Melanocytes are the cells tasked with the production of melanin, the pigment that gives skin its color and shields it from the destructive effects of sunlight. They are found in high numbers in the middle skin layer, the epidermis. Melanin is actually divided into two separate pigments: eumelanin which is dark brown and pheomelanin which is yellow or red. Even if human skin varies in color from white to black, all people actually have the same number of melanocytes in their skin. The different color is caused by the activity level of these cells. Melanocytes are also found in the hair and eyes.
Other less obvious locations of melanocytes are the heart, eyes, brain or inner ears, to name only a few of the many places where these cells are present. Regardless of their position, they can usually be found immediately under the skin or surface of the organ. Environmental factors trigger the production of melanin, not only UV radiation but also the presence of some chemicals. The pigment is released from the cells and raises to the nearest surface in order to provide protection. A constant supply is required, since the pigment breaks down in time and must be replaced.
Melanocytes are initially formed in the neural crest. This is a cluster of embryonic cells that are later replaced by the formation of the pre-spinal cord and neural tube. As the embryo grows, melanocytes migrate to the basal layer of the epidermis, which is their final location. They can transfer the pigment to keratinocytes and other types of skin cells, due to their specific branch-like shape. Keratinocytes are the most common type of structural cells found in the skin, hair and nails.
Melanocytes can be impacted by a number of pigmentation problems, some of which are congenital while others develop later. If the activity of melanocytes is reduced and the production of melanin decreases, patches of white or discolored skin emerge in these locations. Some condition have the opposite effect, causing the skin to become darker than normal due to excess levels of melanin. It is also possible for some medications to boost melanin production and make people look darker.
Melanin plays a key role in the protection of skin. It absorbs the dangerous ultraviolet radiation from sunlight and stops it from penetrating deeper into the body. Humans have adapted to environmental factors, so people from regions with plenty of sun have more active melanocytes and thus darker skin. It is the constant duty of these cells to react to the level of sun exposure and make sure that the skin has the required layer of protective pigment.
Besides its role against UV rays, melanin has other less known functions. Inside the brain, this pigment seems to be one of the building blocks for some neurotransmitters. Studies have revealed that people with a low output of brain melanin also have a decreased level of certain neurotransmitters. Degenerative brain diseases sometimes also kill some of the melanocytes in the brain. This causes the amount of brain melanin to drop, which in turn decreases overall brain functions. The highest level of melanin is concentrated in a particular area of the brain, which is named substantia nigra for this reason.
Melanogenesis is the process of melanin production, which is the only function and purpose of melanocytes. It is controlled by genetic factors that trigger the production of an enzyme named tyrosinase, the first step of the process. This enzyme starts converting tyrosine into dopaquinone. Tyrosine is an amino acid stored in melanocytes after it is produced by the body from meat and other protein sources. Dopaquinone is then transformed into the two pigments that are collectively known as eumelanin and pheomelanin.
Melanogenesis is accelerated when the skin is exposed to sunlight or other sources of ultraviolet radiation. This triggers a series of chemical and hormonal changes that stimulate melanocytes.
It all starts when skin cells are damaged as a result of exposure to a source of ultraviolet light. As the skin repair procedure begins, some chemicals are released into the blood and make their way to the pituitary gland. This gland reacts to the signal and produces MSH, a hormone that stimulated melanocytes and is sent to these cells. When they receive the signal, melanocytes transfer packets of melanin to nearby keratocytes. The final step is the upward movement of these cells that transport the pigment to the superior layers of skin.
Melanin is actually a mix of two compounds, eumelanin and pheomelanin. The amount of each of these compounds determines the actual skin color. For example, light skin types have more pheomelanin than eumelanin but the opposite is true in the case of dark skin tones. In both cases, melanin is located in the epidermis and absorbs the UV light that would otherwise damage tissues. Darker skin has a higher content of melanin and allows less ultraviolet light to reach the deeper layers. Tan is the skin's natural response to sun damage. The human skin needs about a week to adapt and produce the proper amount of pigment for the required level of tanning.
Melanocytes can be affected by three major problems. The first one is albinism, which is a genetic issue affecting tyrosinase. It causes hypo pigmentation, which is a low level or complete lack of melanin in the skin. Melanogenesis happens at a very low level or it can be completely prevented by the low levels of this essential enzyme. As a result, the skin is very light and unable to stop the damage caused by UV radiation.
Vitiligo is an autoimmune disease that causes the skin to lose its normal color. The immune system attacks melanocytes in some skin areas by mistake, which stops the production of melanin. Finally, melanocytes can start a very dangerous form of skin cancer known as malignant melanoma.