Freeze-fracture studies of plasma membranes of astrocytes in freezing lesions.

Epileptic foci are frequently associated with a proliferation and hypertrophy of fibrous astrocytes. Such proliferating astrocytes, which are involved in scar formation and are referred to as reactive, have been characterized by a number of morphological changes involving the cytoplasmic organelles and nucleus. With the development of freeze-fracture techniques, which allow for the splitting and analysis of the macromolecular structure of biological membranes, another morphological characteristic could be used to define reactive astrocytes. This characteristic is the alteration in the normal number and distribution of intramembranous particles. Specifically, there is an increase in the number of orthogonal arrays of intramembranous particles, called assemblies, in the plasma membrane of reactive astrocytes. Besides the presence of assemblies in the plasma membranes of astrocytes, similar particle arrays are also found in the central nervous system (CNS), in cell membranes of ependymal cells, in Müller cells near the inner limiting membrane of the retina, and in satellite cells in spinal ganglia. Assemblies are concentrated in the plasma membranes of astrocytic processes that form the glia limitans at the outer surface of the brain and the perivascular sheath around parenchymal blood vessels in the adult mammalian CNS. Plasma membranes of perineuronal astrocytic processes and astrocytic cell bodies have few, if any, assemblies. The factor responsible for this polarity of assemblies in the normal CNS is unknown. However, this polarity is lost when astrocytes respond to various types of injuries to the CNS. It has been postulated that gliosis, resulting in an epileptic scar, may be a response of cortical astrocytes to changes in the local cellular environment and that this reactivity of astrocytes may be related to plasma membrane changes. However, little is known about the initiation of the astrocytic response or the involvement of the astrocytic plasma membrane. Because of the presence of assemblies, astrocytic plasma membranes are an ideal subject for a morphological investigation of alterations of the cell membrane. Assemblies have a distinctive structure and changes in their number and distribution can be readily identified. Also, since there is polarity of the assemblies in astrocytic plasma membranes, such that their number is greatest in subpial and perivascular areas and least in the perineuronal regions, it is easy to identify when assemblies are added or lost from the astrocytic plasma membranes of a particular brain region.(ABSTRACT TRUNCATED AT 400 WORDS)