Arrays of Particles in Freeze-Fractured Astrocytic Membranes

DENNIS M. D. LANDIS and T. S. REESE. From the Laboratory of Experimental Pathology, NationalInstitute of Arthritis, Metabolism and Digestive Diseases and the Laboratory of Neuropathology andNeuroanatomical Sciences, National Institutes of Health, Bethesda, Maryland 20014INTRODUCTIONPlasma membranes can be split in freeze-fracturepreparations, revealing details of their internalstructure (1, 2, 7, 9). The membrane faces soexposed typically are dotted with globular par-ticles, apparently arranged at random over mostof their surface. At cell-to-cell junctions, how-ever, the particles are often in aggregates coex-tensive with the junction. At gap junctions, forinstance, large particles are closely packed in anhexagonal array (5). In the course of a freeze-fracture study of synaptic junctions in the mam-malian central nervous system, we have found adistinctive type of particle array which charac-terizes the plasma membrane of astrocytes. Thesearrays, which we refer to as assemblies, aredistinguished by the small size of their subunitparticles and by the orthogonal packing of theseparticles. Furthermore, the distribution of assem-blies does not coincide with any junction or316other previously recognized ultrastructural fea-ture of the astrocytic plasmalemma.METHODSMorphine sulfate (20-100 mg/kg) was injected intothe peritoneal cavity of adult mice, rabbits, orchinchillas before cardiac perfusion with 0.08 Mcacodylate or phosphate-buffered formaldehyde-glutaraldehyde for 10 min at 37°C. Selected regionsof brains were immediately dissected, rinsed inbuffer, equilibrated with 20% glycerol, frozen inFreon-22 cooled by liquid nitrogen, fractured in aBalzers 360 M apparatus (Balzers High VacuumCorp., Santa Ana, Calif.) at -119°C, and etched15-90 s. A platinum-carbon replica of the fracturedsurface was subsequently prepared with an electron-beam gun, cleaned in Clorox and mounted on aFormvar-coated slot grid.RESULTSThree regions were selected for study : olfactorybulb, cerebellar cortex, and anteroventral coch-