Rabbit retinal Müller cells in cell culture show gap and tight junctions which they do not express in situ.

Retinae of early postnatal rabbits were enzymatically dissociated and explanted in a culture system. The prospective myelinated region was discarded in order to avoid the presence of astrocytic or mesenchymal cells. After about 14 days in vitro (DIV), outgrowing glial (Müller) cells formed what light optically appeared to be confluent monolayers but by electron microscopy was shown to consist of flat epithelioid cells which overlapped considerably by extension of cytoplasmic tongues. Applying the freeze-fracture technique, apposed membranes of these cells were demonstrated to express infrequently but consistently both gap and tight junctions. This kind of junctions has never been observed on the membrane of rabbit Müller cells in situ. In comparison with Müller cell membranes in situ, the density of intramembrane particles was considerably reduced. Orthogonal arrays of particles which are characteristic elements of Müller cells in situ were not detected. Our results suggest that in homogeneous cell culture, Müller cells form some kind of epithelium-like specialized intercellular junctions. This situation resembles that of closely related glial cell types which form homogeneous layers in situ as e.g. retinal pigment epithelium cells expressing tight junctions, and marginal astrocytes being coupled by extensive gap junctions.