Cerebral Cortex Advance Access published online on January 10, 2007
Cerebral Cortex, doi:10.1093/cercor/bhl142
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The Morphology of the Koniocellular Axon Pathway in the Macaque Monkey
1 Department of Cell & Developmental Biology, 2 Department of Psychology, 3 Department of Ophthalmology & Visual Sciences, Vanderbilt University, Nashville, TN 37232, USA, 4 Current address: Department of Anatomy, Gulhane Military Medical Academy, Faculty of Medicine, 06018 Etlik, Ankara, Turkey, 5 Current address: Department of Neurosurgery, University of Texas Health Science Center 7703 Floyd Curl Drive, MC 7843 Room 102F MED, San Antonio, TX 78229, USA
Address correspondence to V. A. Casagrande, Department of Cell & Developmental Biology, Vanderbilt Medical School, U3218 Learned Lab, Nashville, TN 37232-8240, USA. Email: vivien.casagrande{at}vanderbilt.edu.
The key objective of this study was to determine the distribution and morphology of koniocellular (K) lateral geniculate nucleus (LGN) axons in primary visual cortex (V1) of the macaque monkey. In particular, we were interested in understanding whether subpopulations of K axons exist in this species and, if so, if these subpopulations arise from different K layers of the LGN. Restricted injections of the tracers, biotinilated dextran amine, or Phaseolus vulgaris leucoagglutinin were targeted to specific LGN K layers under electrophysiological guidance and immunocytochemistry was used to visualize labeled axons in cortex that were subsequently reconstructed through serial sections. A total of 36 complete axons and 166 axon segments were reconstructed. Our results identified at least 2 main subpopulations of K axons in macaque V1 based on branching patterns and bouton distribution. Axons that arise primarily from LGN layers K1 and K2 are morphologically simple and tend to branch in cortical layers 1 and 3A. These axons give rise to fewer boutons than seen in axons arising from the dorsal K LGN layers K3–K6. Axons that arise from LGN layers K3–K6 terminate as complex, focused arbors in the cytochrome oxidase (CO) blobs in layer 3B
, with only occasional simple projections to the more superficial layers of cortex. Combined with previous observations, our data suggest that there are at least 3 subclasses of K LGN axons in macaque monkey that are similar to K axons identified earlier in both nocturnal simian owl monkeys (Ding and Casagrande 1997) and in prosimian, bush babies (Lachica and Casagrande 1992) suggesting that the LGN K channels that terminate in the CO blobs and in layer 1 are not unique to macaque monkeys but are a common primate feature.
Key Words: geniculocortical pathway • lateral geniculate nucleus (LGN) • parallel pathway • primate • striate cortex
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