Una cicatriz aparece cuando el tejido epitelial es desgarrado (no cortado). La restitución del tejido conectivo se efectúa mediante el crecimiento de fibroblastos jóvenes; entonces el espacio dejado por la herida es «tapado» con tejido fibrilar (con características similares al que forma los músculos), de ahí que la cicatriz tenga una textura distinta a la piel. Algunas evidencias sugieren que ciertos fibroblastos (los miofibroblastos) están en condiciones de contraerse y así desempeñar un papel en la contracción de la herida. Así pues, una cicatriz es una alteración permanente de la apariencia dérmica consecutiva al daño y reparación colagenosa de ésta.

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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 basketweave 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 randomised alignment. For example, scars in the skin are less resistant to ultraviolet radiation, and 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.

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