The various types of wrinkles.

Wrinkles are one of the revealing signs of skin ageing. They are visible structural changes on the skin's surface, appearing as folds and grooves. They result from a decrease in the number of fibroblasts in the skin, which are key cells in the dermis that produce collagen fibres, responsible for maintaining skin firmness, and elastin, responsible for preserving its elasticity, as well as hyaluronic acid molecules to maintain good hydration. Indeed, as the skin ages, their production by the body decreases. Additionally, keratinocytes divide more slowly with age, causing the thinning of the epidermis which becomes more vulnerable and folds more easily, or even due to a decrease in the subcutaneous fat layer (hypodermis) that shapes the volumes of the face.

They are not necessarily and solely due to skin ageing. Several secondary factors can be the cause of these facial wrinkles, notably theconstant pull of gravity, the frequent and constant positional pressures on the facial skin (for example during sleep), and the repeated movements of the face caused by the contractions of the mimetic muscles of facial expression (for example smiling, lip curling, frowning...). Thus, we distinguish two main types of facial wrinkles, corresponding to two distinct formation processes: static and dynamic.

Expression lines are the first to appear on the face. Indeed, around the age of 30, the production of collagen and elastin begins to decrease by about 1% per year. They form due to the constant movements of the facial muscles in our daily life when we make expression gestures, such as frowning, widening the eyes, squinting, clenching the jaw or smiling. These multiple contractions gradually reduce the skin's elasticity and cause it to fold.

It is important to understand that the face is equipped with 43 facial muscles capable of performing approximately 10,000 contractions per day to convey our various emotions. This type of wrinkle thus reveals our different expressions.

They do not always appear on the skin, but only when we make a facial expression. However, over the years, due to the constant contraction of muscles, these folds eventually become permanent and deepen due to alterations occurring within the skin layers. Depending on their location, there are different types of expression lines, including:

• Forehead Wrinkles : These are horizontal lines that appear each time we raise our eyebrows;

• Lion's Wrinkles (inter-brow or glabellar lines) : These refer to vertical or horizontal furrows located between the eyebrows. They are a result of the contraction of the muscles in this area, particularly when we frown;

• Bunny lines (or witch's wrinkles): This type of wrinkle refers to the folds located on the lateral and upper edges of the nose, a result of the repeated contraction of the muscles that lift the nostril and upper lip. They particularly appear when we express our discontent;

• Crow's feet (or periorbital wrinkles): Located around the eyes, these are the first wrinkles to appear, especially when we laugh or smile. The skin around the eye is heavily used and is 4 to 5 times thinner than the rest of the face, in addition to performing up to 28,000 blinks per day. This could explain the early appearance of small folds. Indeed, our eyes crinkle and deepen these fine grooves.

A lack of hydration tends to promote the appearance of these expression lines. Therefore, it is essential to moisturise your skin several times a day by applying a moisturising cream morning and evening, as well as a serum enriched with hyaluronic acid.

Static wrinkles make their appearance later in life. They are the result of a natural decrease in structural proteins such as collagen and elastin, leading to a loss of firmness and elasticity. This phenomenon can also be linked to the general natural sagging of skin tissues under the influence of gravity, which pulls the skin downwards, thereby accentuating the effects. Indeed, with the loss of collagen and elastin, the skin is less toned to combat this force. They can be found everywhere, even on the body. This is how they form:

• Nasolabial folds (smile lines or nasolabial grooves): This type of wrinkle extends from the wings of the nose to the corners of the mouth. They often result in a sagging of the cheek and are more noticeable when one laughs or smiles;

• Solar Pleated Wrinkles (also known as smoker's wrinkles or barcode lines): These extend vertically below the nose around the contour of the upper lip. They are more pronounced in smokers and in seniors who are often exposed to the sun without protection;

• Marionette lines (labiomental folds or puppet lines): These are vertical wrinkles that extend from the outer corners of the mouth towards the chin;

• Neck wrinkles (or Venus rings) and décolletage: They manifest as horizontal lines. This phenomenon is explained particularly by the loss of subcutaneous adipose tissue which will highlight certain bony areas of the face and thus accentuate the signs of ageing, but also by bone loss. Moreover, the skin of the body is one of the thinnest on the body. As a result, it is more susceptible to external aggressions (sun exposure, etc...), which tend to cause a premature sagging of the skin

• Sleep Wrinkles : The position adopted during the night (face squashed against the pillow) also has an impact on skin ageing. The folds imprinted on the skin night after night eventually become permanent. They are often vertical, most commonly located on the cheeks, neck and décolletage, and can be more pronounced on one side than the other depending on the position in which you sleep during the night.

One of the accelerators of this phenomenon? The sun. Under the influence of solar radiation, fibroblasts become disrupted and their synthesis capacity deteriorates. The elastin and collagen fibres produced are then of poor quality, fragmented: they no longer successfully fulfil their supportive role. Moreover, prolonged sun exposure tends to thicken the skin, making UV-induced wrinkles appear deeper as the formed crease is thicker.

• HOTTA M. & al. The initial study of the correlation between facial movements and wrinkle development.Skin ResearchandTechnology (2012).

• HOSAM W. & al. Forehead wrinkles: a histological and immunohistochemical assessment. Journal of Cosmetic Dermatology (2014).

• LAMBROS V. & al. Sleep wrinkles: facial ageing and facial distortion during sleep. Aesthetic Surgery Journal (2016).