Contrary to a popular belief, the formation of stretch marks is not only associated with the excessive stretching of skin. Stretch marks, also known as striae distensae, largely result from the increase in the level of cortisol in blood, a hormone which is secreted by adrenal glands. The quantity of cortisol increases during the period of pregnancy, adolescence as well as because of obesity or weight gain.
During pregnancy, the increase in the level of estrogens also contributes, to some extent, to an increased possibility of stretch marks formation. Considering the fact that hormonal pills also have the impact on female sex hormones, there is a chance that those pills may be one of the factors conducive to stretch marks appearance.
The formation of stretch marks is also influenced by a regular use of steroids during a longer period of time because they play a part in making epidermis thinner, blocking collagen synthesis in dermis and degrading currently existing collagen fibres, among other things.
As you will learn in a moment, a formation of stretch marks is connected with an incorrect synthesis of collagen in skin. An improper and poor in nutrients (vitamins, microelements and dietary minerals) diet is also named as one of the factors contributing to the formation of stretch marks. What is more, the insufficient quantity of zinc is particularly of great importance. Therefore, women who are frequently on low-calorie diets are prone to stretch marks. Finally, genetic tendencies appear to be another significant factor on which you have no influence whatsoever. By reason of this, there is a chance that if stretch marks formed on  the body of a mother during her pregnancy, they may (but they do not have to) appear on the body of her daughter.

Stretch marks are formed on the level of dermis, whereas their formation is associated with the impairment of fibroblasts, namely cells responsible for the production of collagen fibres and elastines. Collagen and elastine are albumen of a fibril structure and both of them create the arranged network, hence providing the skin with flexibility, firmness, springiness and proper tension. A properly built collagen fibre is distinguished by its remarkable resilience to stretching. You have to apply the load of at least 10 kilograms in order to break a collagen fibre of a 1mm diameter (1).
Fibroblasts which are impaired, for instance due to the increased level of cortisol in blood, start producing modified fibres of collagen and elastine which become more fragile and less prone to stretching. When the skin is excessively stretched, for example during pregnancy, this regular network is broken due to the fragility of fibres, which results in the appearance of stretch marks. Given that epidermis is made thinner at the same time, stretch marks become more visible.

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