Background: A new type of junction-like membrane complex (JMC) was detected between adjacent astrocytes in the optic nerve of Japanese macaque (macaca fuscata). This membrane complex morphologically resembled a cell junction, but a possible role for potassium ion channels could not be denied based on freeze-fracture replica observation. We attempted to determine the chemical nature and function of the novel JMC.Methods: Using an electron microscope, we observed JMCs in the optic nerve astrocyte. In addition, we observed them using a freeze-fracture replica and immunohistochemistry with connexin 43, a gap junction specific protein. Furthermore, immunolocalization of an inwardly rectifying potassium ion channel, K AB -2 (Kir4.1), was studied with a confocal laser-scanning microscope, and an electron microscope using a newly developed pre-embedding method.Results: These JMCs were abundant around the blood vessel in the area just behind the lamina cribrosa. At JMCs the inner leaflet was thicker than the outer leaflet and electron-dense materials were packed in the intercellular space. Freeze-fracture replica observation revealed orthogonal arrays of particles, probably at the place of JMCs, that have been considered a potassium ion channel. No connexin 43 immunoreactivity was detected in JMCs, while K AB -2 was mostly localized on either side of the opposing cell membranes of JMC.Conclusions: These JMCs do not seem to be a simple junction, but relate to a potassium ion channel. The area just behind the lamina cribrosa may be important in terms of conductance of the optic nerve impulse. Anat.
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