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HAIR STRUCTURE AND THE PILIFEROUS FOLLICLE

 

The hair follicle is a tunnel-like segment of the epidermis that extends down into the dermis and has a two layered wall formed by the inner and the outer sheaths. The inner root sheath is three-layered and extends only two-thirds of the wya up the follicle, it is absent above the sebaceous gland opening. The unit is called the pilosebaceous follicle. The arrector pili muscles are attached to the hair follicle walls.

 

THE HAIR FOLLICLE

 

Hair has two distinct structures – first, the follicle itself, which resides in the skin, and second, the shaft, which is what is visible above the scalp.

 

Two sheaths, an inner and outer sheath, surround the follicle. These structures protect and form the growing hair shaft. The inner sheath follows the hair shaft and ends below the opening of a sebaceous (oil) gland. The inner root sheath interlocks with the cuticle of the hair shaft, so that hair and sheath grow together. At its base the follicle is enlarged to form the bulb of hair root. The bulb is pushed in from below, the space form being occupied by the capillary network of the dermal hair papilla. The hair root consists of two regions, the matrix and the upper bulb. The matrix, the lower region, lies over the dermal papilla whose blood capillaries supply the food and oxygen necessary for hair growth.

THE HAIR STRUCTURE

 

Hair, is divided into three parts:

 

1) the HAIR SHAFT, the visible outer part of the follicle (the average thickness for an adult is 5-78 micron);
2) the ROOT, the enlarged part of the hair bulb lies deep in the scalp and is, therefore, not visible to the human eye.
3) the MATRIX, found in the lower region is where the unpigmented cells are all alike and dividing rapidly. As the only germinal cells are found within the matrix it is not true that cutting hair will strengthen its structure. (The only advantage to be gained from cutting hair short is to reduce damage from brushing and drying etc).

 

throughout their reproduction the matrix cells continue to push the older cells, as these cells move upwards they undergo “keratinization” and progressively become more and more rigid. The role of the melanocytes, found immediately above the matrix cells, is to “color” them by releasing melanin, these cells are destined to become the “cortex” of the hair structure.

 

By cutting a section of hair horizontally it is easy to see the 3 principal part of its structure:

THE CUTICLE

 

The cuticle is the external part of the hair structure and it is formed by a single line of fine transparent cells (0.2-0.5 micron thick) which make up the scales and are positioned in the following way:

 

  • vertically at the root;

  • like scales, diagonally. The bottom part of the scale is attached to the cortex, whereas, at shaft level its upper section is loose, rather like a sheaf of wheat.

The scales are then to protect the hair structure. The cuticle, due to its external position, is the first section to be damaged and when the hair structure is over stressed (aggressive shampoos, chemical services and brushing) its scales open and hair feels rough and brittle.

 

 

THE CORTEX

 

The cortex, the largest section, is found in the middle of the structure. It is made up of different sized fibre, which are positioned vertically in parallel lines, called:

 

  • macrofibrils

  • microfibrils

  • protofibrils

 

The cohesion between the fibre is determined by the chemical bonds. The fibres contain, a colored pigment, its production diminuishes as we age and hair turns grey. The cuticle does not contain melamin, therefore, when lifting the natural hair color the chemical reaction will occur within the cortex.

 

 

THE MEDULLA

 

The medulla is the most internal part of the hair structure. It is formed by rounded cells which are positioned in columns, the cells are generally separated by air spaces. The medulla is the central part of the hair, although its specific function is still unknown.

 

 

CHEMICAL COMPOSITION OF HAIR

 

  • Carbon 50%

  • Oxygen 25

  • Nitrogen

  • Hydrogen

  • Sulfur

 

are the most important chemical elements that make up the hair structure.

 

Under chemical analysis hair is also made up of water and:

 

A) KERATIN: a protein mainly found within the cortex (keratin makes up the greater part of it) and it is the principal component of the hair structure. Like all proteins, keratin, is made up of amino acids, which when joined together make up macro-molecules. There are 18 amino acids present in keratin, however, Cysteine is present in greater quantities, cystine (derived from the condensation between two molecules and the formation of disulfide bonds) seranine, glutamic acid, glycine, treonine, arginine, valine and isoleucine.

 

The macro-molecules have a spiral shape and are held together by different types of bonds.

 

  • Disulfide bonds: these are reduced during the chemical action of perming.

  • Salt bonds: bonds that help consolidate the shape of the hair structure. Products that reach very high temperatures or cause excessive swelling can totally destroy the hair structure.

  • Hydrogen bonds: these bonds are vulnerable to water and heat.

 

 

B) LIPIS: made up of triglycerides, wax, phospholipids, cholesterol, squalene and fatty acids, which are difficult to document as the majority of them are derived from sebum.

 

C) MINERALS (oligoelements): represent an essential component of the proteic-enzymatic system. There is a direct correlation between the quantities of oligo-elemensts present in the blood and those present in the hair.

 

  • iron (average 4-12mg/gr): more present in red hair than blonde and black hair.

  • magnesium (average 30-45mg/gr): most present in blakc hair (up to 170mg/gr).

  • zinc (average 150-180 mg/g4): essential for correct germinal cellular activity within the matrix. When there is a deficiency hair becomes weak and the growth cycle slows down

  • copper (averge 16-50 mg/gr): essential for melamin synthesis and to allow cysteine to oxidize into cystine with the formation of disulphide bonds.

  • lead (average 10-30mg/gr): mpst present in chestnut colored hair. As lead is almost absent in our organism the hair shaft is used to measure the presence of this metal in environmental pollution. Under microscopic investigation the bulb of a strand of hair with a protein and/ or mineral defficiency will appear small (if hair is naturally fine the size of its bulb will be normal).

 

D) PIGMENTS: represented by melamin (colored substances), present in hair either granular or diffuse form. These pigments are insoluble in water but soluble in strong acid and can be decoloured with hydrogen peroxide. Melanocytes are found in three principal types of melamin:

 

  • eumelanin, a dark pigment present in black hair

  • tricosendrine, a red pigment present in medium dark hair

  • pheomelanin, a yellow pigment present in light hair

 

 

CLASSIFICATION OF DIFFERENT TYPES OF HAIR STRUCTURE

 

There are 3 different types or classifications which are all visually distinct:

 

A) THE FORM

 

  • Straight hair: hair a round section along which sebum is easily transferred and is able to offer the hair structure protection.

  • Wavy hair: its oval sections make transferring sebum quite easy along the length of the hair structure.

  • Curly or frizzy hair: its ellipsodail shape makes sebum transference very difficult and leaves the hair structured unprotected and fragile.

 

B) THE DIAMETER

 

Visually identifiable and divided into 3 structures:

 

  • Fine

  • Medium

  • Thick

 

The diameter of the hair structure will influence its level of resistance during the various chemical services, and its reactivity to products.

 

C) RESISTANCE

 

Measures the ability of the hair structure to resist traction, hair can bear 80 to 160 gm of traction before breaking. A damaged hair structure is less resistant.

 

D) POROSITY

 

Defines the degree of absorption of the hair structure:

 

 

  • Healthy hair structures have a lower degree of porosity

  • Treated/stressed structures have a greater degree of porosity.

 

 

 

 

 

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