Scars are areas of fibrous tissue (fibrosis) that replace normal skin after injury. A scar results from the biological process of wound repair in the skin and other tissues of the body. Thus, scarring is a natural part of the healing process. With the exception of very minor lesions, every wound (e.g., after accident, disease, or surgery) results in some degree of scarring. An exception to this are animals with complete regeneration, which regrow tissue without scar formation.

Scar tissue is composed of the same protein (collagen) as the tissue that it replaces, but the fiber composition of the protein is different; instead of a random basket-weave formation of the collagen fibers found in normal tissue, in fibrosis the collagen cross-links and forms a pronounced alignment in a single direction. This collagen scar tissue alignment is usually of inferior functional quality to the normal collagen randomized alignment. For example, scars in the skin are less resistant to ultraviolet radiation, sweat glands and hair follicles do not grow back within scar tissues. A myocardial infarction commonly known as a heart attack, causes scar formation in the heart muscle, which leads to loss of muscular power and possibly heart failure. However, there are some tissues (e.g. bone) that can heal without any structural or functional deterioration.

Types

Man with visible face scars after a car accident. All scarring is composed of the same collagen as the tissue it has replaced, but the composition of the scar tissue, compared to the normal tissue is different. Scars differ from other scars in the amounts of collagen overexpressed. Labels have been applied to the differences in overexpression. Two of the most common types are hypertrophic and keloid scarring, both of which experience excessive stiff collagen bundled growth overextending the tissue, blocking off regeneration of tissues. Another form is atrophic scarring (sunken scarring), which also has an overexpression of collagen blocking regeneration. This scar type is sunken, because the collagen bundles do not overextend the tissue. Stretch marks (striae) are regarded as scars by some. High melanin levels and either African or Asian ancestry may make adverse scarring more noticeable.

Hypertrophic

Hypertrophic scars occur when the body overproduces collagen, which causes the scar to be raised above the surrounding skin. Hypertrophic  scars take the form of a red raised lump on the skin. They usually occur within 4 to 8 weeks following wound infection or wound closure with excess tension and/or other traumatic skin injuries.

Keloid

Keloid scarring on the back of Gordon, a man formerly enslaved by John and Bridget Lyons. People with darker skin pigmentation are more prone to the development of keloid scarring.
Keloid scars are a more serious form of excessive scarring, because they can grow indefinitely into large, tumorous (although benign)neoplasms. Hypertrophic scars are often distinguished from keloid scars by their lack of growth outside the original wound area, but this commonly taught distinction can lead to confusion. Keloid scars can occur on anyone, but they are most common in dark-skinned people. They can be caused by surgery, accident, acne  eor, sometimes, body piercings. In some people, keloid scars form spontaneously. Although they can be a cosmetic problem, keloid scars are only inert masses of collagen and therefore completely harmless and not cancerous. However, they can be itchy or painful in some individuals. They tend to be most common on the shoulders and chest. Hypertrophic scars and keloids tend to be more common in wounds closed by secondary intention. Surgical removal of keloid is risky and may excerbate the condition and worsening of the keloid.

Atrophic

An atrophic scar takes the form of a sunken recess in the skin, which has a pitted appearance. These are caused when underlying structures supporting the skin, such as fat or muscle, are lost. This type of scarring is often associated with acne, chickenpox, other diseases (especially Staphylococcus infection), surgery, or accidents.

Stretch marks

Stretch marks (technically called striae) are also a form of scarring. These are caused when the skin is stretched rapidly (for instance during pregnancy, significant weight gain, or adolescent growth spurts), or when skin is put under tension during the healing process, (usually near joints). This type of scar usually improves in appearance after a few years. Elevated corticosteroid levels are implicated in striae development.

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