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A teratoma is a tumor with tissue or organ components resembling ''normal'' derivatives of more than one germ layer. Although the teratoma may be monodermal or polydermal (originating from one or more germ layers), its cells may differentiate in ways suggesting other germ layers. The tissues of a teratoma, although normal in themselves, may be quite different from surrounding tissues and may be highly disparate; teratomas have been reported to contain hair, teeth, bone and, very rarely, more complex organs or processes such as eyes, torso, and hands, feet, or other limbs. Usually, a teratoma will contain no organs but rather one or more tissues normally found in organs such as the brain, thyroid, liver, and lung. Sometimes, the teratoma has within its capsule one or more fluid-filled cysts; when a large cyst occurs, there is a potential for the teratoma to produce a structure within the cyst that resembles a fetus. Because they are encapsulated, teratomas are usually benign, although several forms of malignant teratoma are known and some of these are common forms of teratoma. A mature teratoma is typically benign and found more commonly in women, while an immature teratoma is typically malignant and is more often found in men. Teratomas are thought to be present at birth (congenital), but small ones often remain undiscovered until much later in life. Definitive medical diagnosis of a teratoma is based on its histology. ==Terminology== As is true throughout oncology (the study of tumors, malignant and benign), the nomenclature for these tumors continually evolves on the basis of threads of consensus in the scientific literature regarding the tumors' classification (which is based on shared embryologic origins, clinicopathologic characteristics, and so on). As science advances, more is understood about how one can differentiate tumors that formerly seemed alike. For example, the scientific knowledge of tumor markers, genomics, and proteomics regularly advances. In this environment, tumor names may slip over the years between synonymity, hyponymy and hypernymy, and deprecation. Some terms that have been synonymous with ''teratoma'' include ''dysembryoma'', ''teratoblastoma'', ''organoid tumor'', and ''teratoid tumor''. The words "teratoma" and "mature teratoma" both have been used to refer to a benign growth, while the word "teratoma" may also refer to "immature teratoma", a cancerous growth. Avoiding misunderstanding due to such polysemy is part of why tumor nomenclature changes over decades. The nomenclatural changes are voluntary, based on scientists agreeing or debating in the literature regarding what to call particular neoplastic entities (types of tumors). The term "malignant teratoma" has sometimes been used as a synonym for nonseminomatous germ cell tumor. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「teratoma」の詳細全文を読む スポンサード リンク
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