Three-dimensional organization of the perivascular glial limiting membrane and its relationship with the vasculature: a scanning electron microscope study.

To examine the three-dimensional structure of the perivascular glial limiting membrane (Glm) and its relationship with the vasculature in rat/mouse cerebral cortices, serial ion-etched plastic sections were observed under the scanning electron microscope and their images were reconstructed. In the case of arterioles and venules close to the pial surface, cord-like principal processes predominantly formed the endfeet; whereas in the case of capillaries and venules, sheet-like secondary processes chiefly formed Glm. Moreover, it was found that several plate-like structures protruded from the basement membrane surrounding the arterioles to penetrate into the astrocytic somata. The perivascular Glm was formed by monolayers of astrocytic processes and/or somata irrespective of the types of blood vessel. However, the thickness of the perivascular Glm, varied greatly according to the type of blood vessel. The thickness of Glm decreased in the order of arterioles, venules and capillaries. The outer surface of the perivascular Glm was extremely irregular, and sheet-like processes arising from this Glm infiltrated into the surrounding neuropil.