Deep-etch structure of astrocytes at the superficial glia limitans, with special emphasis on the internal and external organization of their plasma membranes.
The cytoskeletal system in rat subpial astrocytes and the relationship between astrocytic plasma membrane and basal lamina or cytoplasmic components were examined with a quick-freeze deep-etch technique, mainly using chemically fixed tissues. Attention was focused on the way intramembrane particles (IMPs), particularly orthogonal arrays, are organized in the membranes and related to extramembrane components. The basal lamina was composed of a sheet-like network of strands (4-9 nm thick), some, which we have called 'trabecular' strands, extending through the lamina lucida to touch the astrocytic membrane at irregular intervals. The trabecular strands usually formed a bulbous structure where they touched the membrane, but sometimes appeared to intrude directly into the external lipid layer. The orthogonal arrays did not extend to the outer true surface, and no special structure was detectable in association with them. Small spherical protrusions (7-9 nm in diameter), related to neither the trabecular strands nor the arrays, were observed in the outer surface. Judging from their size and distribution, these are probably tops of tall globular IMPs. In the inner or cytoplasmic true surface, protrusions were relatively numerous; some were large, 15-20 nm in diameter, while others were small (8-10 nm). Some of the small protrusions were identified as transmembrane components. Although protrusions were more conspicuous in the inner than in the outer surface, none of them provided images related or similar to the orthogonal arrays. Some protrusions in the inner surface were connected with thin (4-5 nm) or thick (approximately 10 nm) filaments constituting the underlying network. The thin filaments were also anchored to the intermediate filaments which lay parallel with the astrocytic membranes. In the cytoplasm, the intermediate filaments were firmly packed to form bundles. Because the orthogonal arrays are probably embedded within the astrocytic membrane, they may not serve as a transmembrane channel but rather contribute to some stabilizing function for the membrane.
Gotow
T
T
Department of Anatomy, Osaka University Medical School, Japan.
Hashimoto
P H
PH
D000375:Aging; D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000648:ultrastructure*; D001485:Basement Membrane / Q000648:ultrastructure; D002462:Cell Membrane / Q000648:ultrastructure; D005613:Freeze Etching; D007382:Intermediate Filaments / Q000648:ultrastructure; D051381:Rats