Neuron-glia relationship in the bipolar cell layer of the fish retina

Abstract

The present study of the bipolar cell layer of the fish Centropoma reveals details of the cells forming this layer, and the relationship existing between the neurons and the glial cells. The cell types are the horizontal, bipolar, amacrine, and Muller cells. The horizontal cells are organized in three rows which occupy the two external thirds of the layer, the first row being formed by the external horizontal cells and the other two rows by the internal horizontal cells. In cross section, each row appears as a reticulum in which the cells are interconnected by apposition of their processes. The ultrastructure of the horizontal cells suggested that they are glial cells rather than neurons. This is further supported by their electrical and osmotic behavior. Some bipolar cells and the Muller cell processes occupy the free spaces among the horizontal cells. Most bipolar cells are concentrated toward the inner third of the layer. The amacrine cells and the Muller cells, which present their maximal diameter in this zone, are also found in the inner third. The amacrine cells present two different patterns, one neuronal and the other glial. Some retinas were incubated and fixed in anisotonic media for study of the cellular osmotic behavior. In hypotonic media, the ∼60 A intercellular space was not modified by the osmotically induced swelling of the neighboring cells. The osmotically induced shrinkage of the cells produced a ∼15 A increase in the average width of the intercellular space. The horizontal cells were the most affected by the anisotonicity. The characteristic osmotic response of each cellular type shows the structural and functional individuality of the cell membrane. The glial cells, forming the largest part of the extraneuronal space, is considered a highly organized cellular compartment.