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. The total number of carbons (30) reflects its Triterpenoid origin. In some steroids some carbons may be removed (such as carbon 18) or added (such as carbons 241 and 242) in downstream biosynthetic reactions.]] A steroid is a Terpenoid Lipid characterized by a Carbon skeleton with four fused rings, generally arranged in a 6-6-6-5 fashion. Steroids can vary by the Functional Group s attached to these rings and the Oxidation state of the rings. Hundreds of distinct steroids are found in Plant s, Animal s, and Fungi . All steroids are biosynthetically derived either from the Sterol Lanosterol (animals and fungi) or the sterol Cycloartenol (plants). Both sterols are derived from the cyclization of the Triterpene Squalene .http://www.chem.qmul.ac.uk/iubmb/enzyme/reaction/terp/lanost.html ORIGIN (IPP), Dimethylallyl Pyrophosphate (DMAPP), Geranyl Pyrophosphate (GPP) and Squalene shown. Some intermediates are omitted.]] Steroids include Estrogen (U.S spelling) or oestrogen (UK spelling), Progesterone and Testosterone . Oestrogen and progesterone are made primarily in the Ovary and in the Placenta during pregnancy and Testosterone in the Testes . Certain Neurons and Glia in the Central Nervous System (CNS) express the Enzymes that are required for the local synthesis of Pregnane Neurosteroids , either ''de Novo'' or from peripherally derived sources. CLASSIFICATION Taxonomical/Functional Some of the common categories of steroids:
Structural It is also possible to classify steroids based upon their chemical composition. One example of how MeSH performs this classification is available at . SEE ALSO EXTERNAL LINKS
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