Melanocyte cell - function and dysfunctions
Melanocyte is a type of cell prominent in the dermis (middle layer of the SKIN) that produces melanin, the pigment that gives color to the skin as well as protects the skin from ultraviolet light damage. There are two types of melanin: the dark brown pigment eumelanin and the red/yellow pigment pheomelanin. The skin contains the same number of melanocytes no matter what the individual’s natural skin color. The melanocytes in darker skin are more active than those in lighter skin. The eyes and HAIR also contain melanocytes.
Melanogenesis
The exclusive role of melanocytes is to produce melanin (melanogenesis), a somewhat sequential process. To prepare for melanogenesis, the body produces the enzyme tyrosinase. Genetic encoding regulates this process. Tyrosinase initiates conversion of the amino acid tyrosine, which the body synthesizes from dietary proteins such as meats and which the melanocytes store, into dopaquinone. The dopaquinone forms the pigments eumelanin and pheomelanin, which collectively comprise mel-anin.
Exposure to ultraviolet light, notably sunlight, initiates a sequence of hormonal and chemical events that stimulate melanocytes to produce melanin (melanogenesis):
- Sunlight (or other ultraviolet light exposure) damages the cells of the skin. The damage activates the natural repair mechanisms within the cells, which releases chemicals into the bloodstream that travel to the PITUITARY GLAND.
- In response the pituitary gland releases melanocyte-stimulating hormone (MSH), to bind with melanocytes.
- Melanocytes pass packets of melanin molecules to the keratocytes, which carry them to the outer layer of the epidermis as they migrate upward.
The resulting skin color depends on the mix of eumelanin and pheomelanin the melanin contains. The melanin in light skin contains more pheomelanin than eumelanin. In darker skin the balance tips the other way with the melanin in dark skin containing more eumelanin than pheomelanin. In the epidermis, melanin protects the skin from damage by absorbing ultraviolet light. The darker the skin, the less ultraviolet light penetrates the epidermis. In general, it takes about a week of regular sun exposure to generate a tan adequate to begin protecting the skin from further sun damage, though the tan itself signals sun damage.
Dysfunctions of Melanocytes
There are three significant dysfunctions of melanocytes:
- ALBINISM is a deficit or absence of pigmentation (hypopigmentation) caused by a MUTATION in the genetic encoding for tyrosinase. The body may produce little or none of this enzyme, reducing or completely blocking melanogenesis. The dermatologic consequence is extremely light-colored skin that cannot protect itself from ultraviolet light damage.
- VITILIGO is a hypopigmentation disorder of autoimmune origin in which the melanocytes in areas of the skin die, leaving the skin without pigmentation.
- Malignant melanoma is a serious type of SKIN CANCER that arises from melanocytes.
For further discussion of melanocytes within the context of integumentary structure and function, please see the overview section, “The Integumentary System”.
See also AMINO ACIDS; KERATINOCYTE; PHENYLKETONURIA (PKU).