Circumventricular organs

AP= : area postrema; AQP4= : aquaporin-4; AT1R= : angiotensin type 1 receptor; AVP= : arginine vasopressin; BBB= : blood–brain barrier; CVO= : circumventricular organ; IgG= : immunoglobulin G; IL-6= : interleukin-6; NMO= : neuromyelitis optica; NSCC= : nonspecific cation channel; NTS= : nucleus of the solitary tract; OVLT= : organum vasculosum of the lamina terminalis; PVN= : paraventricular nucleus; SFO= : subfornical organ; SON= : supraoptic nucleus; TRP= : transient receptor potential The circumventricular organs (CVOs) are highly vascularized structures located around the third and fourth ventricles and characterized by the lack of a blood–brain barrier (BBB). These specialized areas are points of communication between the blood, the brain parenchyma, and the CSF. Neurons and glial cells of the CVOs express a unique repertoire of receptors and ion channels and receive a wide range of chemical signals from the bloodstream. Via their interconnections with the hypothalamus and brainstem, these sensory CVOs have a critical role in sodium and water balance, cardiovascular regulation, energy metabolism, and immunomodulation. They are also involved in mechanisms of fever, vomiting, and other responses to potentially noxious stimuli; they are sites of access to the CNS for circulating microorganisms, prion proteins, and autoantibodies. These subjects have been recently reviewed.1,–,4 ### Microscopic architecture. The circumventricular organs (CVOs) are specialized structures strategically located along the surface of the brain ventricles. From the structural standpoint, they are subdivided into 2 groups, paraependymal and ependymal; the paraependymal CVOs have been further characterized into sensory and secretory.5 The CVOs have a unique neural and vascular architecture (table 1).1,–,4 They consist of neurons, glia, and ependymal cells combined with leptomeningeal and vascular elements. A common salient feature of the CVOs is presence of fenestrated capillaries and loosely apposed astrocytic processes that create relatively large perivascular spaces. These characteristics allow the ready passage of large molecules, such as peptides and polar substances, between the blood to the perivascular spaces, exposing neurons to peripheral signals. The ependymal lining of the CVOS contains specialized ependymocytes and tanycytes. Whereas ependymocytes are linked by tight junctions that establish a barrier between the CSF and the brain, tanycytes have multiple microvilli or cilia that contact the CSF, potentially providing for a …