Acute appendicitis. Coloured light micrograph (LM) through a section of an appendix showing acute appendicitis. At this early stage, the epithelial lining (mucosa, pink edges) of the appendix becomes ulcerated and is replaced by an inflammatory exudate (thick fluid secreted as a result of disease). White blood cells (small, purple) are present in high numbers. As the disease progresses, the inflammation will spread throughout the appendix until exudate is present on the external surface (peritoneum) of the appendix (not shown). The subsequent inflammation of the peritoneum (peritonitis) is serious and often requires emergency surgery.