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

Abstract

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 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 A thick, which is composed of microfilaments, about 50 A in diameter. These microfilaments were continuous with the inner aspect of the plasma membrane. The tonoflaments 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.In mammalian central nervous tissue, astrocytes are recognized mainly by the relative abundance of intracytoplasmic particulate glycogen and the presence of numerous, variously distributed glial filaments (Peters et al., '70). The latter, about 130 A in diameter, are similar to the tonofilaments characteristic of epithelial cells and often are gathered up into sheaths, which course through the cytoplasm. They are especially prominent in the attenuated cytoplasmic processes of astrocytes.The function of these glial filaments in astrocytes is essentially unknown, yet they increase in number in several degenerative neuronal diseases (Terry and Weiss, '63; Eager and Eager, '66; Luse, '68) and in so-called "glial scars" formed in brain tissue in response to tissue injury (Schultz and Pease, '59; Luse, '68). They are found normally in the astroglial cells forming the external glial membrane (Nominu HistoZogica, '70), which separates the molecular layer of the cerebral cortex from the overlying pia-arachnoid tissues.In vertebrates, generally, the external glial limiting membrane is composed of variably distributed cell bodies and cytoplasmic processes of astrocytes (Held, '09; AM. J. ANAT., 136: 277-296.Penfield, '32; Bohme, '66). In animals subjected to perinatal asphyxia, then permitted to come to term normally, an atrophic cortical sclerosis (ulegyria) results with an associated hypertrophy of cortical astrocytes (Meyers, '69) and presumably an hypertrophy of the external limiting membrane astrocytes.We report here the results of an electron microscopic study of the external glial limiting membrane in normal rhesus monkeys and those subjected to perinatal asphyxia. The submicroscopic distributions of tonofilaments and microfilaments, which previously have been unknown in mammalian astrocytes, are described. The relationships of these two types of filaments to the plasma membrane and its associated basement membrane suggests similar relationships, well known in typical epithelial cells. MATERIALS AND METHODSThis study concern...

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