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Basically, they are an off-color type of scar visible on the surface of the skin. Stretch marks are caused by tearing beneath the top layer of skin, and while they may fade over time, they will never disappear completely. These scars appear in clusters as thin, parallel channels or grooves in the skin.
And unfortunately, stretch marks can appear anywhere that fat is stored, such as the abdomen, back, breasts, buttocks, hips, near the navel, on inner and outer thighs, as well as under arms, and upper arms. They cause no health risks and have no effect on normal body functions. What Causes Stretch Marks Stretch marks are caused by rapid stretching that occurs during periods of growth spurts, rapid weight gain, or possibly rapid increases in muscle mass.
The development of these striations are also significantly influenced by hormonal fluctuations that occur during hormone replacement therapy, pregnancy, puberty, etc.
Stretch marks first appear as reddish or purple in color, but tend to fade gradually to a paler hue. Stretch marks develop beneath the surface in the stretchy middle layer of skin responsible for retaining skin’s shape. Inadequate support within the dermis allows the development of these stripes, which resemble soft grooves or channels.
Where the marks appear and what direction they run are determined by where and how stretching occurs. What Are Stretch Marks – Damaging Hormones Many physicians believe hormones known as glucocorticoids damage the elastin and collagen fibers. These deeply interlacing strings are knit together to form new skin, and normally keep skin tight. However, when these protein strands become damaged, they are no longer capable of retaining skin’s shape.
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From: (why do you have stretch marks) http://www.stretchmarkremovalremedies.com/what-are-stretch-marks.html