The external glial limiting membrane in Macaca: Ultrastructure of a laminated glioepithelium

The external glial limiting membrane in Macaca is a multilaminar epithelium‐like mantle formed by the perikarya and interdigitating cell processes of astrocytes. It is separated from the overlying subarachnoid space by a typical basement membrane. Tonofilaments, about 100 Å in diameter, course through the cytoplasm of the processes in sheaths which run in different planes in successive layers of the mantle, forming a course brushwork. Beneath the surface plasma membrane of the superficial astrocytes is a filamentous layer, 500–700 Å thick, which is composed of microfilaments, about 50 Å in diameter. These microfilaments were continuous with the inner aspect of the plasma membrane. The tonofilaments appeared to terminate in the microfilamentous network. The similarity of these microfilaments to the cytocholasin B‐sensitive microfilaments of epithelial cells suggested contractile properties and their participation in the motility of glial cells at the border of the brain and subarachnoid space.