Assemblies of particles in the cell membranes of developing, mature and reactive astrocytes

Orthogonal arrays of small intramembranous particles characterize freeze-fractured astrocytic plasma membranes. The normal variation of assemblies in plasma membranes of subpial astrocytic processes in mature and developing rats was established and compared with assemblies in plasma membranes of reactive astrocytes. In mature rats, subpial astrocytic processes had the greatest number of assemblies. As the parenchyma was approached, this number decreased in each successive layer of astrocytic processes. The advent of assemblies within the plasma membrane is a new criterion of astrocytic differentiation. The foot processes of subpial astrocytes in foetal rats began to acquire assemblies between day 19 and 20 and continued to mature postnatally by a constant addition and rearrangement of assemblies. In contrast to the paucity of assemblies in deeper laminae of the normal brain, reactive astrocytes comprising the lower lamellae in glial scars had an increased number of assemblies while the most striking feature of the superficial, astrocytic processes within the scar was a rearrangement of assemblies. Although the function of these intramembranous particles in still unknown, it appears from our measurements that they are localized primarily in the outermost astrocytic foot process and to a progressively lesser degree in the underlying astrocytic lamellae.