cellular, tissue and environmental levels [ 21 ].
During melanogenesis, as intermediate products, cytotoxic molecules are synthesized quinones, hydrogen peroxide.Mobile Alabama girl with dark skin
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Skin melanocytes: biology and development