Understanding skin at different ages.

The skin is composed of three layers, from the most superficial to the deepest:

• Epidermis:

  This is the most superficial layer of the skin. It contains several cellular populations (keratinocytes, melanocytes, Langerhans cells, etc...).

  The keratinocytes are present at various levels of the epidermis. They synthesize keratin, a fibrous and water-insoluble protein, which gives the skin its waterproof nature. It thus protects the skin from external aggressions.

  The primary role of the epidermis is its function as a skin protector thanks to keratinocytes and the stratum corneum. Indeed, the stratum corneum is composed of various epidermal lipids that shield the skin from external aggressions and transepidermal dehydration. Additionally, it is covered by a hydrolipidic film that acts as an additional protective barrier. This hydrolipidic film is primarily composed of sebum secreted by the sebaceous glands.

  The skin also contains melanocytes. When exposed to UV rays, melanocytes synthesize melanin, a pigment that serves to protect our skin from UV rays. Therefore, tanning is actually a defense mechanism of the skin!

• Dermis :

  It is at the level of the dermis that new skin cells multiply in order to replace those that are eliminated. Furthermore, the dermis contains the fibroblasts which synthesize collagen fibers, elastin, as well as hyaluronic acid, responsible for the density, elasticity, and firmness of the skin.

• Hypodermis:

  Its primary role is to manage the storage and release of lipids. It also plays a part in thermoregulation.

It is around the age of 20 years that the first signs of aging can begin to appear. We observe a beginning of the decline in the production of collagen fibers of about 1% per year. Hyaluronic acid also decreases starting from the age of 20. This results in the appearance of the first fine lines, dehydration, and loss of firmness. The skin's barrier function gradually weakens, making the skin more vulnerable to external aggressions and transepidermal dehydration.

In our thirties, the skin's barrier function weakens even further due to a decrease in keratin production by keratinocytes. As a reminder, keratin is a molecule that plays a role in protecting the skin. Additionally, the sebaceous glands produce less and less sebum, which weakens the hydrolipidic film and therefore the skin barrier. Skin dehydration increases and the skin progressively loses its elasticity. Fibroblastic activity, and therefore the production of collagen, elastin, and hyaluronic acid fibers, decreases, promoting the loss of skin density, elasticity, and firmness.

At the level of the epidermis, we observe a decrease in lipid levels and a slowdown in skin renewal. As a result, the skin becomes more sensitive due to a weakening of the barrier function, and its structure becomes granular. Additionally, the melanocytes decrease gradually, which leads to poorer protection against UV rays, as well as irregular pigmentation (pigment spots). The Langerhans cells, which are epidermal defense cells, also decrease, causing an increase in the risk of skin infections.

Within the dermis, the activity of fibroblasts becomes increasingly weak, leading to the deepening of expression lines, a less plump skin envelope, and a loss of firmness and elasticity. We also observe a decrease in the skin's blood supply, which results in a loss of skin radiance.

At the level of the hypodermis,the number and size of adipose cells decrease in the fatty tissue. The skin loses volume (hollowing of the cheeks), and under-eye bags and deep wrinkles appear.

We observe a significant weakening of the skin's natural lipid production ability at the epidermal level, exacerbating dehydration, the appearance of deep wrinkles, and skin sagging. The skin's cellular renewal is significantly slowed, which impairs the healing process. The skin is also very vulnerable to infections due to a decrease in immune function by Langerhans cells.

• ORTONNE J. P. & al. Le vieillissement cutané et sa prévention. La Presse Médicale (2003).

• PUIZINA-IVIC N. Skin aging. Acta Dermatovenerologica Alpina Pannonica et Adriatica (2008).

• ELLIS D. A. F. & al. Aging skin: histology, physiology, and pathology. Facial Plastic Surgery Clinics of North America (2011).

• DESMOULIERE A. & al. Skin changes during ageing. Subcellular Biochemistry (2019).