A teratoma composed exclusively of fully differentiated, 'adult-type' tissue elements that are sometimes arranged in a pattern resembling normal tissue relationships. Mitotic activity is low or absent. The more common ectodermal components present in a mature teratoma include skin, brain and choroid plexus. The more common mesodermal components include cartilage, bone, fat and muscle (both smooth and striated). And the most common endodermal components are cysts lined by epithelia of respiratory or enteric type and in some cases pancreatic or hepatic tissue. (Adapted from WHO)