|Schematic drawing of a false diverticulum. A - mucosa; B - submucosa; C - muscularis; D - serosa and subserosa|
A diverticulum is the medical or biological term for an outpouching of a hollow (or a fluid-filled) structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false.
In medicine, the term usually implies the structure is not normally present. However, in the embryonic stage, some normal structures begin development as a diverticulum arising from another structure, such as the thyroid diverticulum that arises from the tongue.
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Diverticula are described as being true or false depending upon the layers involved:
- True diverticula involve all layers of the structure, including muscularis propria and adventitia, such as Meckel's diverticulum.
- False diverticula (also known as "pseudodiverticula") do not involve muscular layers or adventitia. False diverticula, in the GI tract for instance, involve only the submucosa and mucosa.
- The kidneys, originally diverticula in the development of the urinary and reproductive organs.
- The lungs, 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.
Gastrointestinal tract diverticula
- Meckel's diverticulum: a persistent portion of the omphalomesenteric duct present in 2% of the population, making it the most common congenital gastrointestinal malformation.
- Esophageal diverticula may occur in one of three areas of the esophagus:
- Killian-Jamieson diverticulum- is very similar to a pharyngeal esophageal diverticulum, differing in the fact that the pouching is below the cricopharyngeal muscle.
- Colonic diverticula: Although found incidentally during colonoscopy, these diverticula may become infected (see diverticulitis) and can perforate, requiring surgery.
- Duodenal and jejunal diverticul(um|a): are congenital lesions and may be a source of bacterial overgrowth. They may also perforate or result in abscesses.
- Rokitansky-Aschoff sinuses are diverticula in the gallbladder due to chronic cholecystitis
- Gastric diverticula are very infrequent.
Most of these pathological types of diverticulum are capable of harboring an enterolith. If the enterolith stays in place, it may cause no problems, but a large enterolith expelled from a diverticulum into the lumen can cause obstruction.
Genito-urinary tract diverticula
- Calyceal diverticulum: usually asymptomatic, but if a stone becomes lodged in the outpouching they may present with pain.
- Bladder diverticulum: Balloon-like growths on the bladder commonly associated with a chronic outflow obstruction, such as benign prostatic hyperplasia in older males. Usually found in pairs on opposite sides of the bladder, bladder diverticula are often surgically removed to prevent infection, rupture, or even cancer.
- Urethral diverticulum: Acquired diverticula are usually post-infectious in females, whereas they are more likely to be congenital in males.
- Cardiac diverticulum: A very rare congenital malformation of the heart that is usually benign 
- Diverticulum of Kommerell: is an out-pouching(aneurysm) of the aorta where an aberrant right subclavian artery is located. It is unusual nomenclature, in that focal dilatations of a blood vessel are properly referred to as aneurysms.
Bladder diverticula containing stones. Also note that the bladder wall is thickened due to possible transitional cell carcinoma.
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