Perfusion-Induced Oedema Does Not Disrupt Perivascular Glial Sheaths in the Rat Area postrema: Evidence for an Inconspicuous Type of Cell Junction?

After perfusion fixation using phosphate-buffered glutaraldehyde, the rat area postrema always contained some portions with lacunar extracellular spaces in the neuropil. This was interpreted as a sign of local oedema due to perfusion-induced extravasation, made possible by the absence of an endothelial blood-brain barrier in the area postrema. All perivascular spaces were delimited from the nervous tissue by a continuous layer of astroglial processes. The cell appositions in these perivascular glial sheaths were not only seen in the regions of the area postrema displaying conventional morphology, but also persisted systematically in those regions containing lacunar extracellular space after fixation. At these sites, the glial sheaths had presumably endured a net outflow of extravasated oedema fluid in vivo. In the neighbouring neuropil at these locations, certain cell appositions with conventional intercellular clefts also persisted. These phenomena might both be interpreted as non-random, functionally important cell contacts with the inconspicuous ‘intercellular clefts’ containing unstained material. In the case of perivascular glia this might imply a partial restriction of diffusion between blood and brain tissue, allowing certain control or defence functions.