Hair growth
Hair growth

HOW DOES HAIR GROW?

Hair is made up a protein called keratin and follows a specific growth cycle made up of three distinct phases:  anagen, catagen and telogen.  The amount of keratin and the length of the growth cycles determine the length of the hair and the thickness of the hair fibre.

Scalp hair can grow at a rate of between 0.6 cm and 3.6 cm per month. The growth of human hair depends on the continued keratinisation of epidermal cells.  Keratinisation is a process where a living keratin cell loses its cytoplasm and becomes a hardened keratin cell.  This process means cells that were previously growing and dividing can turn into the tough waterproof material that can be continually regenerated and make up the hair fibre.  The keratin that makes up the hair fibre is produced from cells from the base of the dermal papillae.  The bound collection of dead cells that continues to be pushed up the hair follicle ends in the formation of hair that is seen at scalp level.

THE HAIR GROWTH CYCLE

Anagen phase –  The proliferation of the hair shaft starts with the cells of the dermal papilla instigating the anagen phase by signalling to the stem cells in the hair bulge.  The stem cells move down to the germinal matrix and differentiate into keratin cells.  The hair follicle derives nourishment from the dermal layer which feeds the keratinocytes.  The keratinocytes grow larger and continue to divide whilst the keratin cells are biologically viable.  The position of the keratin cells defines what part of the hair fibre it will become.

The growing hair shaft is pushed up by the increasing population of new keratin cells from the basal layer into the upper area of the root bulb.  The keratin cells continue the journey through the hair follicle, still growing and acquiring pigment from the melanin produced by melanocytes.  The keratin cells then enter the fibrillation zone and the framework of the cell skeleton starts to develop.

Once cells enter the lower keratinisation zone, substantial disulphide cross linking starts creates a protein matrix.

Hydrogen bonding of amino acids further stabilises the structure and then more disulphide linking begins to create a rigid structure.

As the keratin cells reach the upper stabilisation zones, the cells begin to harden and the keratin cells produce more and more keratin protein .  At this point the water content of the cell drops to around 20% and disulphide bonds continue to form.  As the cells unite, the absence of water leaves air-filled gaps in the hair shaft.  If the keratin cells have not produced an adequate amount of keratinous protein the gaps will be large and the hair will be fragile and coarse.  These gaps are known as fusi, they are larger at the base of the the hair and the air dissipates near the tip of the hair.  This eventually leads to split ends.

The cell membrane complex binds to the matrix structure acting as an intracellular cement and provides suppleness and flexibility to the hair fibre.  The cell membrane complex is also responsible for the hydrophobicity of the hair fibre.

The hair follicle unit with a hair fibre growing in the hair canal through the zone of protein synthesis, the zone of differentiation, and the zone of keratinisation where the keratin cells start to fill with keratin.  In the region of permanent hair fibre all keratin cells will have lost all their cytoplasm and be full linked by disulphide bonds.  The epidermal layer provides zinc and sulphur for the differentiating (growing) keratin cells.  The bulge area is home to the stem cells where the keratin cells originate.  The dermal papilla is the anchorage site of the hair follicle.

Catagen phase –  The catagen phase is a transitional phase where the follicle will go through a period of renewal.

The hair follicle will begin to shrink as the hair follicle starts to break down and the dermal papilla detaches from the dermis cutting the follicle off from its usual blood supply.  The hair follicle canal is reduced to around 15-20% of it’s original length causing the hair fibre to be pushed upward.

A ‘semi’ catagen stage can be induced if there are low levels of collagen within the epidermal layer as the hair follicle canal will be reduced.  This catagen stage usually lasts about two weeks.

Telogen phase – During the telogen stage the hair follicle is said to be ‘resting’.  It is unable to grow any further as it has no blood supply and may be pulled out easily as it is not anchored by the dermis.  Women with a high number of hairs in telogen stage will find themselves mores susceptible to traction alopecia from tight hairstyles or hair extensions.

At any one time 10-15%  of hairs will be in telogen phase.  During telogen phase, the cells in the hair follicle canal that line the keratin cells can temporarily anchor the cell in position.  when the hair follicle enters early anagen, the new hair follicle will disrupt the anchorage of the resting hair and that hair will be dislodged, around two weeks later the new hair will begin to emerge.

FORMATION OF THE HAIR FIBRE 

Hair is made up of around 80% alpha-keratin, a collection of multilayered, flattened cells connected by rope like filaments that provide the structure and strength of the hair fibre..  Alpha-keratin synthesis begins near the adhesion point of the base of the hair fibre to the cell membrane.  Two polypeptide chains (made up of alanine, leucine, arginine and cysteine) twist together to form a structure called a coiled curl dimer, these coiled curl dimers are bonded together with disulphide bonds that can only occur between the cysteines.

Two coiled curl dimers align with each other using disulphide bonds forming a protofilament.  Through further disulphide bonding, alignment of two protofilaments allow the formation of a protofibril.  Four protofibrils then combine to form an intermediate filament unit, this is the basic alpha-keratin subunit.

The interfilamentous units are embedded in a matrix formed from coiled curl fibres that are dominant in either cysteine, glycine, tyrosine or phenylalanine.  The different amino acid content matrixes account for the variability in strength of the alpha-keratin protein.

Coiled curl fibres high in cysteine will provide the strongest base for an interfilamentous protein.  Properly bound filaments build a thick and smooth hair fibre with high tensile strength resistant to breakage and damage.

Units that are higher in cysteine will form strong, water-repellent fibres less vulnerable to the detrimental effects of shampoos, heated styling tools and chemical treatments.

WHAT DETERMINES HAIR THICKNESS AND LENGTH?

The thickness of the hair fibre is dependant on how large a keratin cell can grow before it dies and hardens.  The length of the hair fibre depends on how many times the stem cells continue to differentiate into keratin cells without cell inhibition or multiple replication errors.

Blood is made up of vital elements for hair growth such as hormones, irons, amino acids, glucose and oxygen.

The level of diffusion of these vital elements from blood vessels to hair follicle is one determinant of how fast or how thick hair will grow.  This is how vasodilators increase hair growth. Other factors that influence the length of hair include the level keratin gene expression and transcription success in the keratin stem cells,  local androgens, growth factors and inflammation responses.  Hair will continue to grow whilst it is has the right hair growth signals, keratin gene activity and nutrient supply.

THE FINAL WORD

Keratinisation is complete when as the keratin cells are pushed to the surface of the scalp to form the shaft of hair that is visible. The external hair is completely dead and composed entirely of keratin.  The quality, thickness and length of your hair will depend on the early stages of keratinisation (the bit you cannot see).