How Cysts Occur
Cysts form through multiple distinct mechanisms depending on their location and type, but most commonly arise from developmental abnormalities, blocked ducts, inflammation, or cellular defects that create enclosed fluid-filled sacs.
General Mechanisms of Cyst Formation
Developmental/Congenital Origins
- Ductal plate malformation is the primary mechanism for hepatic and biliary cysts, occurring when the embryonic ductal plate (a double cylinder of hepatoblasts around portal vein branches) fails to properly mature and instead disconnects from the biliary tree, forming cystic structures 1
- Embryonic development errors lead to gastrointestinal duplication cysts, which form as benign lesions from foregut developmental abnormalities and are lined with gastrointestinal epithelium 1
- Pericardial cysts represent diverticulae or cystic formations from abnormal development, often found in cardiophrenic angles and not communicating with the pericardial space 1
- Arachnoid cysts are thin-walled sacs filled with clear CSF fluid that form during neural development 2
Obstruction and Secretion
- Epithelial cells within cysts retain secretory function, and continued fluid production generates positive luminal pressure that maintains and expands the cyst architecture 1
- Blockages of ducts can trap secretions and lead to cyst formation in various tissues 3
- Ruptured pilosebaceous follicles commonly give rise to epidermoid (sebaceous) cysts in the skin 4
Inflammatory and Reactive Processes
- Chronic inflammation can trigger cyst formation through tissue damage and healing responses 3
- Inflammatory cysts comprise pseudo-cysts and encapsulated/loculated effusions caused by rheumatic disorders, bacterial infection, trauma, or cardiac surgery 1
- Secondary aneurysmal bone cysts (about 30% of ABCs) occur as a reaction to another bone lesion, without characteristic translocation 5
Cellular and Tissue-Specific Mechanisms
Bone Cysts:
- Unicameral bone cysts are benign fluid-filled cavities that enlarge over time, causing bone thinning, though the exact mechanism remains unclear 5, 6
- Aneurysmal bone cysts are expansive, hemorrhagic tumors usually showing characteristic translocation, appearing as multicameral, fluid-filled lesions 5
Ganglion Cysts:
- Extra-articular mucin "droplets" coalesce to form the main body of the tumor, with the cyst wall and pedicle (connecting to nearby synovial joint) forming subsequently 7
Renal Cysts:
- Acquired cystic disease-associated RCC develops from atypical renal cysts that show proliferative changes with multilayered epithelium, thought to be precursor lesions 1
- Duration of hemodialysis and male gender are associated with increased occurrence in end-stage renal disease 1
Parasitic Cysts:
- Neurocysticercosis occurs when Taenia solium eggs are ingested via fecal-oral contamination, with oncospheres establishing in tissues as larval cysts that reach mature size in approximately 3 months 1
- Echinococcal cysts usually originate from ruptured hydatid cysts in the liver and lungs 1
Common Pitfalls
- Not all cystic lesions are benign—mucinous cystic neoplasms have malignant transformation rates up to 30% 1
- Functional ovarian cysts in premenopausal women are extremely common and usually resolve spontaneously, avoiding unnecessary intervention 1
- Inflamed cysts are difficult to excise; postpone removal until inflammation subsides 4
- Most unicameral bone cysts resolve spontaneously by skeletal maturity, making watchful waiting appropriate for many cases 5, 6