Hair is a keratinised (based on keratin) cutaneous element with unique properties to which three main functions can be attributed:
It is estimated that the human body has some 5,000,000 hairs, of which from 100,000 to 150,000 hairs are found in the scalp.
PROTECTIVE
against sunlight, temperature changes, bumps, rubbing, etc.
TACTILE
increasing sensory perception
AESTHETIC
increasing sensory perception
ANATOMY OF A HAIR
The structural and functional unit is called the pilosebaceous follicle. It is composed of the hair itself (visible part), a hair follicle (under the skin where the hair originates), a sebaceous gland (which releases its contents through the canal where the hair comes out), a hair erector muscle (which causes the appearance of “goosebumps” when it contracts) and an apocrine sweat gland.
The hair has an outer part known as the “shaft” and an inner one (anchored in the skin), called the “root”. The hair has a layered structure, formed by three concentric layers (medulla, cortex and cuticle); during the hair’s growth inside the follicle, other layers protecting and anchoring it (epithelial root sheath, accompanying layer, glassy membrane and connective tissue sheath) enclose them.
GROWTH CYCLE
All the hairs of the body grow to a planned length that is characteristic for each body area and for each individual. The hairs are constantly renewed from birth, with others of the same size and length replacing them. This happens thanks to the fact that the follicles have periods of growth and rest, known as the “hair grown cycles”.
ANAGEN PHASE
CATAGEN PHASE
TELOGEN PHASE
This is the phase when active growth of the hair takes place. The matrix cells reproduce and produce the 3 hair layers and the epithelial root sheaths.
The melanocytes initiate the formation of melanin, which gives a dark colour. As these cells ascend, they get bigger, become keratinised and differentiate into the different layers.
At some moment, the distal end of the new hair replaces the old hair (which is in the telogen phase).
The follicle enters this phase when the hair stops growing.
Hair matrix proliferation halts, as does the activity of the bulb melanocytes. What is called “club hair” forms, a keratinised structure made up of cortical cells and cuticle that is enclosed by remains of the inner epithelial sheath.
The cells that will give rise to the new follicle come from the germinal cells in the bulge. The follicle gradually degenerates and is finally reabsorbed.
The “club hairs” of the follicles remain inactive and latent.
The hair is now loose within the follicle and can be pulled out painlessly because the root has lost its attached inner epithelial sheath.
How long each cycle and phase last is intrinsic to each follicle and independent of the others. In the scalp, the average length of the anagen phase is 3 years, the catagen phase lasts an average of 3 weeks and the telogen phase, 3 months.
In a normal scalp about 13% of the follicles are in telogen, 1% in catagen and the remainder (approximately 85%), in a growth phase (anagen).
