A comprehensive knowledge of hair and scalp physiology is required to analyse hair type accurately and choose the correct product for your client's needs.
(see image Electron Microscope View of a Healthy Cuticle)
The cuticle is the outermost layer of hair, protecting the inner structure with a series of fine scales facing upwards and away from the root. The cuticle is made up of approximately 8-10 layers, however this can vary due to wear and tear and also ethnicity.
The cuticle is particularly sensitive to pH. Alkaline products cause the hair to swell, opening the cuticle layer and exposing the inner core, whilst acid-based products have the opposite effect, tightening the cuticle layer and making hair smooth and glossy. Both extremes are bad for the hair, which is itself naturally slightly acidic at about pH 5.5. A high acid value will make hair resistive to treatment and exclude essential moisture; hair in this state can be dry and brittle. If the hair is too alkaline it becomes porous and may break down completely. Porous hair is dull and lifeless with bad retention. Before proceeding with a perm or straightening product always analyse hair condition.
Most form and straightening products contain an alkali, enabling the main chemical ingredient to penetrate quickly and act on the cortex, whilst others maintain a slight acidic value for a more caring effect on the hair.
(see image Cross Section Showing the Hair Follicle)
The cortex determines most of the hair's physical characteristics including:
Determined by pigments.
Moisture content and retention is also important here.
A measure of how healthy the hair is.
Determined by the Arrector pili muscle.
The cortex (see image The Composition of Hair Fibres) makes up 75-90% of the bulk of hair and comprises hundreds of tiny macrofibrils resembling tubes or straws. These are held together by keratin fibres, the main component of surface hair. Within each of the macrofibrils are smaller tubes called microfibrils. These contain the molecular components of hair in the form of three coiled polypetide chains, known as protofibrils. Protofibrils have a spiral shape resembling a spring. It is this structure that gives hair its elasticity, enabling a return to its previous form when stretched. If protofibrils are stretched beyond their breaking capacity the molecular chains are destroyed, causing permanent damage and possible hair breakage. Salt bonds and hydrogen bonds form links between these polypeptide chains, helping to hold the hair together (see image The Molecular Structure of Keratin).
(see image Cross Section Showing the Hair Follicle)
The medulla is a small, hollow tube running down the centre of the cortex; in some fine hair types the medulla is not present at all. The medulla has no bearing on the form process.
