melanocyte

Melanocyte and melanin.

Melanocytes (pronounced /mɛˈlænɵsаɪt/ ( listen)) are 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] that produce melanin which is a pigment.

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 tan which is in contrast to the tan that originates from oxidation of already existing melanin.^[6]

There are both basal and activated levels of melanogenesis; lighter-skinned people generally have low basal levels of melanogenesis. Exposure to UV-B radiation causes an increased melanogenesis due to DNA photodamage.^[7]

Since the action spectrum of sunburn and melanogenesis are virtually identical, it is assumed that they are induced by the same mechanism.^[8] The agreement of the action spectrum with the absorption spectrum of DNA points towards the formation of cyclobutane pyrimidine dimers (direct DNA damage). The acronym 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 fair people and dark people 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.

Stimulation

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 by biologist James D. Watson to have effect on appetite and sexual activity.^[9] Vitamin D metabolites, retinoids, melanocyte-stimulating hormone, forskolin, cholera toxin, isobutylmethylxanthine, diacylglycerol analogues, and UV irradiation all trigger melanogenesis and in turn, pigmentation.^[10] The production of melanin is also initiated by ACTH (an adrenocorticotropic hormone).

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", that 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^[11] 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 cold blooded animals)
• Eye color
• Melanocytic tumors of uncertain malignant potential
• Melanoma
• Tanning activator
• Mole (skin marking)

References

Further reading

• Ito S (June 2003). "The IFPCS presidential lecture: a chemist's view of melanogenesis". Pigment Cell Research / Sponsored by the European Society for Pigment Cell Research and the International Pigment Cell Society 16 (3): 230–6. doi:10.1034/j.1600-0749.2003.00037.x. PMID 12753395. 
• Millington GW (May 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

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