Terminations of individual optic tract fibers in the lateral geniculate nuclei of Galago crassicaudatus and Tupaia belangeri

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Abstract
The morphology and laminar distribution of individual optic fibers projecting to the lateral geniculate nucleus (GL) of Galago and Tupaia were studied following iontophoretic injections of horseradish peroxidase (HRP) into the optic tract. In Galago the GL is composed of three functionally matched pairs of layers, each characterized by cells of a given size, one large, one medium‐sized, and one small. The results show that there is a close correspondence between the size of the afferent fibers and the size of the neurons in the target layer: large axons project to the magnocellular layers, medium‐sized axons project to the parvicellular layers, and small fibers project to the intercalated layers. In Tupaia the GL is composed of two functionally matched pairs and two unmatched layers. Optic fibers that project to the medial matched pair (1 and 2) are only slightly larger than those that project to the lateral matched pair (4 and 5), but both are larger than those that project to the unmatched layers (3 and 6). In both species terminal arbors and the distribution of terminal boutons within layers corresponded closely with the organization of dendritic processes of cells in the target layer. This correspondence was particularly evident in the parvicellular layers in Galago and in layer 6 in Tupaia: parvicellular terminal arbors, like the dendrites of parvicellular cells, are organized in narrow columns oriented along lines of projection, whereas layer 6 terminal arbors, like the dendrites of layer 6 cells, are oriented in elongated strips perpendicular to lines of projection. In both species there was evidence for sublaminar terminations in some layers. These were restricted to the parvicellular layers in Galago and layers 4 and 5 in Tupaia. With the exception of a small number of fine fibers in the intercalated layers in Galago, optic fibers in both species terminated in one and only one layer in a set. The significance of this result depends on the relation between ganglion cell classes and what is being segregated in different GL layers. Lateral geniculate lamination varies even in closely related species and has evolved independently in such distantly related lines as carnivores and primates. It is not surprising, therefore, that what is being segregated varies from species to species.

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