Freeze-fracture evidence for a novel restricting junction at the blood?brain barrier of the cuttlefishSepia officinalis

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Abstract
The blood—brain barrier in the cuttlefishSepia officinalis has been studied with the freeze-fracture technique. Previous thin-section electron microscopy showed that a restricting junction is formed between perivascular glial processes in microvessels and venous vessels, and between pericytes in arterial vessels; the restriction appeared not to be a classicalzonula occludens or septate junction. In freeze-fracture replicas from brain optic and vertical lobe, endothelial cells, pericytes and perivascular glia could be recognized by their morphology and relation to the vascular lumen. In microvessels, endothelial and pericyte membranes showed sparse but uniform distribution of P-face intramembranous particles, with no particular particle aggregations. Perivascular glial membranes had a higher density of intramembranous particles but again no particle alignments characteristic of known restricting junctions were seen, although clusterings of intramembranous particles resembling gap junctions were present. In larger venous vessels, the perivascular glial layer showed a multilamellated organization, but again no arrays of intramembranous particles were detected, although this should be a favourable site for visualization of the restricting junctions. The walls of arterial vessels showed collagen deposits and cell processes with apparent intracellular myofilamentous profiles, but no intramembranous junctional particle arrays. It is concluded that the junctional zone observed in thin section electron microscopy is not associated with aligned aggregations of intramembranous particles detectable in freeze-fracture replicas, strengthening the evidence that this is a novel type of restricting junction.