Morphological organization of thalamocortical relay cells in the dorsal lateral geniculate nucleus of the North American opossum.

The morphology of relay cells in the dorsal lateral geniculate nucleus of the North American opossum was studied by using both Golgi-Cox material and cells stained from retrograde transport of horseradish peroxidase. In general, soma sizes were largest in the part of the nucleus representing the central retina and decreased from the middle third of the nucleus to the anterior to posterior poles. Relay cells labeled with horseradish peroxidase were found to constitute approximately 90% of the dorsal lateral geniculate nucleus cells and have larger soma diameters than most unlabeled cells. From morphometric analysis of several structural characteristics, three classes of relay cells were identified in both Golgi-Cox and horseradish peroxidase material. Type 1 cells, the predominant class, exhibited radially arranged primary dendritic fields, symmetrically organized relative to projection lines. Type 2 cells had relatively few primary dendrites, and complex dendritic fields that were oriented parallel to projection lines. Least numerous were Type 3 cells, which were characterized by relatively sparse dendritic fields oriented perpendicular to projection lines. An additional class of neuron, Type 4 cells, with small somata and sparse dendritic branching, was found only in Golgi-Cox material. Cells with Type 4 dendritic morphology were not found with retrograde horseradish peroxidase labeling and may represent interneurons. The classification of morphologically characterized cells in the opossum dorsal lateral geniculate nucleus was evaluated quantitatively with multivariate discriminant analysis. The classes are compared to physiologically identified Y-, X-, and W-like relay cells in the opossum and to relay cell classes in other species.