Astrocytic membrane morphology: differences between mammalian and amphibian astrocytes after axotomy

Abstract

Previous studies have shown that astrocytes in some nonmammalian species provide a favorable environment for axonal elongation, whereas mammalian astrocytes are thought to inhibit fiber outgrowth. The present study was performed to determine whether any plasma membrane differences exist between these glial elements which could account for their contrasting effects upon axonal outgrowth. Astrocytic scars were formed in optic nerves of rats, newts, and frogs by enucleation. Subsequently, the astrocytic membranes were examined with the freeze-fracture technique. Orthogonal arrays of small intramembranous particles (IMPs) are a prominent component of the plasma membranes of normal mammalian astrocytes; these arrays are most numerous in astrocytic membranes that form an interface between the CNS and nonneural tissue. Astrocytic membranes within the normal CNS parenchyma, however, possess much lower densities of arrays. Following axotomy and Wallerian degeneration, the density of arrays increased threefold within the parenchyma of the optic nerve, while remaining constant at the glia limitans. In striking contrast, only a few aggregates of IMPs that resembled orthogonal arrays could be found in normal and reactive astrocytes of amphibians, although the cytology of these glial cells and density of the scars are otherwise similar to those of their mammalian counterparts. These findings suggest (1) that a proliferation of orthogonal arrays in astrocytic plasma membranes is a prominent feature of gliosis in the mammalian CNS and (2) that differences in the composition of reactive mammalian and amphibian astrocytic membranes may account for variations in axonal-glial interactions within the injured CNS.