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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.
Structure of 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).


Structure of 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).

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

  • In the drawing 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 drawing above). The cortex is made from tiny fibres 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 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 drawing, you can count 6 cuticle layers.

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

  • In the drawing 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 drawing above). The cortex is made from tiny fibres 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 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 drawing above, you can count 6 cuticle layers.

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

Skin and Hair Pigmentation

Skin colour - Inside the germinating layer of the epidermis, cells called melanocytes produce melanosomes (these are small packets that contain melanin granules). The melanosomes then spread upwards through the epidermis and accumulate just above the nucleus within skin cells - this gives protection against UV radiation. Darker skin contains more melanin and so has more protection against the sun.

Hair colour - Within the dermal papilla at the base of a follicle, melanocytes produce melanin granules. Then, as germinal matrix cells are produced, melanin granules pass into the cells which then develop into the cortex (the middle layer) of a hair shaft to give hair its colour.

There are two types of melanin: eumelanin is dominant in brown and black hair, and pheomelanin is dominant in ginger and blonde hair. Without melanin granules, the partly hollow hair appears grey. Hair that is completely absent in melanin is white.

This is page 1 of 3.

Read next page? Hair Growth Cycle.

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Reference:
education.vetmed.vt.edu