The hair (from the Latin capillus) is a particular hair which has the function of protecting the head and the scalp. Composed of keratin, it is very strong and can withstand a weight of 100g without giving way.

The hair is strictly speaking characteristic hairs of the human species. They have the particularity of being long and flexible and of covering the head. In a healthy adult, there are about 150.

Hair is essentially composed of a protein, keratin, responsible for its great strength. But it also contains water, fatty acids, melanin, zinc, vitamins and iron in small amounts.

A hair is made up of a visible part, the stem, and a root buried in a small cavity, the hair follicle.

The rod emerges on the surface of the scalp. Its color varies depending on the individual. It is made up of three layers: the marrow, surrounded by the cortex, itself enveloped by the cuticle. The latter is composed of a simple layer of cells arranged like tiles on a roof: this arrangement allows the separation of the hairs, which prevents them from tangling. The cuticle is the region that contains the most keratin, which strengthens the hair and makes it extremely strong.

The root is implanted under the skin obliquely. It sinks into the hair follicle, where the hair is produced. In its lower part is the hair bulb which has at its base, the hair papilla; It is at this level that the exchanges of nutrients and oxygen necessary for hair growth take place. Above the bulb is the sebaceous gland, which secretes sebum for the lubrication of the hair.

At the base of the follicle, we also find the arrector muscle. It contracts under the effect of cold or fear.

Hair cycle

All hair is born, lives and dies: this is the hair cycle. Not all hair is at the same stage. A cycle lasts on average 3 to 4 years and has 3 stages:

Anagen phase – Growth

85% of the hair is growing. The hair is formed at the level of the bulb and grows by the multiplication of keratinocytes, the cells which synthesize keratin. The keratinocytes move away from the growth zone, harden to form the hair shaft and then die. The hair bulb also contains a second cell type, melanocytes, which synthesize melanin, the pigment responsible for hair color. The hair growth speed is 0,9-1,3cm per month. It varies according to the type of hair, the fastest being the Asian type.

Catagen phase – Rest

Period called “involution”, it lasts 2 to 3 weeks and concerns 1% of the hair. It corresponds to the resting of the follicle: cell division stops, the follicle shortens and decreases in size.

Telogen phase – The fall

It is the complete keratinization of the hair which, in the long term, is expelled from the scalp. It lasts about 2 months for 14% of the hair. Then the cycle resumes, a new hair is produced by the follicle.

Role of hair

The hair has a minor function of protecting the head from blows.

Hair types and color

The hair has various shapes. The section of the hair shaft makes it possible to distinguish them:

• an oval section, which reflects a smooth, silky and wavy hair,
• A flat section that characterizes a frizzy hair,
• A round section that gives a stiff hair, with a rough tendency.

There are also differences between ethnic groups. An African American person will exhibit low hair density, diameter, strength, and speed of growth. For a person of Asian descent, the hair will generally be thicker and stronger.

La Hair color is governed by melanocytes which synthesize melanin. There are different colors – yellow, red, brown and black – which by combination produce the color of the hair. In the case of white hair, the melanocytes no longer work.

Alopecia : refers to hair loss leaving the skin partially or totally bare. There are different forms.

Alopecia in plaque (or alopecia areata): causes hair loss in patches, most often in the scalp. The skin of the skull retains its normal appearance, but is simply devoid of hair in places.

Baldness (or androgenetic alopecia) : refers to hair loss leaving the skin completely bare. It mainly affects men and is mainly determined by heredity.

Scarring alopecia : hair loss caused by permanent damage to the scalp due to a skin disease or infection (lupus, psoriasis, lichen planus, etc.).

Ringworm : scalp and hair disease caused by fungi, dermatophytes. Benign but very contagious infections which mainly affect children under 12 years old. It is the most common cause of alopecia in children, but the hair grows back in most cases.

Effluvium télogène : sudden and temporary hair loss as a result of physical or emotional shock, pregnancy, surgery, severe weight loss, high fever, etc.

Forked tip : the outer layer of the hair, the region richest in keratin, is highly exposed to external aggressions and decreases at the end of the hair. The keratin fibrils then contained in the inner layers rebound, a phenomenon known as split ends.

Greasy hair : shiny appearance of the hair which reflects a disturbance in the functioning of the sebaceous glands located at the level of the follicle. The sebum is produced in excess. They more easily trap dust and pollution, which can irritate the scalp and result in itching.

Dry or brittle hair: hair that has aged too quickly and whose keratin has lost its elastic properties. Therefore, they easily break when brushing, styling, or while sleeping. They are rough to the touch, difficult to disentangle, and the ends become split ends.

dandruff : benign, these are small white scales of skin corresponding to clusters of dead cells that fall from the scalp. This abnormal desquamation is due to an acceleration of the renewal of the cells of the epidermis of the scalp, caused by its inflammation by the fungus. malassezia (present naturally, it proliferates abnormally in this case). Dandruff affects one in two people in France.

Seborrheic dermatitis : benign skin disease characterized by the formation of red patches accompanied by scales (kinds of dandruff) on the surface of the skin. It mainly affects oily parts of the skin, including the scalp.

Sometimes taking certain medications can cause hair loss. This is the case with certain psychotropic drugs. Let us name for example lithium which, prescribed for bipolar disorders, has been recognized as responsible for alopecia.

Certain blood thinners, such as warfarin prescribed for people with atrial fibrillation or venous thrombosis, for example, can cause alopecia in some people. In the majority of cases, stopping the treatment or reducing the dose allows hair to regrow.

Undergoing chemotherapy or radiation therapy to treat cancer is also a condition known to cause hair loss and body hair loss. Usually temporary alopecia, the hair grows back at the end of treatment.

Hormonal imbalances, fatigue, unbalanced diet, sun or stress are all factors that can influence the health of our hair. Preventing alopecia is not possible. Nevertheless, certain measures contribute to the good health of the hair. Adopting a healthy and balanced diet avoids deficiencies and provides important nutrients for the beauty of the hair such as zinc, magnesium or calcium. Consuming foods rich in B6 vitamins (salmon, bananas or potatoes) can prevent or help treat oily hair, for example.

Treatment of alopecia

Minoxidil lotion is a treatment that slows down hair loss and stimulates hair growth. Finasteride slows down hair loss and in some cases promotes hair regrowth.

General visual examination : it is a question of comparing the aspect of baldness with a visual classification, known as Hamilton modified by Norwood. This examination makes it possible to determine which cases can benefit from treatment and which cannot (too advanced forms).

Trichogramme : examination of a hair under the microscope to examine the root, measure the diameter and quantify the fall. Allows the identification of the causes of alopecia in difficult cases.

Toxicological analysis : the hair has the capacity to store the substances which we ingest: alcohol, cannabis, ecstasy, cocaine, drugs, amphetamines, arsenic, pesticides, endocrine disruptors… the list goes on. The detection of drugs and alcohol is used in particular in the judicial field.

Hair Transplant : hair restorative surgery. Possible in people whose baldness has stabilized. This involves taking part of the scalp with hair and root behind the scalp, where the hair is programmed to last a lifetime. These implants are cut into fragments containing 1 to 5 hairs and are then inserted in areas of baldness.

The term “alopecia” comes from the Greek alopex which means “fox”. This term was chosen in reference to the loss of fur that affects this animal each spring (2).

Hair has always been a symbol of seduction in women. Already in mythology, the goddesses are described as having magnificent hair (Aphrodite and her long blond hair, Venus who took care of her own hair…).

In men, the hair is rather a symbol of strength. Let us quote the story of Samson (7) who draws his extraordinary strength from his hair. In the biblical story, he is betrayed by the woman he loves who shaves his hair to deprive him of his strength. A prisoner, he regains all his strength when his hair grows back.