The ultrastructure of the sheath around chronically implanted electrodes in brain

Abstract

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.

Quantitative analysis of the tissue response to chronically implanted microwire electrodes in rat cortex

“…In particular, by 4 weeks following implantation, the reaction has a stereotypical organization consisting of hypertrophic astrocytes, fibroblasts, and meningeal cells [10][11][12][13][14][15][16][17][18][19][20][21][22][23] that surrounds an activated macrophage/microglial/ foreign body giant cell core found immediately adjacent to the implant [15,17,22]. Within this region of reactive gliosis, numerous studies have described a decrease in the neuronal population [10][11][12][13]15,[20][21][22][23][24]. Quantitative studies have shown that the response is limited to the immediate vicinity of the electrode extending a few hundred microns from the interface into surrounding brain tissue [13,15].…”

Cerebral Tissue Response to Electrode Implantation

“…From our observations as well as those of others (Collias & Manuelidis, 1957, Schultz & Willey, 1976, it is evident that implantation of electrodes elicits a tissue response which is definable, reproducibly limited as to involvement, and divided into developmental stages. Although the degree of involvement between traumatic, hemodynamic, and cytotoxic factors remains unclarified, the net effect is definable in tissue response and should not be ignored in conclusions drawn from studies involving implantation.…”

“…A border zone at the electrode/parenchymal interface was composed of a mixture of hemorrhagic, fibrous, phagocytic, and necrotic elements. A clearly defined sheath of a cellular lining as was reported in a study following 30 days chronic implantation (Schultz and Willey, 1976) had not developed in these animals. In the present studies no detectable capillary proliferation was encountered in border zone.…”

“…After an interval of thirty days, the latter workers reported parenchymal response consisting of numerous giant cells and phagocytes as well as changes in neuronal and astrocytic components. Schultz and Willey (1976) further reported an increase in the number of blood vessels in the sheath zone as well as an expanded extracellular space.…”

“…Schultz and Willey (1976) reported ultrastructural changes following chronically implanted electrodes. After an interval of thirty days, the latter workers reported parenchymal response consisting of numerous giant cells and phagocytes as well as changes in neuronal and astrocytic components.…”