The glio-vascular system of cephalopods

Abstract

The branches of the cerebral arteries run to the centre of each lobe of the brain and from there radiate outwards. The arteries are lined by endothelial cells, surrounded by pericytes. Outside these are large extracellular spaces containing collagen. These spaces continue as a system of ‘glio-vascular’ channels among the tissues. These channels contain collagen and other extracellular material and nuclei belonging probably to muscle cells and fibroblasts. This system permeates the neuropil and in the cell layers provides wrappings for the perikarya. The channels and extracellular material form tunnels of ‘trophospongium’ within the neuronal cytoplasm. Glial fingers also penetrate into these channels but are not well seen by light microscopy. The system of spaces among the tissues communicates with an elaborate set of branching ‘lymphoid' channels. These collect into veins either in the membrane around the brain or at the centre of the optic lobe. The veins discharge to the pharyngo-ophthalmic vein and the brain is thus provided with a venous system partly isolated from the main haemocoele. The true neuroglia cells are of two types. The protoplasmic glia cells have much-branched processes, especially in the neuropil. They have abundant end-feet attached to the outside of the perivascular channels and to the glio-vascular strands. They occur in the cell layers and their processes probably penetrate the neurons. The fibrous glia have very long processes, mostly smooth and with little branching. The processes run in bundles accompanying the main tracts of nerve fibres in the neuropil and extending into the cell layers.