The ultrastructure of the sheath around chronically implanted electrodes in brain

Insulated, bipolar stainless steel electrodes were chronically implanted in various regions of the cat brain and the long-term structural changes in the tissue surrounding the electrodes were studied by light and electron microscopy. A sheath surrounded and separated the electrode from normal grey or white matter. A layer of foreign body giant cells of variable thickness was formed adjacent to the electrode. This layer was attenuated in some places so that it was unrecognizable by light microscopy. The bulk of the sheath structure consisted of collagen fibrils, leptomeningeal cells and hypertrophied astrocytes. Areas consisting of modified leptomeningeal cells with long thin processes we designated as spongy areas. These have not been previously reported using the electron microscope. Glycogen bodies were seen in leptomeningeal cells. Astrocytes became greatly enlarged and were more numerous in and around the sheath. Oligodendrocytes contained lamellar bodies, and direct continuity was shown between a lamellar body and an adjacent myelin sheath. Myelin was seen in abnormal sites (around oligodendrocytes and neurons) and in unusual configurations. Neuronal changes near the sheath included whorls and stacks of modified endoplasmic reticulum and the presence of cytoplasmic nucleolus-like bodies. Reactive, regenerative and degenerative axons were observed. Blood vessels were more numerous in the sheath and surrounding tissue than normal. Perivascular spaces were prominent even around capillaries and often plasma cells and monocytes were in these spaces. As compared to normal tissue the extracellular space is noticeably increased. Electrodes passing through ventricles were surrounded with a sheath covered with ependymal cells. This sheath was comparable in structure to the sheath present around the electrode in other locations.