Cell junctions and intramembrane particles of astrocytes and oligodendrocytes: a freeze-fracture study

The plasma membranes of astrocytes and oligondendrocytes in the white matter of the cat were studied with the freeze-fracturing technique. The intramembrane particle profiles differ in the two type of cell. Orthogonal, small particle assembles and isolated globular particles 5-18 nm in diameter characterize the astrocytic plasmalemma, whereas the plasma membrane of oligodendrocytes shows large, tall globular particles, small globular particles, small ellipsoidal particles and previously undescribed, thin, short, rectilinear strands composed of fused subunits. Using these distinct differential features we can identify partners of glial cell junctions. We confirm the existence of interastrocytic gap junctions. Moreover, we identify numerous heterologous gap junctions between astrocytes and oligodendrocytic cell bodies, processes and the outer turn of myelin sheaths. Interoligodendrocytic gap junctions are not observed. Adjacent oligodendrocytes, however, form tight junctions consisting of linear P face strands and rows of particles; tight junctions are a reliable marker for oligodendroglial membranes. Connexons of interastrocytic gap junctions are packed in a crystalline array, while astrocyte-oligodendrocyte junctional connexons are closely packed but not crystalline. This study indicates that gap junctions between glial cells are pleomorphic and non-randomly distributed. The junctions between astrocytes and those between astrocytes and oligodendrocytes may had different roles in interglial and neuron-glia cooperation.