In medicine or biology, a diverticulum is an outpouching of a hollow (or a fluid-filled) structure in the body. Depending upon which layers of the structure are involved, diverticula are described as being either true or false. In medicine, the term usually implies the structure is not normally present, but in embryology, the term is used for some normal structures arising from others, as for instance the thyroid diverticulum, which arises from the tongue. The word comes from Latin dīverticulum, "bypath" or "byway". Classification Diverticula are described as being true or false depending upon the layers involved: False diverticula (also known as "pseudodiverticula") do not involve muscular layers or adventitia. False diverticula, in the gastrointestinal tract for instance, involve only the submucosa and mucosa. True diverticula involve all layers of the structure, including muscularis propria and adventitia, such as Meckel's diverticulum. Embryology The kidneys are originally diverticula in the development of the urinary and reproductive organs. The lungs are originally diverticula forming off of the ventral foregut. The thymus appears in the form of two flask-shape diverticula, which arise from the third branchial pouch (pharyngeal pouch) of the endoderm. The thyroid gland develops as a diverticulum arising from a point on the tongue, demarcated as the foramen cecum. Human pathology Gastrointestinal tract diverticula Esophageal diverticula may occur in one of three areas of the esophagus: Pharyngeal (Zenker's) diverticula usually occur in the elderly, through Killian's triangle above the cricopharyngeal muscle. Midesophageal diverticula Epiphrenic diverticula are due to dysfunction of the lower esophageal sphincter, as in achalasia. A duodenal diverticulum can be found incidentally in 23% of normal people undergoing imaging. It can be either congenital or acquired, but the acquired form is more common and is due to the weakness of the duodenal wall, which causes protrusions. It is usually found at the second or third part of duodenum, around the ampulla of Vater. Food debris may enter the diverticular outpouchings, causing inflammation or diverticulitis. On CT or MRI imaging, it appears as a sac-like outpouching. If the diverticulum is filled with contrast agents, the wall would be thin and may contain air, fluid, contrast material, or food debris. If the food debris is broken down by bacteria, the outpouching may show "faeces sign". Inflammation of the duodenal wall shows thickening of the wall. Rarely, on barium studies in congenital duodenal diverticula, the contrast material fills up the true lumen,