FINE-STRUCTURE OF THE BLOOD-BRAIN INTERFACE IN THE CUTTLEFISH SEPIA-OFFICINALIS (MOLLUSCA, CEPHALOPODA)

The blood-brain interface was studied in a cephalopod mollusc, the cuttlefish Sepia officinalis, by thin-section electron microscopy. Layers lining blood vessels in the optic and vertical lobes of the brain, counting from lumen outwards, include a layer of endothelial cells and associated basal lamina, a layer of pericytes and a second basal lamina, and perivascular glial cells. The distinction between endothelial cells and pericytes breaks down in small vessels. In the smallest microvessels, equivalent to capillaries, and in venous channels, the endothelial and pericyte layers are discontinuous, but a layer of glial cells is always interposed between blood and neural tissue, except where neurosecretory endings reach the second basal lamina. In microvessels in which cell membranes of the entire perivascular glial sheath could be followed, the glial layer was apparently 'seamless', not interrupted by an intercellular cleft, in ca 90% (27/30) of the profiles. Where a cleft did occur, it showed an elongated overlap zone between adjacent cells. The walls of venous channels are formed by lamellae of overlapping glial processes. In arterial vessels, the pericyte layer is thicker and more complete, with characteristic sinuous intercellular clefts. Arterioles are defined as vessels containing 'myofilaments' within pericytes, and arteries those in which the region of the second basal lamina is additionally expanded into a wide collagenous zone containing fibroblast-like cells and cell processes enclosing myofilaments. The 'glio-vascular channels' observed in Octopus brain are not a prominent feature of Sepia optic and vertical lobe. The organization of cell layers at the Sepia blood-brain interface suggests that it is designed to restrict permeability between blood and brain.