Circumventricular organs (CVOs) (circum-: around ; ventricular: of ventricle) are structures in the brain characterized by their extensive and highly permeable capillaries, unlike those in the rest of the brain where there exists a blood–brain barrier (BBB) at the capillary level. Although the term "circumventricular organs" was originally proposed in 1958 by Austrian anatomist Helmut O. Hofer concerning structures around the brain ventricular system, the penetration of blood-borne dyes into small specific CVO regions was discovered in the early 20th century. The permeable CVOs enabling rapid neurohumoral exchange include the subfornical organ (SFO), the area postrema (AP), the vascular organ of lamina terminalis (VOLT — also known as the organum vasculosum of the lamina terminalis (OVLT)), the median eminence, the pituitary neural lobe, and the pineal gland.
The circumventricular organs are midline structures around the third and fourth ventricles that are in contact with blood and cerebrospinal fluid, and they facilitate special types of communication between the central nervous system and peripheral blood. Additionally, they are an integral part of neuroendocrine function. Highly permeable capillaries allow the CVOs to act as an alternative route for peptides and hormones in the neural tissue to sample from and secrete to circulating blood. CVOs also have roles in body fluid regulation, cardiovascular functions, immune responses, thirst, feeding behavior and reproductive behavior.
CVOs can be classified as either sensory or secretory organs serving homeostatic functions and body water balance. The sensory organs include the area postrema, the subfornical organ, and the vascular organ of lamina terminalis, all having the ability to sense signals in blood, then pass that information neurally to other brain regions. Through their neural circuitry, they provide direct information to the autonomic nervous system from the systemic circulation. The secretory organs include the subcommissural organ (SCO), the pituitary gland, the median eminence, and the pineal gland. These organs are responsible for secreting hormones and glycoproteins into the peripheral blood using feedback from both the brain environment and external stimuli.
Circumventricular organs contain capillary networks that vary between one another and within individual organs both in density and permeability, with most CVO capillaries having a permeable endothelial cell layer, except for those in the subcommissural organ. Furthermore, all CVOs contain neural tissue, enabling a neuroendocrine role.
Although the choroid plexus also has permeable capillaries, it does not contain neural tissue; rather, its primary role is to produce cerebrospinal fluid (CSF), and therefore is typically not classified as a CVO.
Sensory organs
Area postrema
Anatomy
The area postrema is located in the caudal medulla oblongata near the junction of the brainstem and the spinal cord. In humans and in most other mammals that have been studied, it consists of swellings on either wall of the fourth ventricle. In rodents and lagomorphs, however, the area postrema forms a midline structure dorsal to the obex. When viewed histologically for its capillary distribution and morphology, the area postrema has numerous subregions separated according to capillary permeability, rates of blood flow, and duration of blood transit through respective capillary beds.
Function
Relatively little is known about the function of the area postrema in humans. However, there is strong evidence that the area postrema acts as the chemoreceptor trigger zone for vomiting, which is triggered by the presence of noxious stimulation from the blood. There is also evidence that the area postrema is the site at which angiotensin stimulates glucose metabolism, presumed efferent neural activity, blood pressure control, and thirst. The area postrema also has integrative capacities that enable it to send major and minor efferents to sections of the brain involved in the autonomic control of cardiovascular and respiratory activities.