[The subfornical organ today: morphological aspects and functional role].

The recognition of the role played by the subfornical organ (SFO) in the central regulation of body water balance has recently aroused new interest in this anatomical formation which remained ignored for a long time. The SFO is included in the group of the circumventricular organs. In higher vertebrates it is adherent to the ventral surface of the fornix and protrudes into the third ventricle at the level of the interventricular foramina, partially covered by the choroid plexus. The SFO appears as a small nodule, rounded or ovoidal in shape, consisting of highly vascularized nervous tissue and lined by ependyma at the ventricular surface. Its structural organization is fundamentally constant and presents only minor differences in the various species. The SFO neuronal perikarya show different aspects which have been classified in four types. However, it is not yet clearly defined if such aspects refer to distinct cell types or to different transitional features. Nerve and glial cell processes form a dense plexus through the SFO and the subependymal area, as well as in the connective tissue perivascular spaces. These may be narrow or wide and surround fenestrated and non-fenestrated capillaries, assuming sometimes a labyrinthine aspect. The ependymal lining of the SFO ventricular surface shows large variations and regional differences concerning the cell height, the number and development of microvilli, the cilia distribution. The structural properties of SFO, which is characterized by a rich and highly permeable capillary bed, by a wide surface area of contact and exchange with the cerebrospinal fluid, by direct and indirect neural connections with a number of regulatory structures, have been considered as the basis for the role of neurohumoral integration that SFO plays in regulating physiological and behavioral responses to water-mineral changes. Much experimental evidence substantiates this function. However, the studies on SFO are increasingly enriching the literature with new experimental, especially physiological and cytochemical, data which may suggest for this organ connections even more extensive and functions even more complex than those until now ascertained.