Plasticity of astroglia: evidence supporting process elongation by "stretch"

The structural plasticity of cerebral astroglia was investigated in vivo by implantation experiments. Immunocytochemical markers for glia filament protein were used to identify the astrocytes. First it was established that implanted nitrocellulose filters provided a substrate for astrocytes from different brain regions of young rats. Astrocytes attached to the filter and projected fine processes into it. Longer implantation times increased the density and length of glial processes within filter spaces. Astrocytes that penetrated the filters implanted in the tectum exhibited more processes than those in the cortex, suggesting regional differences of astrocyte distributions. Second it was observed that astrocytes that attached to the filter formed elongated processes when they were tethered within an expanding matrix. This was shown by implanting the nitrocellulose filter together with PC12 cells that continued to grow. The implantation of neither PC12 cells without filters nor nitrocellulose filters alone induced the formation of elongated astroglia with parallel aligned processes, resembling radial glia. Such glial forms only occurred in the filter/PC12 cell cografts. This indicates that processes of astrocytes adherent to nitrocellulose filters could be stretched in response to expansion of the surrounding tissue.