The causes of the appearance of stretch marks.

Stretch marks are micro-lesions, visible on the skin's surface, very common and comparable to scars. They are the result of a change in the structure of the dermis and a rupture of collagen fibres.

Often localised on specific parts of the body such as the thighs, abdomen, buttocks, hips, chest, lower back, upper arms, and the inner side of the knees, they appear on the skin as elongated lines or streaks.

Upon their appearance, stretch marks are red/purple and have a somewhat raised appearance. This is the so-called inflammatory phase (immature stretch marks). The blood vessels are dilated and the skin can be painful. After their healing, the stretch marks become white and painless. No treatment can act against this type of stretch marks (mature stretch marks), they are permanent.

The primary cause of stretch mark formation is asudden distension of the skin. They can particularly manifest during a rapid and significant weight fluctuation. The skin tissues then undergo high traction, leading to a rupture of collagen fibres and the appearance of stretch marks. It's worth noting that collagen is a protein naturally present in the dermis and contributes to its structure. A decrease in the amount of collagen or damage to these fibres can accelerate the skin ageing process. The skin then loses its suppleness and elasticity.

Stretch marks can also be due to ahormonal dysfunction. Stress, in particular, can intensify the production of cortisol. This hormone plays a significant role in regulating blood glucose levels and in releasing sugar from the body's reserves. When produced in excess, it can reduce collagen production, thereby promoting the appearance of stretch marks. At high concentrations, cortisol is indeed responsible forthe inhibition of the TGF-β1 growth factor, which is involved in tissue development and stimulates collagen synthesis by fibroblasts.

Finally, genetics play a role in the development of stretch marks. Some people have very thin skin due to a lack of production of elastic fibres, that is, collagen and elastin. More fragile, this type of skin is more likely to develop stretch marks. This is also why men are less prone to this phenomenon than women: men's skin is approximately 20% thicker.