Are close contacts between astrocytes and endothelial cells a prerequisite condition of a blood-brain barrier? The rat subfornical organ as an example*

The microvessels of the rat subfornical organ (SFO) are heterogeneous: those of the caudal part lack a blood-brain barrier (BBB) unlike those of the rostral part. The astroglial environment of these microvessels has been studied by combining an immunocytochemical technique employing an anti-GFAP (glial fibrillary acidic protein) antiserum with the morphological detection of a barrier to the protein-silver complex.

All the SFO microvessels are surrounded by astrocytes characterized by a tumescent aspect; however, the relative proximity between the astrocytic feet and the endothelial cells varies considerably. The capillaries provided with a barrier (rostral SFO) are contiguous with the astrocytes from which they are only separated by a basement membrane. The capillaries devoid of BBB (caudal SFO) are surrounded by a pericapillary space that keeps the astrocytes at a short distance (capillaries with a very rich vesicular entothelium) or at a long distance (capillaries with a fenestrated endothelium).

The astrocytes are absent in the choroid plexus where all microvessels are fenestrated and lack a barrier.

These data suggest that the astrocytes release one or more signals which in their vicinity inhibit the expression of endothelial morphological characteristics (fenestrations, vesicles) responsible for the leakage of plasmatic proteins from the blood to the cerebral parenchyma of the circumventricular organs.