Light micrograph section through scar tissue. Scar tissue is a collection of cells and collagen that covers the site of an injury. People can develop scar tissue on the skin as a result of an injury, surgery or acne. There are various types of scar tissue, such as, keloid, hypertrophic scar (the most common form) and contractive scar. Other areas of the body can also develop scar tissue, such as the heart muscle after a heart attack.