Morphological analysis of astrocytes in the bullfrog (Rana catesbeiana) spinal cord with special reference to the site of attachment of their processes.

An attempt was made to elucidate the morphological features of astrocytes in the bullfrog spinal cord by means of a combination of electron microscopy, the Golgi method, and the intravascular dye injection method. Astrocytic somata are densely concentrated both in the wall of the central canal and in its proximity, and diffusely distributed in the intermediate and the ventral part of the gray matter. The most complicated and densest vascular network is found in the dorsal part of the gray matter. There is little correlation between the density of the distribution of astrocytic somata and that of the vascular network. Each astrocyte emits one process and ramifies by repeated bifurcations as it approaches the white matter or enters it. All these branching processes reach the pial surface of the spinal cord (the principal processes). Total rostrocaudal extent of their ramification is within 400 micron. A great number of small lateral offshoots (the secondary processes) arise both from the somata and the principal processes. Electron microscopy of vessel walls and the pial surface revealed that the principal processes attach to the subpial basement membrane with a specialized structure, an electron-dense layer, while the secondary processes merely surround blood vessels in a mode of juxtaposition. Comparison between amphibian and mammalian astrocytes is made regarding the site of attachment of their processes.