Necrosis

Necrosis, the death of cells or tissues, takes place if the blood supply to tissues is restricted; poisons produced by microbes, chemical poisons, and extreme heat or electricity also may cause necrosis. The rotting of the dead tissue is known as gangrene. Atrophy of animal tissue involves a process of tissue wasting, in which a decrease occurs in the size or number of functional cells. Hypertrophy-an increase in the size of the cells in a tissue or an organ occurs in heart muscle during diseases involving the heart valves, in certain pneumonias, and in some diseases of the endocrine glands. Aplasia is the term used when an entire organ is missing from an animal; hypoplasia indicates arrested or incomplete development of an organ, and hyperplasia an increase in the production of the number of cells—e.g., the persistent callus that forms on the elbows of some dogs. Metaplasia is used to describe the change of one cell type into another; it may occur in chronic irritation of tissues and in certain cancerous tumours.