Beginner’s guide to hair growth & anatomy

Reading time: 9 minutes (1564 words)

We all have to deal with our hair.

Maybe you braid it.

Maybe you wash it every day with the fanciest of shampoos.

Maybe you’ve dyed it every shade of the psychedelic rainbow.

Ooooorrrrrr maybe it’s been a hot minute since you’ve grown a full head of hair, so it’s not exactly a daily concern….

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I’m sure Patrick Stewart saves mad money on celebrity hairstyling.

Either way, we have all dealt with hair at some point in our lives. 

Yet we know very little about it.

Before I actually started studying hair, I probably spent a good 20 years of my life not knowing anything about it.

Clearly, I survived.

So I’m not gonna try and tell you that this is essential knowledge or something.

BUT, knowing something about what hair actually is and how it grows may help you make more informed decisions about the way you cut/dye/pluck/wax/treat/do whatever to it!

And (BONUS!) for those of you who want to understand the science of hair, a basic introduction to hair growth & anatomy will definitely come in handy.

Think of this post as the ‘How it’s made‘ of hair:

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Disclaimer: this is not how hair is made. I just really like this pasta making machine. How it’s made has excellent pasta GIFs, just saying.

Really, the point of this post is to teach you the basics and answer questions like:

  • “What is hair, even?”
  • “How does hair grow?”
  • “If I shave my mustache, will it grow back thicker? And if so, can I prep for Movember this way?”

I don’t know what your hair questions are… I’m just guessing here.

But without further ado, let’s get on with today’s lesson!

Overview of the stuff you’re going to learn:

  • Hair is part of the integumentary system and it is an appendage on the skin.
  • There are two parts to “hair”: the hair shaft and the hair follicle.
  • The layers of the hair shaft are the cuticle, the cortex, and the medulla.
  • The hair shaft is produced by the hair follicle, a complex mini-organ in the skin.
  • The hair growth cycle has an anagen (growing), catagen (resting), telogen (shedding) phase.
  • (If you’re really keen and want to read more, there are scientific references at the bottom that are cited throughout the post with little numbers)

What is hair even?

The thing we call “hair” can more precisely be described as a keratinized appendage that grows out of a mini-organ in the skin called a hair follicle.1

And now, a longer explanation in less fancy language:

Some of you may know that skin is the largest organ in your body (well, I guess on your body is more accurate).

But did you know that there are organs within this organ???

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One might say, there’s layers to this.

Your skin is part of the integumentary system. The integumentary system is an organ system like the circulatory system and the nervous system etc. In these organ systems, a bunch of different organs work together to do stuff.

The integumentary system is made up of skin and its appendages (i.e. the stuff that sticks out of it). For humans, this includes mainly hair and nails, but if you have a more avian or reptilian inclination, feathers and scales are also important appendages.

Hair and nails are keratinized structures. This means that the cells from which they came have become filled with keratin (a tough protein) and have died in the process.

And as a little bonus did-you-know, integument comes from the Latin integumentum, which means “cover” or “enclosure”. Perfect name for the stuff that keeps our insides from spilling out!

The hair shaft – the thing we usually think of when we talk about hair

When we talk about hair, we usually mean the stringy stuff that’s growing out of our bodies – the hair shaft. But this is only half the story! If we really want to understand how hair works, we also have to talk about where this stringy stuff grows, which is the hair follicle.

BUT, first things first, the hair shaft that we are all so familiar with, is actually not a simple fiber!

The hair shaft is frequently described as having three main parts (from outside to inside): a cuticle, a cortex, and a medulla.

hair-shaft

  • The cuticle is the outermost layer which is often described as consisting of scale-like structures that cover the hair fiber like tiles on a roof.
  • The cortex represents the majority of the hair and is made up of keratin bundles that form long and fiber-like structures (filaments).
  • The medulla is the innermost part which is essentially made up of empty space (more specifically, vacuolated cells). It may often be discontinuous or even missing in hair fibers.2

The hair follicle – the thing that actually makes your hair

The hair follicle is the mini-organ which produces the hair shaft. Essentially, it is the pasta machine to our spaghetti:

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I honestly didn’t realize how relevant pasta-making GIFs were to hair…

However, a slightly more accurate biological depiction might look like this:

hair-follicle

  • The hair shaft exits the hair follicle through the epidermis (the top layer of your skin).
  • The sebaceous gland (produces oil in the hair follicle) and the arrector pili muscle (responsible for ‘raising your hair’ when you get goosebumps) are also associated with the hair follicle.
  • The inner root sheath (IRS) consists of a cuticle, Henle and Huxley layer these are involved in shaping the hair3. The outer root sheath (ORS) is the outermost part of the hair follicle and the ‘bulge’ is a thickened portion of the ORS where some stem cells of the hair follicle are found.
  • At the bottom of the hair follicle, we find the portion known as the hair bulb. It contains the dermal papilla which is essential for signaling in the hair growth cycle 4. The matrix surrounding the dermal papilla is full of proliferative (dividing) cells which differentiate into the various layers of the hair mentioned above (medulla, cortex, cuticle, and IRS) 5.
  • So, as the cells of the matrix divide, older cells are pushed upwards by newer ones. At a certain point, these cells reach the keratogenous zone which is where the cells of the hair shaft keratinize and become hard and dead.

As you just learnt, once your hair is out of the follicle, it’s dead, so whatever you do to the hair shaft (e.g. cutting/shaving) won’t have any effect on the hair growing underneath.

So there’s also no point in covering your hair with avocados and bananas for the healthy minerals and vitamins! (Although you’ll probably smell like a delicious smoothie!)

And, if you trim your hair often because you think it will grow faster, I’m sure you’re feeling pretty scammed right about now.

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Real footage of a hairdresser telling you that your hair will grow faster if you just come have it trimmed with her every 3 weeks.

But if you wanna give your awesome hair follicles some love as they do the hard work of producing those hair fibers, go for it!

The hair growth cycle – the circle of life, but for hair

Now, this is the part where I ruin your dreams of becoming Rapunzel.

ynlqwrm

If you’ve been avoiding cutting your hair for years and years because you’re hoping to reach fairytale princess/metalhead lengths, you may be disappointed to learn that your hair will not grow forever.

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Still waiting for someone to propose long hair evolved because of sexual selection on metalheads.

Your hair grows in a cycle! This means it doesn’t grow forever, but it grows for a while, then stops, then falls out. Luckily, human hairs aren’t synced up, or you would have awkward moments of hairlessness every now and then…

But the hair growth cycle for scalp hair looks something like this:

Growth cycle pie chart

  • During anagen (growth), the matrix cells are proliferating and keratinizing as they move up – the hair grows. The duration of this phase is variable among people, but it is usually about 4-6 years for scalp hair. While human hair does grow asynchronously (which means hairs in the same region may be at different points in the growth cycle), about 90% of the hair will usually be at some point in the anagen phase.
  • During catagen (resting), the cells stop proliferating and programmed cell death begins to destroy the lower (cycling) part of the hair follicle. The matrix hardens and the hair follicle is loosely attached to the dermal papilla (which you will remember is a mesodermal derivative, unlike the rest of the hair follicle). This phase only lasts a few weeks.
  • In the telogen (shedding) phase, the hair follicle is not doing much and the hair is just waiting to be shed (which can take around 4-6 months)6.
  • The anagen phase is reinitiated by signaling between the ‘bulge’ (in the permanent portion of the hair follicle) and the dermal papilla, prompting the growth of a new hair bulb and restarting the entire process3.

So if you glanced over the hair growth cycle chart, you may have gotten worried for a second that your eyebrows and leg hair are growing for years.

Don’t worry, they have much shorter growth cycles! Scalp hair has the longest duration of hair growth on the body.

Other hair on the body has shorter cycles and there’s quite a bit of variation in the duration of different parts of the cycle depending on the region of the body and on the individual – but that’s a story for another time!

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That was it: the beginner’s guide to hair growth & anatomy!

So, out of curiosity, did you know any of this? Or was this all new to you?

Are there any other (science) questions/shower thoughts you’ve had about hair?

Please share them in the comments so I can make sure I give you some answers in future blog posts!

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References from the scientific literature:

  1. Awgulewitsch, A. Hox in hair growth and development. Naturwissenschaften 90, 193–211 (2003).
  2. Fujimoto, S., Takase, T., Kadono, N., Maekubo, K. & Hirai, Y. Krtap11-1, a hair keratin-associated protein, as a possible crucial element for the physical properties of hair shafts. J. Dermatol. Sci. 74, 39–47 (2014).
  3. Schneider, M. R., Schmidt-Ullrich, R. & Paus, R. The Hair Follicle as a Dynamic Miniorgan. Curr. Biol. 19, R132–R142 (2009).
  4. Schmidt-Ullrich, R. & Paus, R. Molecular principles of hair follicle induction and morphogenesis. BioEssays 27, 247–261 (2005).
  5. Rosenquist, T. A. & Martin, G. R. Fibroblast growth factor signalling in the hair growth cycle: expression of the fibroblast growth factor receptor and ligand genes in the murine hair follicle. Dev. Dyn. 205, 379–86 (1996).
  6. Sachs, H. Theoretical limits of the evaluation of drug concentrations in hair due to irregular hair growth. Forensic Sci. Int. 70, 53–61 (1995).

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