Reactive astrocytes in myelin-deficient rat optic nerve reveal an altered distribution of orthogonal arrays of particles (OAP).

Reactive astrocytes are a common feature of various pathological conditions within the CNS. Morphological changes of reactive astrocytes include an altered nucleus-cytoplasm relationship, nuclear indentations, an increased amount of intermediate filaments, and an immunologically immature phenotype. Additionally, the number of orthogonal arrays of particles (OAP) was found to be increased within parenchymal membranes of reactive astrocytes. This observation prompted us to investigate the distribution of astroglial OAP in the amyelinated CNS of the myelin-deficient (md) rat in which reactive astrocytes prevail. In the present freeze-fracture study, astroglial OAP were determined within endfoot and nonendfoot (parenchymal) membranes in the developing optic nerve of md rats and normal littermates at the age of 12, 23, 40, and 64 days postnatally (dpn). The endfoot OAP density in md astrocytes remained constant during the entire period of investigation. In myelinated littermates, OAP densities continuously increased up to adult values. In contrast, the parenchymal OAP density in md astrocytes increased during the entire period of investigation. In normal littermates, OAP densities remained constant during the first 40 dpn and thereafter increased rapidly. These observations suggest that the absence of myelinogenesis in the md mutant may be a stimulus for parenchymal membranes of reactive astrocytes to insert OAP or to assemble OAP subunits into complete arrays.