Skin

In a few figures:

1.75 m² surface area
3.5 kg without grease. Heaviest organ
0.5 à 5 mm thick depending on the area of the body
20% of the body’s total water
Skin contains 70% water

In 1 cm² of skin: 10 hairs, 10 nerves, 100 sebaceous glands, 150 sweat glands and 3 million cells.

Some information

The skin is composed of three main layers: the epidermis, the dermis and the hypodermis.

The epidermis is the first protective layer: 0.04
mm on the eyelid to 1.6 mm on the palm of the hand and sole of the foot. Composed of keratinocytes and melanocytes.

The stratum corneum (0.01 to 0.02mm) is made up of corneocytes, which are keratinocytes that have lost their nucleus. They are flattened and superimposed, forming keratin and filagrin lamellae whose cohesion is ensured by a lipid cement.

This lipidic cement is made up of ceramides, cholesterol, enzymes and free fatty acids from the cells and sebum. It is the hydrolipidic film that allows us to keep our skin moisturised and helps it to be supple. It is also essential to protect the skin from microbial and mycotic aggressions.

The stratum corneum peels permanently (desquamation). This is cell renewal. It occurs every 3 to 4 weeks, not simultaneously.
The beauty and radiance of the skin depend closely on the hydration of the horny layer and the quality of its hydrolipidic film.

The PH of the skin in equilibrium is about 5.5, therefore slightly acidic to be able to eliminate the microbial agents which do not support acidic environments

The dermis is composed of a network of blood vessels, nerve endings and fibroblasts which produce collagen and elastin, molecules capable of resistance and elasticity. The matrix in which all these elements are immersed is made up of hyaluronic acid, which binds water like a sponge. It is from this water reservoir that the epidermis pumps the water necessary for its hydration.

The dermis also contains the sweat glands and the pilosebaceous follicles.

The hypodermis is the deepest part and is the energy reserve. It is the location of fat cells, blood vessels and nerves.
It shapes the body, cushions shocks and regulates our temperature.

Very often thinner people are more cold than others.

Sebaceous glands are small sacs whose walls produce sebum which accumulates in the centre.
The sac communicates with the surface of the skin through the channel of a hair or fuzz. The sebum flows to the surface of the skin and mixes with sweat and epidermal debris to form the hydrolipid film. Sebum is protective.
The sebaceous glands are mostly found on the upper body. These are the acne-prone areas: head, neck, shoulders, décolleté and upper back. These are the acne-prone areas. They are sensitive to hormones. In particular testosterone, which stimulates their secretion.
When the skin degreases, the gland immediately regreases. Sweating also increases sebaceous secretion.

The sweat glands produce sweat.

Sweat is a colourless liquid that is secreted continuously.
There are two types of sweat on the body:

Eccrine sweat is present on the forehead, chest, hand and foot areas. It increases with physical exertion and increased temperature. It contains mainly water and substances such as urea, lactic acid, organic acids, amino acids, mineral salts, enzymes, vitamins, sugars and is mixed with the hydrolipidic film. It has an acid PH

Apocrine sweat is present in the armpits, mammary, umbilical, pubic and anal regions. Apocrine glands have an intermittent secretion: fear, sexual excitement. They only become functional at puberty. They contain odorous molecules and pheromones. Microbes on the surface of the skin degrade the sweat and change the smell, which can make it unpleasant.

The skin is permeable because many substances can pass through it. There are two main routes of passage: inter- and intracellular, then through the pilosebaceous follicles and sweat glands.

The most permeable areas of the body are the scrotum, the inside of the wrists, the eyelids and the retroauricular area (behind the ear).

If the stratum corneum is damaged (depilation, de-lipidised, over- or under-moisturised etc), it may exceed the expected absorption of certain chemical ingredients.

The stratum corneum functions as a reservoir and distributes the applied substances over a certain period of time. Smaller molecules pass more easily.

The nature of the active ingredient (water-soluble, fat-soluble) as well as its concentration and permeability coefficient, the excipient (vehicle) alcohol, vegetable oil, liposome, etc.) are determining factors for penetration.

The application on a large surface, the occlusive mode (to put a film on the substance forces the natural brake of the corneal layer by increasing the temperature and the degree of hydration), the massage also facilitates the penetration of the active principles.

Thanks to the lipidic component of the horny layer, the skin is impermeable to water. To moisturise the skin, water must be contained in a fatty cream.