Melanocytes are cells located in the bottom layer (the stratum basale) of the skin's epidermis and in the middle layer of the eye (the uvea).

Melanogenesis

Through a process called melanogenesis, these cells produce melanin, which is a pigment found in the skin, eyes, and hair.

There are both basal and activated levels of melanogenesis; lighter-skinned people generally have low basal levels of melanogenesis, and exposure to UV radiation generally causes increased melanogenesis.

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 micrometers 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.

Albinos lack an enzyme called tyrosinase. Tyrosinase is required for melanocytes to produce melanin from the amino acid tyrosine.

Embryology

Embryologically, melanocytes are derived from the neural crest, which is a completely different source than that of the surrounding skin cells (keratinocytes).

All melanocytes have the capacity to migrate widely in the embryo. Therefore, a cancer of a melanocyte (which is called a melanoma) will spread (metastasize) very easily. For this reason, melanomas are often fatal. When melanomas are surgically removed, much of the surrounding tissue must be taken as well.

Stimulation

Main article: Melanocortin

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. Vitamin D metabolites, retinoids, melanocyte-stimulating hormone (ie: Melanotan), forskolin, cholera toxin, isobutylmethylxanthine, diacylglycerol analogues, and UV irradiation all trigger melanogenesis and in turn, pigmentation.^[1] 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 ^[2] 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
• Melanoma

References