Any of any of the secretory or sensory organs located in the brain region around or in relation to the ventricular system that are characterized by extensive vasculature and a lack of a normal blood brain barrier (BBB) and allow for the linkage between the central nervous system and peripheral blood flow.

http://upload.wikimedia.org/wikipedia/commons/7/7d/Gray734.png

circumventricular organ, circumventricular organ of neuraxis, CVO

uberon

UBERON:0005408

http://purl.obolibrary.org/obo/uberon.owl, http://purl.obolibrary.org/obo/cl.owl

CVOs contain neural tissue and are an integral part of neuroendocrine function; all of the CVOs, besides the subcommissural organ, contain extensive vasculature and fenestrated capillaries which leads to a leaky BBB at the site of the organs; the lack of a BBB allows CVOs to act as an alternative route for peptides and hormones in the neural tissue to the peripheral blood stream, while still protecting it from toxic substances

All vertebrates possess at least 4 circumventricular organs (including the hypothalamus) which can monitor the contents of the cerebrospinal fluid and secrete additional substances into it. (Butler, 1996, p. 330)