Estimated reading time: 7 mins (1295 words)
Have you ever gotten into a debate about someone’s hair color?
“Remember that blonde?”
“Which blonde? You mean, Jessie? Her hair is light brown”
“No, it’s clearly dark blonde”
“You need to get your eyes checked, that’s light brown”
Same goes for dark brown to black. And I’m pretty sure I’ve heard people argue that they’re not ‘redheads’ but actually they’re ‘strawberry blonde’.
But how can hair color be so debatable?
Well, it’s because hair color categories are an illusion.
Hair color categories are arbitrary groups that we distinguish and name.
And clearly, they can be highly subjective…
There is no absolute way to categorize someone’s hair color.
Who is blond/brunette or strawberry blond/red-haired depends on how you decide to define those categories.
To show you why hair color categories are a lie and how we get the amazing palette of hair colors we see today, I’ve decided to share with you guys how hair gets its color.
Overview of today’s post:
- Hair gets its color from a pigment called melanin, which is found in a red/yellow form (pheomelanin) and a brown/black form (eumelanin). The ratio and absolute amount of these pigments determine the color of hair.
- Melanin is produced in melanosomes (organelles) of the melanocytes (melanin-producing cells) in hair follicles during the anagen (growth) phase. (Feel free to check out the beginner’s guide to hair growth & anatomy if you need a refresher.)
- Melanosomes are then transferred from the melanocytes to keratinocytes (keratin-producing cells) and produce pigmented hair shafts.
If this overview went too fast for you, no worries, we’re going to break it down below:
You may have heard of the word melanin before. On this blog, you can expect to hear it all the time because I’m all about the pigment (and so is my research)! In this first post on melanin, I will introduce you to a number of melanin-related vocabulary as I give you a basic description of melanin production.
You may be wondering, for someone studying human hair diversity, why would I be interested in the process of hair pigmentation? The answer is: all the steps in the process of hair pigmentation are points at which we can have variation.
At each step, something can go wrong/right/differently. This means that the final result (i.e. hair color) can change because of any of this reasons. So, for someone who is interested in how and why humans vary in hair color, understanding all the things that could possibly cause the differences we observe is pretty important!
Hopefully, this will motivate you to bear with me through the explanations of the process so we can understand the how all of this diversity came to be!
(I am a firm believer in the positive power of exclamation marks so I have no doubt you have been bathed in motivation by the last few sentences – you are totally welcome!)
Melanin-producing cells called melanocytes are responsible for the pigmentation of keratin-producing cells called keratinocytes. Melanocytes and keratinocytes are both found in hair, as well as in skin.
There is a special set of melanocytes called the hair follicle pigmentary unit that is responsible for pigmenting the hair shaft. These melanocytes are found in the hair bulb, above and around the dermal papilla.1
Melanin production + vocabulary
Melanogenesis is the production of melanin. In pigmented human hair, we may find both pheomelanin (a red-yellow pigment) and eumelanin (a brown-black pigment). The entire process of melanogenesis takes place in melanocytes (cyte = cell). Inside the melanocyte, melanin is produced inside melanosomes (organelles related to lysosomes).
Now, the biological pathway for melanin synthesis is complex (although I have to say, I don’t think I’ve ever come across a biological pathway that was not complex). This means that we can ease into that topic in a later blog post! For now, all you need to know is that the melanosomes mature in the melanocytes before being transferred into the keratinocytes of the forming hair shaft.2
As mentioned above, pigmented hair shafts get their color from the melanin-filled organelles (melanosomes) that are transferred to the keratin-cells of the hair shafts (keratinocytes). So, how did these melanosomes end up in keratinocytes?
Over the years much knowledge has been generated by researchers who have looked into the black box process that is melanosome transfer.
Over these many years, thoughts about how these organelles end up keratinocytes have evolved, and so has the technology we have available to us. For example, now we can see cells ‘in action’ and, as you can imagine can give us a completely different picture of the dynamic process that is melanosome transfer.
Taking this important bit of history into consideration, we can describe four types of hypotheses put forward about how exactly melanosomes end up in keratinocytes3,4:
- Membrane fusion. Melanocytes are dendritic cells, which means they have tentacle-like protrusions sticking out of the body of the cell. This hypothesis suggests that melanosomes may be transferred by a fusion of the cell membranes of melanocytes (and their dendrites) to keratinocytes, after which the melanosomes pass through this ‘channel’.
- Shedding-phagocytosis. Another hypothesis is that melanosomes are packaged in little membrane-bound vesicles that are ‘shed’ by the melanocyte and are then phagocytized (“eaten”) by the keratinocyte.
- Exocytosis-endocytosis. This hypothesis proposes that melanosomes are released without being packaged and bound into the intercellular space and then incorporated by the keratinocyte.
- Cytophagocytosis. Lastly, it has been suggested that the keratinocyte may be actively phagocytizing melanosome-rich dendrites.
There is the most evidence for the shedding-phagocytosis mechanism.4 Regardless, once the melanosomes are transferred to keratinocytes, these cells are pigmented, and as the hair shaft grows from the hair follicle, a pigmented hair will appear.
Same process; different colors
The same process of melanogenesis described above can produce very different hair colors.
Regardless of the final color of the hair shaft, all pigmented hairs get their color from melanin.
This melanin comes in two forms: pheomelanin (red-yellow) and eumelanin (brown-black).
And the funny thing is that all pigmented hairs have BOTH types of melanin. The lightest strawberry blond hair will contain brown-black eumelanin and the darkest black hair will contain red-yellow pheomelanin.
So, as you can see, understanding hair color is not a simple matter of looking at a hair and describing its color.
So, if the process of pigmenting hairs is the same for all hair colors, and the building blocks are the same – why do we see differences?
The reason for this is that the absolute amount of melanin and the ratio of the two melanins can be different. But, ultimately, combining pheomelanin and eumelanin in various amounts and ratios is what produces all hair colors!
So, next time you get into a debate about the precise cut-off between dark blond and light brown, remember that those categories are absolutely arbitrary and this is a pointless debate.
Although, as someone who feels very strongly about the pronunciation of GIF, I completely understand if hair color is something you want to fight about…
- Schneider, M. R., Schmidt-Ullrich, R. & Paus, R. The Hair Follicle as a Dynamic Miniorgan. Curr. Biol. 19, R132–R142 (2009).
- Slominski, A. et al. Hair Follicle Pigmentation.
- Wasmeier, C., Hume, A. N., Bolasco, G. & Seabra, M. C. Melanosomes at a glance. J. Cell Sci. 121, (2008).
- Wu, X. & Hammer, J. A. Melanosome transfer: it is best to give and receive. Curr. Opin. Cell Biol. 29, 1–7 (2014).