Specialization of astrocytic membrane at glia limitans in rat optic nerve: freeze-fracture observations.

The ultrastructure of astrocytic foot processes at the glia limitans in rat optic nerve was studied by freeze-fracture. Two classes of astrocytic end-foot processes were observed. Most astrocytic foot processes in this region show a high density of orthogonal arrays of P-face particles (assemblies) that are randomly oriented. However, astrocytic foot processes with highly organized columns of assemblies are also observed. These columns are 1-4 assemblies wide (approximately 0.1 micron), with the center-to-center distance between the columns being relatively constant (approximately 0.17 micron). Columns of at least 4 micron length have been observed. The regional specialization of the astrocyte membrane and the differentiation of subpial astrocytes into two structural classes may have important functional implications.