Blood-brain barrier and cerebrospinal fluid circulation.

There are two distinctive regions in the central nervous system of all vertebrates: the common brain region possessing the blood-brain barrier, and a group of several brain regions (circumventricular organs (CVOs) including the choroid plexus) devoid of the blood-brain barrier. Of the first, the morphological basis in vertebrates higher than teleosts is the brain-type capillary endothelium, and in elasmobranchs the perivascular astrocytic end-feet sealed by tight junctions. Of the second, the morphological basis is the fenestrated capillary with perivascular space. The pial connective tissue directly continues to the propria of circumventricular organs, and thus offers sites of cerebrospinal fluid drainage through CVO venous capillaries of which the fenestrated endothelia are anchored to perivascular tissues by the microfibrils of elastic fibers. The arachnoid granulations, which increase in size and number in accordance with the increasing volume of the skull are safety balloons against a sudden rise in intracranial pressure. They are possibly also sites of immune responses.