How Hair Grows
Understanding what goes on in terms of hair growth
What is hair?
The simplest, scientifically correct definition of hair is “a Keratin protein filament”. Keratin is fibrous protein that can be hard, flexible or both depending on how it is arranged. Fingernails are also keratin as are claws and horns in other mammals.
Starting from the exterior and working inward a hair is composed of 3 layers:
- The cuticle is composed of layers of thin flat cells that look like roof shingles
- The cortex contains the long thin strands of Keratin
- The medulla is a loose and often open area at the centre of the hair.
Although most diagrams of human hair will show a round structure like in the first photo, in fact, most human hair is oval in cross-section as in the second photo. Only people of Asian descent have perfectly round hairs. The 3 hair layers do not necessarily grow evenly and, in an oval shaped hair, the result is curls. The round shape of Asian hair, however, is stable enough to overcome such uneven growth leaving hairs always straight and untangled.
Hair colour is derived from the same pigment as skin colour: melanin. There are 2 types of melanin in human hair: eumelanin and pheomelanin.
Eumelanin is either black or brown but is largely responsible for the lightness or darkness of hair. Pheomelanin is either orange or yellow and responsible for the subtle variety in hair colours. Blonde hair is not caused by any particular combination of the melanin types but simply from having very low levels of melanin in the hair.
Although hair on different parts of our bodies appears to be quite different this is simply to do with density and growth rate. Under a microscope, only 2 types of hair are identifiable: Terminal hair and Vellus hair.
Terminal hair is the thick stranded coloured hair concentrated most densely on the head while vellus hair is the colourless down you will find growing on almost the entire body other than hands, feet and lips.
When we are newly born almost all our hair, even on our heads, is vellus hair. Over time, growth and sex hormones cause vellus hair follicles to switch to producing terminal hair and eventually the male and female patterns of body hair.
Terminal hair follicles will never switch back to producing vellus hair once this change has been triggered so we are stuck with any terminal hair we acquire. The one mysterious exception to this is male pattern baldness. Curing male pattern baldness or deliberately triggering it on other parts of the body is a puzzle science has been trying to solve for a long time.
All hair goes through 3 phases of growth. The time a follicle spends in each phase determines the length and apparent growth rate of hair on different parts of the body.
The phases are named:
- Anegen – The growth phase. Hair will grow 1cm every 28 days. Scalp hair will continue to grow for 2-7 years while eyebrows will only grow for 4 months.
- Catagen Transition – Part of the hair bulb ‘dies’ and fuses with the hair in order to keep it anchored to the skin. This stage only lasts 2-3 weeks.
- Telogen – The resting phase lasting around 3 months. At the end of the telogen phase the hair will fall out, the bulb will regenerate and the cycle will begin again. We normally shed around 75 hairs a day from our scalp, however, extreme stress can throw up to 70% of our scalp hair into premature telogen and shed it in less than a month. This does not mean the hair is gone forever as the follicles are capable of recovering to Anegen phase once the stress is removed.
Of course, without looking inside the follicle, it is impossible to tell the difference between the catagen and telogen phases so we commonly refer to hairs as either growing or resting. Why these phases are so important will become apparent when discussing methods of hair removal.
Each hair on our body grows from a follicle. The base of the follicle is around 3mm below the skin. The diagram below shows the 6 major structures making up a follicle.
- Papilla – The papilla is a large structure at the base of the follicle made up of connective tissue to anchor the hair base and provide a capillary loop to supply blood.
- Matrix – Around the papilla is the hair matrix made up of epithelial cells and pigment producing cells. Cell division in the matrix produces the cells that form the hair fibre.
- Bulb – The bulb sheaths the papilla and matrix and contains all the cell structures that divide and grow to produce the hair shaft. Cells in the bulb divide every 23 – 72 hours and are the fastest dividing cells in the body. The bulb is critical when it comes to long-term hair removal. The more damage done to the bulb the longer it will take to recover and the longer it will be before hair regrows.
- Bulge – The bulge produces the stem cells used to reinvigorate the bulb at the beginning of each growth cycle and feed it with cells capable of the high division rate needed for hair production. The bulge is critical to achieving permanent hair removal. The bulge is capable of regrowing a completely new hair bulb so no amount of damage to the bulb can stop the hair eventually regrowing. Destroy the bulge, and it is impossible for hair to regrow.
- Sebaceous Gland – This gland is responsible for producing oil to lubricate and waterproof your skin.
- Apocrine gland – This is the sweat gland.
- Arrector Pili Muscle – This is the muscle that makes hair stand on end when you get cold or scared.
- Hair Receptors – These nerve endings are too small to show in this diagram but are revealed in this photo of a human ear hair. This ring of nerves around the hair shaft lets you know when your hair is standing on end and provide the feeling when an object brushes past.