In medicine, melanocytes are the pigment-producing cells in the basal cell layer of the skin. They synthesize melanins, which give skin and hair their color. The best-known disease related to melanocytes is black skin cancer.
What are melanocytes?
Melanocytes migrate out of the neural crest during the embryonic developmental phase and thus into the skin as derivatives of the neuroectoderm. This migration occurs during the third month of fetal life. In the basal cell layer, the cells are located at the basement membrane and are connected to the membrane by hemidesmosomes. Each melanocyte has about six keratinocytes that are loosely connected to each other. All melanocytes contain multiple mitochondria and are equipped with a Golgi apparatus and a rough endoplasmic reticulum. The cells lie on the skin proper, as well as on the oral mucosa, choroid, and iris. In addition, melanocytes lie in the bulb and root sheath of the hair follicle. The density of these cells is about 1,300 per square millimeter of tissue.
Anatomy and structure
Melanocytes migrate out of the neural crest during the embryonic developmental phase and thus into the skin as derivatives of the neuroectoderm. This migration occurs during the third month of fetal life. In the basal cell layer, the cells are located at the basement membrane and are connected to the membrane by hemidesmosomes. Each melanocyte has about six keratinocytes that are loosely connected to each other. All melanocytes contain multiple mitochondria and are equipped with a Golgi apparatus and a rough endoplasmic reticulum. The cells lie on the skin proper, as well as on the oral mucosa, choroid, and iris. In addition, melanocytes lie in the bulb and root sheath of the hair follicle. The density of these cells is about 1,300 per square millimeter of tissue.
Function and tasks
The function of melanocytes is to produce melanins. This process is also known as melanogenesis. The first step in this process is the synthesis of tyrosinase. This is an enzyme that contains copper. The synthesis of the enzyme takes place in the rough endoplasmic reticulum of melanocytes. The Golgi apparatus is where the synthesized enzyme is collected. From the apparatus, the synthesized enzyme is released in the form of round vesicles. The enzyme is inactive until now. It is only activated when it comes into contact with UV light. The vesicles continue to ripen, forming crystalline inclusions. These inclusions turn the vesicles into premelanosomes. The amino acid tyrosine migrates into the premelanosomes, which converts the interior into a precursor of melanin within the framework of a tyronsinase. With the help of the protein Trp-1, the conversion is completed and the premelanosome becomes a mature melanosome. These cells immigrate into the cytoplasmic extensions of the melanocytes and from here are delivered to the five to eight surrounding keratinocytes. The keratinocytes take up the mature melanosome and store it in their cytoplasm. UV radiation plays a major role in this process. The fact that human skin tans under the sun is due to the increased activity of melanocytes caused by UV radiation. Like UV radiation, the hormone melanotropin stimulates melanocytes, thus tanning the skin. Melanocytes thus have a direct link to solar radiation. Pigments take on a protective effect in this context. Darker skin colors, for example, reduce the risk of skin cancer. Light-skinned people are basically more sensitive to UV radiation and develop black skin cancer more easily.
Diseases
Hypopigmentation is below-average coloration of the skin and is usually due to either a low number of melanocytes or decreased melanin synthesis. In vitiligo, for example, patchy hypopigmentation of the skin is present. In this case, there is simply a lack of melanocytes on the pigmentless areas of skin. A better known phenomenon in connection with hypopigmentation is albinism. This is a congenital disorder in the biosynthesis of melanins, which is associated with an unusually light skin and hair color. Hyperpigmentation of the skin can also occur in the context of various diseases. In Addison’s disease, for example, too much melanotropin is produced.This overproduction of the stimulating hormone leads to increased activity of melanocytes and thus to dark skin coloration. Even more familiar hyperpigmentation occurs in the context of moles. For example, nevus cell nevi are clearly demarcated patches of nevus cells. The nevus cells are similar to melanocytes and can form pigments just like them. However, because of the lack of dendrites, they cannot release the pigment produced to the surrounding skin cells. Dysplastic moles are associated with some risk of degeneration and may develop into malignant melanoma. Melanomas can occur on the conjunctiva, choroid, skin, mucous membranes, internal organs, or central nervous system. This cancer is equivalent to black skin cancer and is an extremely malignant tumor of melanocytes. Melanomas spread metastases through the lymphatic system or bloodstream at an early stage. Dysplastic moles are therefore removed as early as possible to prevent degeneration. Regular moles, on the other hand, are not considered a threat.
