Morphology of the organum vasculosum of the lamina terminalis (OVLT) of the sheep.

Examination of the ventricular surface of the organum vasculosum of the lamina terminalis (OVLT) of sheep with the scanning electron microscope revealed an elongated protuberance occupying most of the frontal wall of the third ventricle below the level of the anterior commissure. This protuberance lacked ciliated ependymal cells. Examination of horizontal sections with the transmission electron microscope revealed an apparent lack of regularly apposed ependymal cells, suggesting that ependyma is either greatly modified or absent. The surface was composed of numerous intertwining cell processes with some scattered cells situated on this surface. The body of this structure was composed of many cell processes separated by a network of extracellular channels sometimes extending to the ventricular surface. Towards the base of this protuberance, a plexus of blood vessels was observed. Some of these vessels exhibited fenestrated endothelium. Neuronal processes were also apparent in this region. These unusual anatomical features suggest a specific function for this brain region in sheep.