cyst, abnormal sac in the body, filled with a fluid or semisolid and enclosed in a membrane. Cysts can be congenital but are usually acquired, the most common locations being the skin and the ovaries. Sebaceous cysts of the skin, known as blackheads or whiteheads, occur when dirt or other material blocks the oil glands of the skin, preventing secretions from escaping. Retention cysts develop in glandular organs when ducts are blocked, commonly in kidney tubules, mammary glands, and sweat glands. Most cysts can be aspirated for treatment and/or cytology (diagnostic purposes); often cysts require surgical removal.

Any of several types of cysts in the kidneys. Some are present at birth; others are caused by tubular obstruction. Large cysts can cause backaches and a dragging sensation. Multiple cysts occur as a result of various disorders, including kidney vascular diseases, lymphatic vessel blockage, congenital diseases, and tapeworm infestation. The most serious is medullary cystic disease, which has no warning symptoms but causes anemia, low blood sodium levels, and uremia. The kidneys become shrunken, grainy, and scarred. Cysts should usually be checked surgically to rule out cancer. Seealso urogenital malformation.

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Enclosed sac within body tissues. It has a distinct membrane and generally contains liquid. Most cysts are benign, but several kinds may be malignant or precancerous. Benign cysts often press on nearby organs and require removal. Formed by overproduction of epithelium (surface tissue of anatomical structures), cysts may become detached from surrounding structures and move freely. They can contain natural secretions, abnormal breakdown products, or, in infections, bacteria, larval parasites, and microbial products. Some organs, including the kidney, liver, and breast, can become filled with cysts as a result of cystic diseases that may be dangerous or may hide more serious diseases. Seealso tumour.

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