Corpus: Circumventricular organ

Circumventricular organs, or CVO for short, are narrowly circumscribed areas in the wall of the third and fourth ventricles of the brain where the blood-brain barrier is interrupted. As they enable the largely free exchange of substances between the vascular system and the nervous tissue, they are also referred to as neurohemal regions.

Note: In some textbooks, the term "neurohaemal region" is only used for the eminentia mediana.

The surface of the circumventricular organs forms a thickened layer of ependymocytes. The ependymal cells take on a special shape: they are conspicuously elongated, which is why they are also known as tanycytes. They are closely connected to the ventricular lumen by means of tight junctions (zonulae occludentes), so that a separate, closed extracellular space is created subependymal for the exchange of substances. This space contains a plexus of fenestrated capillaries.

The circumventricular organs include:

• Neurohypophysis
• Eminentia mediana
• Organum vasculosum laminae terminalis (OVLT)
• Organum subfornicale (subfornical organ, SFO)
• Organum subcommissurale (subcommissural organ, SCO)
• Glandula pinealis
• Area postrema
• Choroid plexus

The circumventricular organs represent neurohaemal communication contacts that enable the networking of blood, endocrine and nervous systems.

They are involved as control centres in:

• the central nervous control of hormone balance (pituitary gland and eminentia mediana) by means of hormone release and receptive measurement of hormone concentration in the sense of negative feedback
• the regulation of body temperature and blood osmolarity via corresponding receptors and hypothalamic projections of OVLT and SFO
• dopaminergic triggering of the vomiting reflex via receptors of the area postrema.