A<unk> , also  called a dermoid or mature cystic teratoma, is an abnormal relatively rare cyst that usually affects women during their childbearing years (15-40; the average age is 30), is usually benign, and can range in size from half an inch to 17 inches in diameter.  It is similar to those present on skin tissue, and can contain fat and occasionally hair, bone, nails, teeth, eyes, cartilage, and thyroid tissue.  Up to 10-15% of women with them have them in both ovaries.  It develops from a totipotential germ cell (a primary oocyte) that is retained within the ovary.  A CT scan and MRI can show the presence of fat and dense calcifications.  Though it often does not cause any symptoms, it can on the other hand become inflamed, and can also twist around (a condition known as ovarian torsion), causing severe abdominal pain and imperiling its blood supply, which is an emergency and calls for urgent surgery.   These cysts can generally be removed easily, which is usually the treatment of choice, with either conventional surgery (laparotomy; open surgery) or laparoscopy.  Removal does not generally affect fertility.  The larger it is, the greater the risk of rupture with spillage of the contents, which can create problems with adhesions and pain.  Although the large majority (about 98%) are benign, the remaining fraction (about 2%) becomes cancerous (malignant) -- those are usually in women over 40.
Dermoid cyst
Epidermoid cyst
Nabothian cyst
Sebaceous cyst
Trichoblastoma
Cylindroma
Baker's cyst
Acrospiroma