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Stretch marks or striae (singular stria), as they are called in dermatology, are a form of scarring on the skin with an off-color hue. They are caused by tearing of the dermis, and over time can diminish but not disappear completely.
Stretch marks are often the result of the rapid stretching of the skin associated with rapid growth (common in puberty) or weight gain (e.g. pregnancy, muscle building, or rapid gain of fat) or, in some cases, severe pulling force on skin that overcomes the dermis’s elasticity. Stretch marks may also be influenced by hormonal changes associated with puberty, pregnancy, muscle building, hormone replacement therapy for transsexuals, etc. Medical terminology for these kinds of markings includes striae atrophicae, vergetures, stria distensae, striae cutis distensae, striae gravidarum (in cases where it is caused by pregnancy), lineae atrophicae, linea albicante, or simply striae.

Symptoms and signs
They first appear as reddish or purple lines, but tend to gradually fade to a lighter range. The affected areas appear empty and are soft to the touch. Stretch marks occur in the dermis, the resilient middle layer that helps the skin retain its shape. No stretch marks will form as long as there is support within the dermis. Stretching plays more of a role in where the marks occur and in what direction they run. Stretching alone is not the cause. Stretch marks can appear anywhere on the body, but are most likely to appear in places where larger amounts of fat are stored. Most common places are the abdomen (especially near the navel), breasts, upper arms, underarms, back, thighs (both inner and outer), hips, and buttocks. They pose no health risk in and of themselves, and do not compromise the body’s ability to function normally and repair itself.

Causes
A number of causes promote the appearance of stretchmarks: one study of 324 women just after giving birth demonstrated that low maternal age, high body mass index, weight gain over 15 kg (31 pounds) and higher neonatal birth weight were independently associated with the occurrence of striae. Teenagers are at highest risk of developing severe striae. The glucocorticoid hormones responsible for the development of stretch marks affect the dermis by preventing the fibroblasts from forming collagen and elastin fibers, necessary to keep rapidly growing skin taut. This creates a lack of supportive material, as the skin is stretched and leads to dermal and epidermal tearing. Skin subjected to more stretching force than it can handle will tear. Hormonal changes and genetics influence the skin’s capacity to withstand stretching forces, as do diet and [possibly] exercise.