Heart attack scar. Light micrograph of sectioned scar tissue (pink) formed to repair the damage caused by a heart attack (myocardial infarction). Normal heart muscle (myocardium) is brown. In myocardial infarction an obstruction in a coronary artery means a portion of the heart muscle does not receive the required amount of oxygen, leading to the death of the muscle in that area. This is often fatal. In patients that survive, the damaged area is replaced by scar tissue, formed largely of the fibrous protein collagen. Heart attacks are the leading cause of death in the developed world. Magnification: x50 at 35mm size.