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The human subfornical organ: an anatomic and ultrastructural study.

The subfornical organ (SFO), one of the circumventricular organs (CVO) and a thirst-regulating structure, was examined in humans. In 21 autopsy specimens, the SFO was identified as one mm grey nodule on the ventral surface of the fornix at the foramina of Monro. The SFO is a neuronal-vascular organ on a loose glial background, lined by ependyma. Ultrastructural examination reveals deep, narrow invaginations in most neuronal nuclei, characteristically seen in the SFO of other species, but unlike neuronal nuclei in the rest of the human brain. Ovoid, clear vesicles in synaptic complexes and dense-core granules in non-synaptic neuronal process are seen. Ependymal channels are observed. Capillaries have luminal tongue-like projections and pinocytotic vesicles in the endothelial wall, as well as both tight and non-tight junctions between endothelial cells; endothelial fenestrations are not found. These specializations may permit access of macromolecules to receptor sites in the SFO, facilitating its functions as a chemoreceptor organ in drinking behavior. The anatomy of the human SFO is consistent with that of other CVO's and with that of the SFO in other species.