The term organum vasculosum refers to one of seven predominantly cellular structures of the anterior hypothalamic region as defined by Nissl stain. Located within the lamina terminalis in the optic recess at the anteroventral end of the third ventricle, it is bounded dorsally by the median preoptic nucleus and laterally by the anteroventral periventricular nucleus. Its neuronal composition is similar to that of those adjacent structures. As one of the circumventricular organs, the organum vasculosum is highly vascular and lacks a blood brain barrier ( Saper-2004 ). It is considered the primary detector of dehydration in the control system of drinking ( Graebner-2015 ). It, together with the subfornical organ and the area postrema, constitute the humerosensory system ( Swanson-2004 ).