الفهرس 
MelanocyteOnly 14 pages are availabe for public view
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Abstract
Melanocytes are cell of neural crest origin which is distributed in various organs, including the skin, eye, cardiac valves, inner ear and leptomeninges of the entire brain. Melanocytes are key components of the skin’s pigmentary system through their ability to produce melanin. They are found in the hair follicle and the basal layer of the interfollicular epidermis (at the dermal-epidermal interface) and occasionally in the dermis.
All melanocytes, whether resident in the basal epidermis or in the matrix of the hair, have migrated there during embryonic life from a region known as the neural crest. Each epidermal melanocyte is associated with a group of neighbouring keratinocytes (keratin-synthesizing epidermal cells) into which its dendrites transfer pigment. This structure is known as an epidermal melanocyte unit. The melanin produced by melanocytes is of two kinds: dark brown eumelanin and pale red or yellowish phaeomelanin. Both are formed within the melanocytes by the initial oxidation of the amino acid tyrosine with the aid of the enzyme tyrosinase.
Melanocytes are very difficult to grow in culture. Indeed, such cultures not only need nutrients but also require several growth factors for their proliferation and their survival. Such fundamental considerations shed light on the fact that in the skin, melanocytes are surrounded by many kinds of cells that constantly produce cytokines, hormones and growth factors that not only modulate the melanin production but also control the proliferation and the survival of melanocytes.
Melanocytes form a network of dendrite cells in the basal layer of the epidermis. They are also found in the external hair root sheaths and in the bulbs of hair follicles. The secretory melanocytes behave as unicellular glands producing melanosomes that are transferred to surrounding epidermal keratinocytes. Some do not transfer their melanosomes but redistribute them from the perinuclear zone into the dendrites and then back again.These non-secretory melanocytes are melanophores.
Melanosomes are specialized cytoplasmic membrane-coated organelles within melanocytes in which melanin is formed. They are formed by the interaction between two components: Premelanosomes: they are derived from the blebs of smooth endoplasmic reticulum and contain structural proteins, and tyrosinase enzyme.
The efficiency of melanosome transfer to keratinocytes is markedly affected by the dendricity of melanocytes and their cell–cell communications with those cells. melanocytes are stimulated to proliferate and to become more dendritic by factors secreted by keratinocytes.
Melanocytes may act as stress sensors in the epidermis and provide communicatory links with several different systems.Their close anatomic associations with nerve endings and their ability to produce neuropeptides and neurotransmitters suggest a role as a neuroendocrine cell , thus, as a key component of a communication pathway between the skin and the central nervous system.
Melanocyte passes through three different stages during embryogenesis, melanocyte migration, melanocyte proliferation, melanocyte differentiation and functional development. Each stage is under control of several melanocyte mitogens.
Cell surface and nuclear receptors are important regulators of critical pathways in normal and pathologic melanocytes. Melanogenesis are regulated by number of hormone such as MSH and ACTH where are derived from POMC. C-Kit is a proto-oncogene with intrinsic tyrosine kinase activity. The gene product of c-Kit plays a pivotal role in the normal growth and differentiation of embryonic melanoblasts.
The effects of UV on the skin include direct damage to DNA, apoptosis, growth arrest, and stimulation of melanogenesis. Long-term effects of UV include photoaging and photocarcinogenesis. Melanin, particularly eumelanin, represents the major photoprotective mechanism of the skin. Melanin limits the extent of UV penetration through the epidermal layers and scavenges reactive oxygen radicals that may lead to oxidative DNA damage. The extent of UV-induced DNA damage and the incidence of skin cancer are inversely correlated with the total melanin content of the skin.
Pigment disorders include hypopigmention may have genetic bases such as oculocutaneous albinism (OCA), piebaldism and Waardenburg syndrome (WS). Other disorders of hypopigmention also include Griscelli syndrome. Vitiligo vulgaris is a common depigmentation disorder resulting from the destruction of functional melanocytes (MCs) in the affected skin. The three prevailing pathomechanisms of vitiligo are the immune hypothesis, the neural hypothesis and the autocytotoxic hypothesis.
Disorders of hyperpigmentation include generalized hyperpigmentation as hemochromatosis and localised hyperpigmentation as Postinflammatory pigmentation Hyperpigmented pityriasis versicolor , melasma, Café-au-lait spot and Becker nevus. Malignant melanoma is the eighth most common cancer in the United States and causes 1 to 2 percent of all cancer deaths. Melanoma is a proliferation of transformed melanocytes or pigment producing cells.
In conculsion, the melanocytes are denderitic cells and neural crest origin. Not simply melanin producing cells but may have a number of functions as secretory functions and may act as stress sensors in the epidermis and other functions.
Melanocytes are the site of melanin synthesis, and
keratinocytes are the recipients of melanosomes, melanin-containing organelles. The wide variation in constitutive pigmentation among humans is caused by differences in the rate of synthesis of the two forms of melanin, eumelanin and pheomelanin, and the rate of transfer of melanosomes to keratinocytes.
Cutaneous pigmentation is regulated by a wide of factors, some of which are endocrine, many of those factors regulate constitutive pigmentation and also participate in the UVR induced hyperpigmentation.