melanocyte

Melanocyte
Melanocyte and melanin.
Latin	melanocytus
MeSH	Melanocytes
Code	TH H2.00.03.0.01016

Melanocytes ⁱ/mɛˈlænɵsaɪt/ are melanin-producing cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea),^[1] the inner ear,^[2] meninges,^[3] bones,^[4] and heart.^[5] Melanin is a pigment that is responsible primarily for the color of skin.

Melanogenesis

This section requires expansion.

Through a process called melanogenesis, these cells produce melanin, which is a pigment found in the skin, eyes, and hair. This melanogenesis leads to a long-lasting pigmentation, which is in contrast to the pigmentation that originates from oxidation of already-existing melanin.

There are both basal and activated levels of melanogenesis; in general, lighter-skinned people have low basal levels of melanogenesis. Exposure to UV-B radiation causes an increased melanogenesis. The purpose of the melanogenesis is to protect the Sucutis, the Hypodermisas, the layer under the skin, from the UVB light, that can give it damage it (DNA photodamage). The color of the melanin is dark and it absorbs all the UVB light and it blocks it from passing the skin layer.^[6]

Since the action spectrum of sunburn and melanogenesis are virtually identical, it is assumed that they are induced by the same mechanism.^[7] The agreement of the action spectrum with the absorption spectrum of DNA points towards the formation of cyclobutane pyrimidine dimers (direct DNA damage). The initialism for cyclobutane pyrimidine dimers is CPDs.

Human differences

There are typically between 1000 and 2000 melanocytes per square millimeter of skin. Melanocytes comprise from 5% to 10% of the cells in the basal layer of epidermis. Although their size can vary, melanocytes are typically 7 micrometres in length.

The difference in skin color between lightly and darkly pigmented individuals is due not to the number (quantity) of melanocytes in their skin but to the melanocytes' level of activity (quantity and relative amounts of eumelanin and pheomelanin). This process is under hormonal control, including the MSH and ACTH peptides that are produced from the precursor proopiomelanocortin.

Albinos lack an enzyme called tyrosinase^{[dubious – discuss]}. Tyrosinase is required for melanocytes to produce melanin from the amino acid tyrosine.

Stimulations

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. Melanocortins have been discussed to have effect on appetite and sexual activity in mice.^[8] Vitamin D metabolites, retinoids, melanocyte-stimulating hormone, forskolin, cholera toxin, isobutylmethylxanthine, diacylglycerol analogues, and UV irradiation all trigger melanogenesis and, in turn, pigmentation. Also producing chronic diseases when UV Rays hit open sores.^[9] The production of melanin is also initiated by ACTH (an adrenocorticotropic hormone).^{[citation needed]}

Once made, melanin is moved along arm-like structures called dendrites in a special container called a melanosome, which is shipped to the keratinocytes. Melanosomes are vesicles or packages of the chemical inside a plasma membrane. The melanin is in organelles called "melanosomes", which 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^[10] and cause the melanocyte to produce melanosomes, which are then transferred by dendrite to the top layer of keratinocytes.

See also

• Chromatophore (the pigment cell type found in poikilotherm animals)
• Eye color
• Follicular melanocyte
• List of human hair color genes
• Melanocytic tumors of uncertain malignant potential
• Melanoma
• Mole (skin marking)
• Nevus depigmentosus
• Tanning activator
• Vitiligo

References

Further reading

• Ito S (2003). "The IFPCS presidential lecture: a chemist's view of melanogenesis". Pigment Cell Research 16 (3): 230–6. doi:10.1034/j.1600-0749.2003.00037.x. PMID 12753395. 
• Millington GW (2006). "Proopiomelanocortin (POMC): the cutaneous roles of its melanocortin products and receptors". Clinical and Experimental Dermatology 31 (3): 407–12. doi:10.1111/j.1365-2230.2006.02128.x. PMID 16681590. 

External links

• Melanocyte at eMedicine Dictionary
• Histology at BU 07903loa - "Eye: fovea, RPE"
• Histology at BU 08103loa - "Integument: pigmented skin"