The death of tissue cells. Necrosis can arise as a result of ischaemia (insufficient blood supply), which can lead to gangrene (tissue death); damage by extreme heat or cold, noxious chemicals, or excessive exposure to forms of radiation, including X-rays and infection.
The appearance of the dead tissue is reliant on the cause of the necrosis and the particular tissue that is affected. In necrosis due to tuberculosis, the dead tissue is soft, cheeselike, and dry. Fatty tissue underneath the skin that has died as a result of infection or damaged turns into a tough scar tissue that can then form into a firm nodule.