A novel junction‐like membrane complex in the optic nerve astrocyte of the Japanese Macaque with a possible relation to a potassium ion channel

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. Rec. 250:465‐474, 1998. © 1998 Wiley‐Liss, Inc.