Hair is a living element that grows, stops growing and falls: this is the hair cycle.

The hair cycle in humans is mosaic: the hair of the same individual is not at the same phase of the cycle.

For the same hair follicle, the cycle is repeated 20 to 30 times in a man's life.

The cycle is variable depending on the anatomical location, nutrition, age, existing treatments, seasonal factors (light), hormonal factors (androgens). Whatever the stage of the cycle, the infundibulum, isthmus, sebaceous gland, the upper part of the outer epithelial sheath, the fibro-conjunctive sheath and the dermal papilla are permanent structures.

The bulb, the GEI and the lower part of the GEE are transitional structures.

The duration of a cycle is 3 to 6 years and has 3 phases:

1. Anagen phase (2 to 4 years in men, 4 to 6 years in women):

85% of the hair on a normal scalp is in the anagen phase.

During the anagen phase, the hair grows from 1 to 1.5 cm per month or 0.35mm per day. Growth is faster in women, children and the vertex.

The anagen phase is characterized by an intense proliferative activity of the cell populations of the pilosebaceous follicle.

The anagen phase itself breaks down into several sequences.

Anagen stage I: Stem cells migrate from the GE and go to the dermal papilla which they envelop.

These stem cells begin to proliferate and the papilla migrates deep.

Anagen stage II: GEE accompanies this migration to depth.

Anagen stage III: Reaching its maximum size, the hair matrix forms the IEG and the hair shaft. The cup-shaped hair matrix envelops the dermal papilla.

Stage IV anagen: The melanocytes surrounding the dermal papilla become active, produce melanin and develop dendrites.

At this stage the hair shaft is already present inside the GEI.

Stage V anagen: the hair shaft then leaves the GEI

Anagen stage VI: the new hair appears on the surface of the skin.

Under 20 to 40 magnification, anagen hair has a large, pigmented bulb.

During the early anagen phases, the bulb is very large and has a pyramidal appearance.

During late anagen phases, the bulb becomes narrower and quadrangular with sustained pigmentation.

The mechanism of initiation of a new anagen cycle is not yet understood: interaction signals dermal papilla-multi-potent keratinocyte cells of the EGE?

2. Catagen phase (3 to 4 weeks):

1% of the hair on a normal scalp is in the catagen phase.

During this phase, the lower portion of the follicle goes through a phase of involution or also called the phenomenon of apoptosis.

Thus, during the catagen phase, the dermal papilla condenses, migrates superficially and is placed under the base of the telogen follicle. the matrix condenses, detaches itself from the dermal papilla and migrates towards the surface to be at the level of the secondary epithelial residual germ cells of the bulge going to under the hair in the telogen phase: this is the beginning of the catagen phase. , the dermal papilla migrates upward with the hair bulb to level with the residual secondary epithelial germ cells of the EGE in the region of the bulge.

The glass membrane thickens and wrinkles.

The hair matrix is gone.

The GEI begins to disappear.

The GEE is refining.

The GEE, during the catagen phase, becomes thinner and shortens until it forms the epithelial sac which will envelop the completely keratinized bulb.

The bulb will itself change appearance and thus forming a club.

GE presents a phenomenon of apoptosis (condensation of the cytoplasm and phagocytosis of epithelial or macrophagic cells.

Stopping the formation of pigments, stopping the growth of epithelial parts of the hair follicle, restructuring of the matrix. Apoptosis of keratinocytes.

Under 20 to 40 magnification, the catagen hair is recognizable because its bulb is narrower and begins to depigment. The epithelial sheaths begin to shorten.

3. Telogen or exogenous phase: (about 3 months):

15-20% (10-15% in women) of hair follicles in a normal scalp in men are in the telogen phase.

The hairs are characterized by a longer telogen phase duration.

The hair gradually rises in its sheath towards the surface before being replaced by the next. During the telogen phase, the hair follicle will regress over half of its initial length.

The telogen hair presents itself with a rounded bulb, globular, totally keratinized with clear melanin and an appearance of cotton swab or club

The matrix, the keratinization zone, the medulla and the sheaths have disappeared.

The GEE has involuted and has become the epithelial sac which may exhibit pigmentations.

The adhesion of the hair to the bag is very strong. When the hair falls spontaneously, the rod is detached from the bag. When the hair is pulled out the rod comes with the bag. (It is easier to extract the bag from other structures than to extract the rod from the bag.

In recent telogen, the epithelial sac may present a threadlike extension.

The early telogen hair is surrounded by an epithelial sac.

Mature telogen hair no longer has an epithelial sac. He falls on his own. This represents a daily loss of 25 to 60 hairs.

Washing or brushing results in removal of mature telogen hair.

The telogen phase undergoes intense metabolism.

After an inhibition phase, the stem cells located at the level of the GEE under the action of still unknown stimuli, begin to proliferate and to migrate towards the dermal papilla. This migration would be guided by the vitreous membrane.

A new anagen hair is therefore being formed.

Lag phase: some authors have recently described a resting phase or “lag phase” during which certain follicles are empty without an occupying or developing hair shaft.

This phase can last from 2 to 5 months.