(in case you’re wondering, the featured pic above is eye candy for my female viewers. Enjoy! LOL)
That fascinating molecule – no, group of molecules. So much more to melanin than I realised…
Melanin is the collective name for a group of pigments the body produces. Not just the human body, but almost all other animals* and even plants!
* Arachnids being among the very few exceptions. Hope you’re not arachnophobic!
In humans it’s abundant in the skin (where its main role is protecting against solar UV damage. This has been shown to have no effect on vitamin D production), hair, eyes & brain, and exists in four different forms:
BLACK EUMELANIN: Produced by melanocytes* in the stratum basale (bottom layer of the skin’s epidermis),
and the middle layer of the eyeballs (uvea/ uveal tract), only a small part of which we see as the iris.
High quantities of it leads to extremely dark colouration. The name black eumelanin is a slight misnomer because if you look closely enough you realise no-one has literally black hair, skin or eyes. They’re just very very dark brown. If in low enough levels in the hair (but more than other pigments), it leads to silver hair.
* Weirdly, melanocytes are also present in the bones, heart, inner ear and meninges (the membranes that cover the brain and spinal cord)! They have a common origin with nerve cells – the embryonic dorsal ectoderm – and as such look and behave similar to them. They have lots of branch-like outgrowths called dendrites that connect to other tissues, exactly as nerves have dendrites that connect to each other & to other tissues. They pass on melanosomes, melanin-filled vesicles, to other cells (eg. keratinocytes in the skin), exactly as nerve cells carry vesicles full of acetylcholine to each other.
BROWN EUMELANIN: Produced in the skin, hair and eyes along with the black form. This one is more obviously brown, though if in low concentrations in the hair (combined with low amounts of other pigments) it leads to blond hair! This may explain why a lot of people who were blond in youth turn brunette in later life; their brown eumelanin production has increased.
PHAEOMELANIN: also spelt pheomelanin, but I’m English so I use the British spelling. This one gives a pink-red-orange colour, which gives rise to red hair if it’s the predominant pigment there. In the skin it’s most concentrated in the nipples, lips and penis head/ vagina. I once heard that women usually have more in their skin overall than men regardless of ethnicity, but I don’t know how true that is. It’s true that red hair is most common among ‘white’ people, but…
They aren’t and never have been the only ones.
NEUROMELANIN: as the name suggests this form exists only in the brain. Interestingly, the levels of this we make are unrelated to how much of the other forms we make, hence even albinos produce normal amounts of it. Furthermore, humans are the ONLY animals that make so much; it’s much lower in primates & absent in all other animals! Scientists are still working out its functions, but so far it’s known low levels of it increase risk of Parkinson’s. It protects the nerves against free iron, lipids, pesticides & MPP+ (an unstable molecule that depletes ATP, cardiac adrenaline & dopamine, and increases cell death). It’s expressed in the adrenaline & dopamine-containing cells of the midbrain’s substantia nigra, contributing to its dark colour. It’s often claimed that it’s produced in the pineal gland but those claims might be confusing it with melatonin, a hormone that regulates sleep. The process by which neuromelanin is made is similar to the other forms but is turned into the neurotransmitter dopamine beforehand (probably explains why its levels are unrelated to eu & phaeo levels) and occurs much more slowly – years, as opposed to days/ weeks.
Melanin is ultimately made from the non-essential amino acid tyrosine, which is then oxidised (“burnt”, if you will) in different stages. The two eu & phaeo forms are what I’ll focus on most. If you’re more scientifically minded and want to see the production pathways, see the picture at the top of this post.
Note: in biology, a non-essential item means the body doesn’t need to get it from the diet. It makes enough by and for itself, eg. tyrosine, cholesterol, glycine, glutamine.
There are other substances that contribute to colour, at least in the skin & eyes. That’s partially why we don’t get blonde or ginger eyes – unless we’re anime characters.
Eye colour is furthermore also determined by light; there is no gene that directly produces blue/ green eyes. They’re caused by a combination of congenitally low melanin and a phenomenon called Rayleigh scattering. That’s when light waves are scattered into their constituent wavelengths, most are absorbed into the pigments but the blue wavelengths are reflected back out, which is what we see. This is also what makes the sky look blue. Green, hazel & amber eyes are caused by that plus a higher ratio of phaeo:eu. This dependence on external light sources also means light eyes can turn different colours in different lights.
In case you’re curious, pink & red eyes have no melanin at all, the colour comes from the blood vessels inside the eyeballs. Violet eyes are caused by extremely low melanin, and is the rarest human eye colour.
In the skin, colour is not determined so much by how many melanocytes we have (since we all have pretty much the same amounts) but by their activity. They may produce loads or tiny amounts, and different ratios of eu:phaeo types. On top of that, there are melanin-inhibiting enzymes that can interrupt the tyrosine oxidisation process. Then there’s genetics, which is surprisingly simple as there are less than 6 alleles that account for skin colour. There are 50 different genes that control when and where melanin is deposited, which may explain why colour isn’t always uniform across the same tissue.
The master enzyme that gets the ball rolling with oxidation of tyrosine is tyrosinase. If that is knocked out the result is albinism. If it’s inhibited and/or releases melanin in an inactive form (as happens with a certain allelic switch) the result is fair/ light skin, which can be offset by long-term exposure to sunlight. Tanning, in other words. Apart from albinos, most people regardless of ethnic background are genetically able to produce enough tyrosinase to become very dark!
Note that the reverse is also possible, that certain substances (including foods & food components) can decrease melanin production by inhibiting tyrosinase too! This may explain why when Buddha was in his ascetic phase, he claimed his skin was getting lighter beacuse of eating so little.
And apparently, the darker your skin the less vulnerable you are to hearing loss! WHAT?!?
So there you have it. Melanin is fascinating stuff, especially in the medical industry.