THE DISTRIBUTION AND MORPHOLOGY OF LGN-K PATHWAY AXONS WITHIN THE LAYERS AND CO BLOBS OF OWL MONKEY V1

The lateral geniculate nucleus (LGN) of primates contains three classe s of relay cells, the magnocellular (Mi, parvocellular (P), and konioc ellular (K) cells. At present, very little is known about either the s tructure or function of the K relay cells in New or Old World monkeys (simian primates). In monkeys, K cells are located between the main LG N layers and adjacent to the optic tract. For convenience, these inter calated cell layers are numbered K1-K4 starting closest to the optic t ract with K1. The objective of this study was to examine the details o f K axon morphology in the primary Visual cortex (V1) of owl monkeys a nd to determine if different K layers give rise to distinct axon types . For this purpose, injections of WGA-HRP or PHA-L were made into spec ific K LGN layers and the distribution and morphology of the resulting labeled axons were analyzed. Injections of fluorescent tracers also w ere made within the superficial layers of V1 to further document conne ctions via analysis of the patterns of retrogradely labeled cells in t he LGN. Our main finding is that K axons in owl monkeys terminate as d elicate focused arbors within single cytochrome oxidase (GO) blob colu mns in cortical layer III and within cortical layer I. Overall, the mo rphology of the K axons in these monkeys is quite similar to what we d escribed previously for K geniculocortical axons in the distantly rela ted bush baby (prosimian primate), suggesting that the basic features of this pathway are common to all primates. Our results also provide e vidence that the axon arbors from different K layers are morphological ly distinct; axons from LGN layer K1 project mainly to cortical layer I, while axons from LGN layer K3 chiefly terminate in cortical layer I II. Taken together, these results imply that the basic features of axo ns within the K pathway are conserved across primates, and that the K axons from different K layers are likely to differ in function based u pon their different morphologies.