Glial and perivascular structures in the subfornical organ: distinguishing the shell and core.

The subfornical organ (SFO) is a circumventricular organ with a chemosensitive function, and its vessels have no blood-brain barrier. Our study investigated the glial and vascular components in the SFO to determine whether their distributions indicate subdivisions, how to characterize the vessels and how to demarcate the SFO. To this end, we investigated glial markers (GFAP, glutamine synthetase, S100) and other markers, including vimentin and nestin (immature glia), laminin (basal lamina), β-dystroglycan (glio-vascular connections), and aquaporin 4 (glial water channels). We determined that the 'shell' of the SFO was marked by immunoreactivity for S100, GFAP and aquaporin 4. Nestin immunoreactivity was characteristic of the 'core'. Vimentin was almost evenly distributed. Glutamine synthetase immunoreactivity occurred in the shell but its expression was sparse. Vessels in the core were decorated with laminin but showed a discontinuous expression of aquaporin 4. Vimentin and GFAP staining was usually in separate glial elements, which may be related to their functional differences. Similar to other vessels in the brain, β-dystroglycan was detected along the shell vessels but laminin was not. The gradual disappearance of the laminin immunopositivity was attributed to the gradual disappearance of the perivascular space. Thus, our findings suggest that the shell and core glio-vascular structures are adapted to different sensory functions: osmoperception and the perception of circulating peptides, respectively.