Hair Structure, Hair Growth and Hair Loss!


Hair structure and hair growth are surprisingly complex!

And perhaps that complexity partly explains why the exact mechanism for hair loss has remained a mystery for so long.

But knowledge is a powerful tool.

So, to help fully understand the hair loss process, quickly learn all about the structure of hair and how it grows.

As you read the bullet points below, please refer to the diagrams whenever you see words in bold – some of these have been colour-coded to highlight that they are very important hair structures or areas within the skin.*


*
Hair growth is basically just a collection of dead cells being pushed along a pouch (follicle) in your skin. So it’s also important to learn about skin structure too…

Skin Structure

  • 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 dermis.

  • 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.

Hair Structure


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 follicle.

  • 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.
Basic structure of the hair follicle.
  • 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).

  • The germinal matrix also contains stem cells - these grow the hair shaft through constant cell division which continuously push older cells upwards.

  • 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 diagram below). By the time these cells are a third of the way up the follicle, they have died and fully hardened (keratinised).

  • A sebaceous gland lies within each follicle. This produces an oily substance called sebum from a duct that 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 (this is explained on page 2). When the arrector pili muscles contract, they make your hair stand on end (this is what causes goosebumps).


Basic structure of the hair follicle.


  • 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).

  • The germinal matrix also contains stem cells - these grow the hair shaft through constant cell division which continuously push older cells upwards.

  • 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 diagram below). By the time these cells are a third of the way up the follicle, they have died and fully hardened (keratinised).

  • A sebaceous gland lies within each follicle. This produces an oily substance called sebum from a duct that 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 (this is explained on page 2). When the arrector pili muscles contract, they make your hair stand on end (this is what causes goosebumps).

Hair Shaft

Cross section of a blonde hair shaft showing the hair structure.
You can clearly see the cortex fibres and cuticle layers in this split end.

  • In the diagram above, you can see that the hair structure of 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 colour (blonde in the diagram above). The cortex is made from tiny fibres 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.

Cross section of a blonde hair shaft showing the hair structure.

  • In the diagram above, you can see that the hair structure of 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 colour (blonde in the diagram above). The cortex is made from tiny fibres of keratin running parallel to each other along the length of the hair shaft (as shown in the photo of the split hair end below).

  • 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 below. And in the diagram above, you can count 6 cuticle layers.

You can clearly see the cortex fibres and cuticle layers in this split end.

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 sulphur content (the other being methionine).

The significance of all this is that the mineral sulphur is known to be beneficial for hair growth.  Clearly then, sulphur 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 colour-coded hair follicle diagram).

As you can see, these two important components of the hair loss process seem to be somewhat scattered across the various structures of the hair follicle. And perhaps not surprisingly then, this only adds to the complexity this puzzling type of hair loss presents.

So, now you know all about the various structures which form the hair follicle and the hair shaft, find out on the next page how they all interact to create the hair on your head over and over again throughout life...


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Reference:
education.vetmed.vt.edu