Percentage of relay cells in the cat's lateral geniculate nucleus.

Guillery ~ used the Golgi method to study 4 classes of cells in the cat's lateral geniculate nucleus. His 3 largest classes (classes 1, 2, and 4) of cells were identified as relay ceils projecting to visual cortex, and cells in his smallest class (class 3) were identified as interneurons. Since Golgi methods selectively and somewhat capriciously stain less than 2 ~ of the available neurons 21, it is almost impossible to derive a clear estimate from such data of the relative ratio of relay cells versus interneurons. The recent development of the horseradish peroxidase (HRP) marking technique offers a much less selective means of identifying lateral geniculate relay cells. That is, HRP injected into visual cortex is picked up by geniculocortical axons and/or terminals and transported back to the cell body where it is visualized by a reaction product t°. Norden and Kaas l~,z0 made use of this technique in three primate species (M. mulatta, A. trivirgatus, and G. senegalensis) to determine that lateral geniculate relay cells comprise at least 94-98 ~ of the neurons in that nucleus. However, previous estimvtes from the cat suggest perhaps a lower percentage of lateral geniculate relay neurons. Chow and Dewson 3 concluded from retrograde degeneration following cortical lesions that only 65 ~ of these neurons projected to cortex, but because of the possibility of sustaining geniculocortical collaterals, this should be viewed as a minimum estimate. HRP injected into single cortical areasS,7, H never produced more than 70~,, labeled neurons in the lateral geniculate nucleus. However, these experiments were not designed to study the percentage of relay cells since injections were limited to small cortical loci, and it appears that different populations of lateral geniculate neurons project to different cortical regions4,7,14,24, 25. Also, estimates based on the HRP technique must be viewed as a minimum estimate because not all cells with axons projecting to an area of HRP injection (i.e. lateral geniculate relay cells) will transport the enzyme 17. This raises the question as to whether the lower estimates of lateral geniculate relay cells in the cat compared to the monkey represent a true species difference or an artifact due both to less efficient labeling of cat relay cells for unknown reasons and also to a failure to label completely the geniculocortical projection. We sought to answer this question by applying a potentially more sensitive HRP technique and by extensive HRP injections throughout …