Dermoid cyst of the ovary. Gross specimen of an ovarian dermoid cyst (or ovarian teratoma), a form of tumour which arises when an unfertilised egg begins to divide. Because eggs have the potential to give rise to all cell types, dermoid cysts can form mature tissues. They may contain structures such as hair, oil glands, bone, and teeth (as in this case, upper right). Dermoid cysts are benign (non-cancerous) in 98% of cases. They may cause illness by rupturing or causing twisting of the ovary. They are treated by surgery, either to remove the cyst or the whole ovary.