Melanocytes definition - What are melanocytes?

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What are melanocytes?

Melanocytes are a type of epidermal cells which produce melanin pigment.

Melanocytes are present in stratum basale, the deepest layer of the epidermal skin. Up to 10% of the cells present in the basal cell layer are pigment producing cells. These pigment cells are also present in retinal pigment epithelium, stria vascularis of cochlear duct in inner ear, heart, meninges and substantia nigra pars compacta of the midbrain. Three types of melanins are produced and eumelanin and pheomelanin are present in the skin while neuromelanin is found in the brain.

The number of melanocytes present per square millimeter of skin is between 1000 and 2000. It is not the number of pigment cells present in the skin that contributes to the lighter or darker color of skin. The level of melanin producing activity as well as the types of pigments and their relative amounts contribute to an individual's skin color. The melanin produced by the melanocytes protects the skin from the harmful effects of ultraviolet radiation. The absence of melanin gives rise to skin conditions like albinism and leucism. The total or partial loss of melanin production may cause vitiligo.

Definition of melanocytes

Definition by medterms.com: "A cell in the skin that produces and contains the pigment melanin."

Definition by merriam-webster.com: "an epidermal cell that produces melanin."

Definition by about.com: "Melanocytes are cells located in the epidermis that are responsible for producing melanin, a brown pigment that helps screen against the harmful effects of UV light."

Melanoblasts

The precursor cells of melanocytes arise from the neural crest formed during the gastrulation of the embryo. These neural crest cells migrate to various sites and differentiate to form precursor cells of neurons, endocrine components, bone, cartilage, connective tissue and melanocytes. The precursor pigment cells are known as melanoblasts. The migrating melanoblasts transform into melanocytes and establish at the epidermal-dermal junction to commence melanogenesis. When there is dysregulation in the melanoblast development and migration, skin conditions like piebaldism may occur.

Melanocyte stem cells

Melanoblasts also enter the newly forming hair follicles to form hair matrix melanocytes and the stem cells. The stem cells are found as a small pool of immature cells in the lower permanent portion including the bulge region of the hair follicle. Apart from maintaining a supply of pigment cells for hair pigmentation, these stem cells are also a source for melanocyte replacement in the epidermis.

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Greying of hairs in humans was thought to be due to free radicals causing oxidative stress and cytotoxic damage of the mature melanocytes in the matrix cells. However recent studies indicate that hair graying is due to loss of pigment stem cells. Melanoma occurs when melanocyte development and proliferation pathways are undermined and destabilized.

Melanocyte structure

These pigment cells are regularly dispersed among the basal keratinocytes in the basal layer of epidermis at an ratio of about 1:10. They appear larger than the keratinocytes. They have elongated dendrites which are in contact with the neighboring basal and suprabasal keratinocytes. The nucleus is ovoid in shape. These cells contain clear premelanosomes as well as many melanosomes at various maturation stages.

Melanocyte function

Many functions of these pigment cells and their product can be listed out. Their main function in many species of animals is camouflage from prey and predators. In humans, protection against UV radiation has evolved as one of the primary functions. Skin pigmentation being the most apparent display of human polymorphic traits, defines ethnicity and race.

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Heat regulation, redox signaling and antioxidant activity are some of the other functions of melanin. The presence of the melanocytes and melanin is necessary for the proper functioning of inner ear, eyes and midbrain.

Melanin biosynthesis

The biosynthesis of melanin by the melanocytes is known as melanogenesis.

diagram of melanocyte
showing dendrites and melanosomes

The melanogenesis normally occurs in a basal level. Under certain conditions, the synthesis may occur in an active manner. In the lighter-skinned people, the basal level of melanogenesis is quite low when compared people with darker skin tone.

The mechanism of active melanogenesis is triggered by the keratinocytes on exposure to ultraviolet radiation by synthesizing and secreting endothelin-1 (ET-1) an amino acid peptide. The production of the precursor of melanocyte-stimulating hormone (MSH), proopiomelanocortin (POMC) is also stimulated. The rate of mitosis in the basal keratinocytes is also increased. Through a number of biosynthetic cascading pathways, the rate of proliferation of melanocytes and keratinocytes, the melanogenesis activity, and synthesis and transfer of melanosomes is increased.

Melanocytes and keratinocytes

As described earlier, melanocytes with the help of their dendrites are in contact with nearly forty basal as well as suprabasal keratinocytes. The pigment cell with its dendrite-connected keratinocytes is termed "the epidermal melanin unit.” These melanin producing cells transfer the melanosomes to the keratinocytes through their multiple dendrites. Once inside the keratinocyte, multiple melanosomes are packed into single membrane pigment globules and transported by motor protein dynein to perinuclear region. Then there is membrane degradation of the pigment cell globules to form a UV-radiation protective cap above the keratinocyte nucleus.

Melanocytes and hyperpigmentation

Skin pigmentation is an inherited trait, being regulated by genetic factors on the epidermal cells. However the endocrine secretions, certain medications, chemicals and environmental factors can modulate the distribution of the melanin as well as its amount and type. Endocrine factors induce melanocyte activity and hyperpigmentation occurs temporarily in pregnancy and permanently as age spots. The use of diuretics, levodopa, hydantoins, sulfonamides, tetracyclines, chloroquine and oral contraceptives is associated with hyperpigmentation. Postinflammatory hyperpigmentation of the skin occurs after the resolution of the inflammatory dermatitis, especially in patients with darker skin tone due to enhanced melanocyte activity.

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Reference: 1.Osawa M. Melanocyte stem cells. 2009 Jun 30. In: StemBook [Internet]. Cambridge (MA): Harvard Stem Cell Institute; 2008-. Available from: http://www.ncbi.nlm.nih.gov/books/NBK27077/ 2.Audrey Uong, Leonard I. Zon. Melanocytes in Development and Cancer J Cell Physiol. Jan 2010; 222(1): 38–41. 3.Kvam E, Dahle J. Pigmented melanocytes are protected against ultraviolet-A-induced membrane damage. J Invest Dermatol. 2003 Sep;121(3):564-9. 4.Iozumi K, Hoganson GE, Pennella R, Everett MA, Fuller BB. Role of tyrosinase as the determinant of pigmentation in cultured human melanocytes. J Invest Dermatol. 1993 Jun;100(6):806-11.

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