Immature, or solid, teratomas are the most common type of ovarian germ cell tumor, making up 40–50% of cases. Teratomas are characterized by the presence of disorganized tissues arising from all three embryonic germ layers: ectoderm, mesoderm, and endoderm; immature teratomas also have undifferentiated stem cells that make them more malignant than mature teratomas (dermoid cysts). The different tissues are visible on gross pathology and often include bone, cartilage, hair, mucus, or sebum, but these tissues are not visible from the outside, which appears to be a solid mass with lobes and cysts. Histologically, they have large amounts of neuroectoderm organized into sheets and tubules along with glia; the amount of neural tissue determines the histologic grade. Immature teratomas usually only affect one ovary (10% co-occur with dermoid cysts) and usually metastasize throughout the peritoneum. They can also cause mature teratoma implants to grow throughout the abdomen in a disease called growing teratoma syndrome; these are usually benign but will continue to grow during chemotherapy, and often necessitate further surgery. Unlike mature teratomas, immature teratomas form many adhesions, making them less likely to cause ovarian torsion. There is no specific marker for immature teratomas, but carcinoembryonic antigen (CEA), CA-125, CA19-9, or AFP can sometimes indicate an immature teratoma.