Basic Hair Structure, Function and Diagrams
Hair structure and hair growth are surprisingly complex. And perhaps that complexity partly explains why the exact mechanism behind hair loss has been a mystery for so long.
But knowledge is a powerful tool. So to help fully understand the hair loss process, it makes sense to also learn about the structure of hair and how it grows.
you read the bullet points below, please refer to the basic hair structure and hair follicle diagrams whenever you see words in bold – some of these have also been color-coded to highlight that they're very important hair structures, or areas within the skin.
Since hair growth is basically just a collection of dead cells being pushed along a
pouch (follicle) in your skin, it’s important to learn about
skin structure too.
- Your skin is the largest organ in your body. It
has various functions including: temperature regulation (sweat glands to cool down; goosebumps to keep warm), excretion (the skin is sometimes referred to as the "third kidney") and
protection (against sun, rain, bugs, infection, etc).
- Skin has two main parts - the epidermis and
- The epidermis has five
layers. The uppermost layer forms the
surface of the skin and is made from dead cells which are continuously being
shed and replaced from below. Excessive shedding can, of course, produce dandruff.
- The deepest layer of the epidermis is the germinating
layer. This region is constantly growing and
dividing into new cells, pushing the old cells up towards the
surface of the skin.
- Below the epidermis is the dermis.
The dermis is the thickest part of the skin and contains blood vessels
to supply the nutrients needed for skin cells to grow.
Before hair growth can begin, a hair follicle must first be created:
- The germinating
layer of the epidermis
starts growing down into the dermis, and forms the outside of each hair
- The dermis then grows upwards into the base of the
follicle to form the dermal papilla. This allows capillaries (blood vessels) to enter the papilla and provide nutrients for the hair
shaft to grow.
- The bottom part of the follicle enlarges into an
area of actively growing cells. This is called the hair bulb.
- At the base of the hair bulb, the germinating
layer merges into the outer root sheath
(which forms the inner wall of the follicle).
- The outer root sheath then forms the germinal
matrix (hair root) which surrounds the dermal papilla.
- The germinal matrix grows the inner root sheath (this is the white bit at the end of a hair if it's pulled out).
germinal matrix also contains stem cells - these grow the hair shaft
through constant cell division which continuously push older cells
- Hair shaft cells are similar at first. But as they move up through the
follicle, they begin to change shape, and a protein called keratin develops inside the cells.
- Three different types of hair cell then form
(see hair shaft structure diagram below). By the time these cells are a third of
the way up the follicle, they have died and fully hardened (keratinized).
- A sebaceous gland lies
within each follicle. This produces an oily substance called
sebum from a duct which opens up into the hair follicle about halfway down
from the skin surface.
- The follicle also has a bulge directly below the sebaceous gland in the outer root
sheath at the attachment point of the arrector pili
muscle. The bulge produces
stem cells that regenerate the follicle during the next hair growth cycle. When the arrector pili
muscles contract, they make your hair stand on end (this is what causes goosebumps).
- In the basic hair structure diagram above, you can see that the shaft has three layers: the cuticle (outer layer), cortex (middle
layer) and medulla (inner layer).
- The medulla is a honeycomb
keratin structure with air spaces inside.
- The cortex gives
flexibility and tensile (stretching) strength to hair and contains melanin granules,
which give hair its color (blond in the diagram above). The cortex is made from tiny fibers of keratin running parallel to each other along the length of the hair shaft (as shown in the photo of the split hair end above).
- The cuticle is made from 6
to 11 layers of overlapping semi-transparent keratin scales (which make the
hair waterproof and allow it to be stretched). Someone with thick,
course hair will have more overlapping layers of cuticles than someone
with fine hair. You can see the cuticle layers in the photo above. And in the diagram, you can count 6 cuticle layers.
Take a Closer Look
Beneath the skin of keratin
As already stated, hair cells are mostly made from keratin (up to 95%). And, like all proteins, keratin is made from amino acids. Sixteen amino acids form keratin, with cysteine having the highest concentration. And cysteine is one of just two amino acids that have a high sulfur content (the other being methionine).
The significance of all this is that the mineral sulfur is known to be beneficial for hair growth. Clearly then, sulfur is a very important component of hair structure.
Different type of hair - Same type of hair structure
There are three types of hair - lanugo, vellus and terminal. All of them share a similar basic hair structure, but each plays a different role in the body:
- Lanugo hair starts growing when you’re still in the womb, but falls out either just before or just after birth. (So, in other words, you might experience hair loss before you’ve even been born!)
- After your lanugo hair falls out, it gets replaced by vellus hair - this is the tiny, near invisible hair that covers your body.
- However, a lot of vellus hair turns into terminal hair (which includes eyebrows, eyelashes and scalp hair). And male hormones (androgens) also stimulate vellus hair to "mature" into facial hair, body hair and pubic hair.
Paradoxically, the very same androgens associated with hair growth, have also been associated with hair loss (androgenetic alopecia). These include testosterone and dihydrotestosterone (DHT). So, let’s take a look at where within the hair follicle these androgens act.
Which hair structures are connected to hair loss?
People with hair loss tend to have higher levels of 5-alpha reductase
and androgen receptor sites in the hair follicles within the hair loss
region of their scalp. 5-alpha reductase is an enzyme that
converts testosterone into DHT. And androgen receptor sites are
essentially attachment points for androgens (especially DHT).
In the hair follicles, 5-alpha reductase is found within the sebaceous glands and outer root sheath, and androgen receptor sites are found in dermal papilla cells (please refer back to the color-coded hair follicle diagram).
As you can see, these two important components of the hair loss process (5-alpha reductase and androgen receptor sites) seem to be somewhat scattered across the various structures of the hair follicle. So, perhaps not surprisingly then, this only adds to the complexity this puzzling type of hair loss presents.
However, it's a puzzle that has, I believe, now been solved - please take a look at the skull expansion section to find out.
Now you know all about the various structures which form the hair follicle and hair shaft, learn on the next page how they all interact to create the hair on your head over and over again throughout life.
This is page 1 of 3.
page? Hair growth cycle.