Circumventricular organs

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Circumventricular organs (CVO) are specialised sites in the brain, positioned at sites around the margin of the ventricular system, which have an incomplete blood-brain barrier. ^[1]

Two of these sites, the median eminence and the posterior pituitary gland, are sites of neurosecretion: neuroendocrine neurons whose cell bodies are within the hypothalamus project axons to these sites to secrete their hormonal products directly into the blood. Three other sites, the subfornical organ, and the organum vasculosum of the lamina terminalis (OVLT) adjacent to the third ventricle and the area postrema at the base of the fourth ventricle contain neurons specialised to detect blood-borne substances ^[2]. The subfornical organ and OVLT contain some osmosensitive neurons, and other neurons that detect a wide range of blood-borne peptides whose actions influence body fluid and electrolyte homeostasis in particular. These peptides include angiotensin, endothelin, atrial natriuretic factor, and relaxin. ^[3] The pineal gland - secretes the hormone melatonin and is associated with circadian rhythms.^[4] The area postrema is intimately connected with the underlying nucleus tractus solitarii and senses, amongst other things, peptides and hormones related to energy intake and utilisation. ^[5]

The choroid plexuses are structures specialised to import blood-borne factors such as leptin across the blood-brain barrier.^[6]

The subcommissural organ is a phylogenetically ancient and conserved glandular structure formed by ependymal and hypendymal cells highly specialized in the secretion of proteins. The ependymal cells secrete various proteins into the cerebrospinal fluid; its physiological function is still unknown. ^[7]

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