The functioning of the skin
- General: 2m² of surface area, 16% of the mass of the body with an average thickness of 1.2 mm
- Its role: barrier function + thermoregulation + transmission of information + vehicle of the immune response
- The epidermis: keratinocytes + melanocytes
- The dermis: fibroblasts + elastin
- The hypodermis: adipocytes
Our skin is an organ in its own right of our organism. And it impresses with its surface area, although it is not as large as that of our intestines or lungs. It obviously depends from one individual to another but is on average 2m². What is impressive, imagine a piece of leather 2m long by 1m wide and 1.2 mm thick. It is also heavy because it represents 16% of the weight of the body! After these few figures we quickly understand that our skin is large and heavy but what is its role?
2. Its role
Its main role is its barrier function, which we already mentioned in episode 5 on hyaluronic acid. Indeed, the main role of the skin is to make a physical barrier between the outside of our body and the inside, thus it prevents foreign bodies from entering our body. Similarly, it also prevents water loss from our body to the outside.
In addition to performing the function of a barrier, the skin also serves to regulate the temperature thanks to the sweating process, it also receives sensory information and is involved in the immune response process.
But what is it for? And how is it organized?
Now that we know what it is used for, how is it organized?
The skin is organized into three layers, the closer the layer is to the surface, the thinner it is. The most superficial layer is the epidermis.
3.a 1st cell type
The epidermis is the most superficial layer of the skin, it does not contain blood vessels and is composed of 4 types of cells. But we will only concern ourselves with two for simplicity. Keratinocytes are cells that organize themselves like bricks on a wall. They are renewed continuously at the level of the basal part of the epidermis.
Then the new cells push the old cells more and more towards the superficial part. The morphology of keratinocytes changes a little depending on their depth in the epidermis. They will be more elongated towards the basal part and crush and die on reaching the superficial part of the epidermis.
Besides, the superficial part is the stratum corneum because the keratinocytes become corneocytes by dying and approaching the surface of the skin. They are also said to be dander .
So I'm sure you see the link with what is called desquamation , which refers to the loss of our scales, therefore our corneocytes, also called dead skin cells.
To come back to keratinocytes, its lifespan, from its production to its desquamation, is around 28 days, which is why I would always advise you to evaluate the effectiveness of a cosmetic product in 28 days because this is the time it takes for the epidermis to renew itself .
3.b 2nd cell type
Then the second type of cells in the epidermis are the melanocytes. Melanocytes are not in the form of bricks like keratinocytes but in the form of stars.
These are the cells that pigment the skin . Their number, their sizes and the type of pigments they produce will influence the skin phototype. The phototype is a classification of skin colors and their sensitivity to the sun. For example, white skin that burns in the sun is phototype I, while dark brown skin that never burns in the sun is phototype VI.
The second layer under the epidermis is the dermis, which we will see in the next part.
4. The dermis
Deeper in the epidermis, there is the dermis. The dermis contains an abundant network of blood vessels that help regulate temperature in particular.
It also contains collagen and elastin. Collagen is a protein that supports tissues and elastin is a protein that gives skin elasticity. The quantity of these two proteins decreases with age and explains the sagging of the tissues.
They are produced by cells called fibroblasts and since collagen and elastin are proteins, they are not cells and they are part of the outside, the outside of the cells, which is called the extracellular matrix.
The dermis also contains the bulbs of the hairs and the glands which secrete the sebum which are connected to them. It contains sensory receptors to receive sensory information, that is to say so that we feel the pressures applied to us on the skin as well as the temperatures.5. The hypodermis
And then the hypodermis is the deepest layer that contains the fatty cell called adipocytes, they help regulate temperature, shock absorber and act as an energy reserve.
The skin diagram