Melanocytes are melanin-producing cells found mainly in the lowest part of the top layer of your skin or to put it in science-speak: the stratum basale of your epidermis (see our article on the morphology of the skin). Melanin is a pigment, best known and primarily responsible for giving our skin colour (light, medium, dark skin tones). There are two types of melanocytes: differentiated melanocytes and retinal pigment epithelium (RPE). The former are found in various parts of the body including the skin whereas the latter as present only as a single layer of cells lying behind the retina. While differentiated melanocytes originate from embryonic neural crest cells in the growing embryo, RPE develop in situ from the optic cup of the brain.

While best known as loyal and hard-working residents of our skin, differentiated melanocytes are found in all sorts of places around the body. From parts of the eye and hair follicles, to the inner ear, to the two innermost layers of the meninges in the brain (the leptomeninges), to the valves and septa of the heart, the lungs and even in adipose tissue. Why pigment-producing cells would be found so ubiquitously is a subject up for much debate. We’ll go further into this in the upcoming articles.

Here, we’ll look at the role melanocytes and melanin play in the body particularly their role in various skin lesions, reactions, diseases and conditions.

What Is Melanin and What Does It Do?

Melanin is the pigment that gives hair, skin and eyes their colour. Darker-skinned people have more melanin than those with lighter skin. There are three types of melanin: dark brown eumelanin found in the hair, skin and eyes and guards against UV radiation (lower amounts in the hair follicles is responsible for blonde hair!); neuromelanin in the brain and pale red or yellowish pheomelanin found in the hair and skin (responsible for red hair!).

Pigment production in the skin is regulated by a pituitary gland peptide hormone called melanocyte-stimulating hormone (MSH). Melanocytes produce and store melanin in organelles called melanosomes in response to MSH secretion. The process by which melanin is created is called melanogenesis.

The biochemical pathways of melanogenesis that result in melanin production involves a cascade of protein activations driven by phosphorylation. The pigment is then transferred from the melanocytes by melanosomes. These little pockets  travel to the long, tendrils of the cell, known as the dendrites. The melanin is then secreted into neighboring keratinocytes.  The result is a long-lasting pigmentation. Usually one melanocytes is associated with several epidermal keratinocytes. Together they form what is called epidermal-melanin units (EMU, not to be confused with the aggressive Australian bird!).

What Gives You a Tan?

What gives you a tan you might ask? There are both basal and induced or activated levels of melanogenesis. Generally, lighter skinned people have lower basal levels of melanin production than darker-skinned folks.

When your skin is exposed to UV-B rays, you melanocytes kick into overdrive and produce higher levels of melanin. The darkened skin is better protected from DNA photodamage as darker colours absorb light. This is why tanning can be dangerous as before your body makes the pigment, you’re exposing your cells to UV radiation, and prolonged sun exposure is a factor that increases your likelihood of getting skin cancer. Post-tanning the skin is better protected but UV damage is still possible and just as dangerous so be sure to wear your summer hat at the beach!

Fun fact! Melanocytes are not the only cells in the body capable of producing melanin! Cells of the pigmented epithelium of retina, epithelia of the iris and ciliary body of the eye, some neurons, and adipocytes can too!

What Happens When Melanin Production Goes into Overdrive or is Absent?

Aberrant melanin production, either levels that are too high, too low to absent is responsible for or plays a role in many skin-related features and diseases including: birthmarks, moles, vitiligo, albinism, phenylketonuria, Waardenburg syndrome, age spots, freckles, gray hair and melanomas.

Let’s take a look!

Skin Anomaly The Part Played by Melanin

Birthmarks and moles

There are many types of moles and birthmarks and each are caused by a combination of blood vessel, melanocyte, smooth muscle, fat, fibroblast, and keratinocyte overgrowth.  Interestingly, animals can actually get them too!

Vitiligo

In this pigmentation disorder, melanocytes have been destroyed by the body’s immune system. This can affect any part of the body where these cells are present but is best recognized by the characteristic loss of color in patches of the skin.

Albinism

The body is unable to produce or distribute melanin. The severity depends on the degree of albinism and this condition can occur in those of any ethnic origin.

Phenylketonuria (PKU)

As the body cannot breakdown the amino acid phenylalanine, melanin production is impaired. A lifelong restricted diet for PKU is needed to minimize the destructive health effects.

Waardenburg Syndrome

Melanocytes are functionally impaired and thus melanin production is reduced. This affects the eyes, skin and hair color and interestingly also negatively impacts hearing. A great intro to this subject is given by a youtuber with WS, Stef Sanjati.

Age (Liver or Brown) Spots and Freckles

Liver spots and freckles may form in response to sun exposure and are the result of localized increases in melanin production. (insert link or illustrations?)

Gray and White Hair

Gray hair is caused by a reduction in the amount of melanin produced by melanocytes near the hair follicles. As the production rates gets slower and slower, the hair loses it color going from whatever you normally have to grey and eventually to white. A combination of intrinsic and extrinsic factors play a role including age, hormones and chemical exposure. Recent research has found a gene linked to grey hair so the end may be in sight for those who’d prefer to keep their color!

Melanomas

This form of cancer develops in melanocytes and is caused mainly by UV exposure. It’s important to know the ABCDEs of moles to catch this form of cancer as early as possible.

Next time, we’ll look at melanocytes in more depth and answer some questions like: why is the skin on our palms lighter than the rest of our body? Why do we have melanocytes in the layers of the brain, the lungs and the heart? It’s unlikely these spots are going to be exposed to the sun (and even if they did, I’m guessing sunburn is the least of your worries) so why on earth are they there?!

Take a guess in the comments section below!


Article by Olwen Reina. Contact Olwen at olwen@tempobioscience.com.