The subcommissural organ (SCO) is a small ependymal gland of the circumventricular system, located in the dorsocaudal region of the third ventricle, at the entrance of the mesencephalic aqueduct (aqueduct of Sylvius). The SCO is a phylogenetically ancient and conserved structure of the vertebrate phylum. It has its name because of the localization in the brain. The organ's main functions are unknown; nevertheless, some evidences suggest that the organ may participate in different process, such as the clearance of certain compounds and the circulation of the cerebrospinal fluid, and it could also play a role in some morphogenetic mechanisms.