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The physiology of the scalp

The scalp refers to the part of the skin covering the skull. This continuous, flexible and resistant covering is composed of several layers. It is the living part of the hair that is implanted in its surface layer.

In adults without hair loss, the scalp measures between 600 and 700 cm². With 2,500 receptors per square centimeter, it's an extremely sensitive part of our body.

It houses 100,000 to 150,000 keratinous hairs. This figure varies from person to person.

It’s made up of a protective tissue on the surface (the epidermis) and a nourishing support tissue deep down (the dermis), which contains numerous blood vessels from which the hair root (the hair follicle) has developed an exchange zone to draw the hair's vital energy.

It’s very important that the scalp, the area in which the hair bulb is implanted, is healthy and provides the nutrients necessary for hair growth.

A healthy and balanced scalp is capable of growing beautiful hair full of vitality. It’s also a scalp that’s able to perform its functions as a physical barrier and thermal insulator.

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The life of a hair

It is important to know the life cycle of the hair in order to understand and promote the factors that are essential for good hair health.

Each individual's hair capital is made up of 25 hair cycles that are programmed to last a lifetime.

How do the 3 consecutive phases of a cycle take place: the anagen phase, the telogen phase, and the catagen phase? And what are the challenges of a regular and constant cycle?

The scalp: a complex balance

A cross-section of the scalp shows several layers.

The epidermis is a very thin layer measuring 1/10th of a millimeter. It combines with the dermis beneath it to form the external epithelial sheath, which is therefore continuous with the epidermis itself. Like the epidermis, it’s composed of several layers of cells.

The dermis is a very vascular area of the scalp. It is thick and measures on average 2.5 millimeters. This is where the root of the hair is located: the hair follicle. Surrounded by a rich vascular network, it benefits from a large supply of energy, which is essential for hair growth and for promoting survival and cell renewal.

The hypodermis is made up of fatty tissue in which the hair bulb is located deep down (3.5 centimeters from the surface).

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Hair: from root to tip

The hair follicle is located in the dermis. It can be described as a “sack” containing the hair. It is the matrix of the hair, the area where cell division is active. A true miniature organ, it constantly produces cells whose accumulation and keratinization give rise to the hair shaft.

The bulb lies obliquely in the scalp.

The bottom of the bulb is hollowed out by a richly vascularized and innervated follicular papilla. It receives hormones, transmits signals to the keratinocytes and provides the hair with vitamins and oxygen.

When the circulation is active in the papilla, hair growth is rapid. If the circulation is not active, growth slows down. Destruction of the papilla leads to permanent hair loss and the death of the hair follicle.

The hair follicle is programmed to produce 25 hairs, which corresponds to the 25 hair cycles an individual has in their lifetime.

Attached to the bulb is the sebaceous gland, which produces sebum.

This complex emulsion of water, sweat and sebum is the essential component of the HLF (hydrolipidic film). The surface of the hair and scalp is enveloped by this protective film. It combats the penetration of foreign substances (UV rays, pollution, etc.) and regulates hydration by preventing transepidermal water loss (TEWL). It is also responsible for the soft and silky appearance of the hair.

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The hair shaft is the free and visible part of the hair. Its diameter ranges from 70 to 100 microns.

It’s made up of 95% keratin, a fibrous protein formed by chains of amino acids synthesized by cells located at the bottom of the hair follicle. Twisted into a helix, it forms the skeleton of the hair. It is this that gives hair its exceptional resistance and elasticity. Thanks to it, the hair, despite its thinness, is extremely strong.

The hair shaft has several characteristics: elasticity, low resistance to chemical agents, good mechanical resistance, high water absorption capacity, resistance to biological agents and elastic properties.

Inside, the presence of melanin, the natural pigment of the skin and hair, gives the hair its color.

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The hair shaft is broken down into three concentric layers: the medulla surrounded by the cortex, which is itself enveloped by the cuticle.

• The inner layer: the medulla is located in the center and has no specific physiological properties. It is present in mature hairs.

• The middle layer: the cortex is the most important and thickest part of the hair shaft. It itself is composed of several layers of pigmented keratin cells that are held together by intercellular cement, which ensures the strong cohesion of the hair cells and the impermeability of the hair. Making up 80% of the hair, the cortex gives it its structure and color.

• The outer layer: the cuticle. The hair is protected by the cuticle, which is made up of protective scales that interlock with each other, like tiles on a roof. They are sheathed by a thin layer of sebum, which ensures the shine of the hair shaft. The scales act as transparent protection and give the hair its shine and silkiness.

The cell renewal of the scalp takes 14 days. However, it can be accelerated in event of inflammation.

The physiological roles of the scalp

The scalp plays 2 major roles.

 It houses the hair follicles from which the hair is born and which ensure its growth.

It acts as a real physical protective barrier. 

The location of the hair follicles

The scalp differs from the skin of the body in the abundance of hair follicles it contains, the size of the hairs, and the number and volume of sebaceous and sweat glands. It also has a rich nervous and vascular network, which is necessary to supply the substances that are essential for the growth of strong hair.

It is a true reservoir of hair follicles, which are housed in very large quantities in the superficial layer. On average, there are between 100,000 and 150,000.

At the base of the hair follicle is a multitude of small blood vessels called the microcirculation. Their function is to transport blood to the hair bulbs to supply them with nutrients and oxygen, which are essential for the growth of strong, healthy and bouncy hair.

It is this narrow passage that allows the bulb cells, the keratinocytes, to continuously produce the components of the hair fiber.

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A real physical protective barrier

Just like the skin, the scalp performs a variety of functions.

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• A physical barrier: the scalp effectively protects the internal environment from the external environment and from dehydration. The hydrolipidic film on the surface of the scalp acts as a shield against aggression and regulates hydration. It limits skin dryness and lubricates the hair. It gives a soft, silky and shiny appearance to the hair.

• Immunological protection: the HLF has a naturally acidic pH, which protects against aggression, radiation and micro-organisms.

• A thermal insulator: the scalp is involved in the body's temperature regulation mechanism. In fact, it produces and dissipates heat in order to maintain a constant internal temperature of 98.6°F.

• Sweat actions: sweating contributes to this thermal insulation. It is a phenomenon that essentially consists of dissipating heat thanks to the mechanism that evaporates water from the surface of the skin and therefore from the scalp.

• The sebaceous function: the sebaceous gland, attached to the hair, secretes sebum, the essential component of the surface’s hydrolipidic film (HLF). By limiting scalp dryness, it ensures a bactericidal function and lubricates the hair. It therefore plays a protective role for the scalp.

The scalp is truly an organ that needs to be cared for. The health and beauty of the hair depend on it.

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