Melanocyte Structure Function and Role in Pigmentation

What are melanocytes meaning? Melanocytes constitute melanin-producing neural crest-derived cells that can be found in the epidermis' bottom layer (the stratum basale), the uvea's middle layer, the inner ear, meninges, vaginal epithelium, bones, and the core. Melanin is a black pigment that is responsible for the colour of your skin. Melanin is synthesised and stored in special organelles known as melanosomes, which can then be transferred to neighbouring keratinocytes to cause pigmentation. As a result, darker skin tones contain more melanosomes than lighter skin tones. Melanin is a pigment that protects the skin from UV rays. Melanocyte cells play a part in the immune system as well.

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What are Melanocytes Function?

Major melanocytes function includes the manufacture of melanin, a pigment contained in the skin, eyes, nasal cavity, hair, and inner ear, via a process known as melanogenesis. In comparison to pigmentation that results from the oxidation of already-existing melanin, this melanogenesis produces long-lasting pigmentation.

Melanocyte Role: Melanogenesis has both basal and active levels; lighter-skinned individuals have lower basal concentrations of melanogenesis. Increased melanogenesis is caused by UV-B radiation exposure. The goal of melanogenesis is to defend the hypodermis, or layer beneath the skin, from UV damage. Melanin is black in colour, which allows it to absorb the bulk of UV light and prevent it from getting through the epidermis.

Sunburn and melanogenesis are thought to be caused by the same mechanism because their action spectrums are nearly similar. The formation of cyclobutane pyrimidine dimers (CPDs) - direct DNA damage - is suggested by the agreement of the action spectrum along with the absorption spectrum of DNA.

In the basal layer of the epidermis, melanocytes make up between 1000 and 2000 cells per square millimetre of skin, or around 5% to 10% of all cells. Melanocytes are usually 7μm long, but their size varies.

The disparity in skin colour between lighter and darkly coloured people is due to the degree of activity of their melanocytes, not the number (quantity) of melanocytes in their skin (quantity and relative amounts of eumelanin and pheomelanin). Such a mechanism, along with the MSH and ACTH peptides formed from the precursor proopiomelanocortin, is regulated by hormones.

Pigmentation Levels: Vitiligo is a skin condition in which people lack melanin in some parts of their skin. Phototherapy, or exposure to UV rays, is used to treat vitiligo. It may be UVB or UVA. When Melanocytes are deep in the skin and UVB cannot reach them, UVA is required. Vitiligo is commonly treated with UVA and Psoralen. Whenever the spots are on the hands and legs, it is much more powerful than narrowband UVB 313 nanometer.

Albinism: Oculocutaneous albinism is characterised by a reduced level of melanin development. Albinism is often linked to the TYR gene, which codes for the tyrosinase enzyme, though not always. Melanocytes need tyrosinase to generate melanin from the amino acid tyrosine. Albinism can also be induced by certain other genes, such as OCA2, SLC45A2, TYRP1, and HPS1, to name a few. There are currently 17 forms of oculocutaneous albinism recognised. Each gene is linked to a specific protein involved in pigment production.

Due to irregular microtubule function, people with Chédiak–Higashi syndrome have an accumulation of melanin granules.

Role in the Immune System

Melanocytes constitute immune cells and, in contrast to their function as UV radical scavengers, are indeed part of the immune system. Even though the exact function of melanocytes in the immune response is unknown, they share common similarities with dendritic cells, including phagocytic capabilities, branched morphology, antigen presentation to T-cells, and cytokine development and release. Despite the fact that melanocytes have a dendritic appearance and share certain features with dendritic cells, they are descended from two distinct cell lineages. Dendritic cells, including Langerhans cells, are made up of bone marrow hematopoietic stem cells. Melanocytes, on the other hand, are stem cells from the neural crest.

When stimulated through interactions with antigen or cytokines, melanocytes can express MHC Class II, a form of MHC found only in some antigen-presenting immune cells. MHC is expressed by all cells in every vertebrate, however, a majority of the cells mainly express MHC class I. Class II MHC can only be found on "trained" antigen-presenting cells including dendritic cells, B cells, macrophages, and melanocytes. Notably, cytokines induce melanocytes to produce surface proteins like CD40 and ICAM1 in addition to MHC class II, enabling T cells to be co-stimulated.

Melanogenesis

Tyrosine is a non-essential amino acid that helps melanin form. The enzyme tyrosinase converts tyrosine to Dihydroxyinephenylalanine (DOPA). DOPA is then polymerized to form melanin.

Stimulation

A variety of stimuli can affect melanogenesis, or the development of melanin by cultured melanocytes, though the mechanism isn't fully understood. In laboratory tests, some melanocortins were found to affect appetite and sexual behaviour in mice. Melanogenesis and pigmentation are triggered by eicosanoids, diacylglycerol analogues, melanocyte-stimulating hormone, retinoids, endothelins, oestrogens, psoralens, isobutylmethylxanthine, forskolin, hydantoin, cholera toxin, and UV irradiation. Enhanced melanin synthesis can be seen in Addison's and Cushing's disease, as well as other conditions where adrenocorticotropic hormone (ACTH) levels are high.

This is mostly due to the fact that alpha-MSH is secreted alongside the hormone correlated with primate reproductive tendencies. Alpha-MSH is an ACTH cleavage product that binds to the MC1 receptor on melanocytes with the same affinity as ACTH.

Melanosomes are vesicles inside a plasma membrane that bundle chemicals. The melanosomes form a protective cap around the keratinocyte's nucleus. As ultraviolet rays disrupt DNA on the skin, thymidine dinucleotide (pTpT) fragments from the damaged DNA cause melanogenesis, causing the melanocyte to release melanosomes, which were then passed to the top layer of keratinocytes by dendrites.

Stem Cells

The melanoblast is the precursor to the melanocyte. Adult stem cells are found in the bulge region of the hair follicle's outer root sheath. The stem cells are stimulated when a hair is lost as well as the hair follicle regenerates. Such stem cells differentiate into keratinocyte precursors and melanoblasts, which include both hair and skin. Furthermore, there is proof that melanocyte stem cells exist in cutaneous nerves, and that nerve signals cause such cells to differentiate into skin melanocytes.

Clinical Significance

• Melanoma - Melanocytic tumors

• Melanocytic tumors of uncertain malignant potential

• Vitiligo

• Albinism

• Melasma (Chloasma)

• Addison disease

• Nevus depigmentosus