Layer I of striate cortex of Tupaia glis and Galago senegalensis: projections from thalamus and claustrum revealed by retrograde transport of horseradish peroxidase.

We have examined the origin of the subcortical projections to the superficial layers of the striate cortex in Tupaia glis and Galago senegalensis by using the retrograde transport of HRP. Crystals of HRP were laid directly on the moist pial surface of the cortex which had been gently pricked with a small glass pipette. The diffusion of HRP was limited to layers I and II by restricting the length of time that the HRP was in contact with the surface. Following the application of HRP to the striate cortex, labeled cells were found in restricted regions of the lateral geniculate body of both species. Layers 4 and 5 of galago and layer 3 of tree shrew contained dense clusters of labeled cells. Labeled neurons were also found in the zones between the layers of the lateral geniculate body in both species and these cells were always in register with the labeled cells within the layers. In galago, curved columns of labeled cells were observed in the inferior and superior subdivisions of the pulvinar nucleus. These columns were arranged in the shape of two arcs, joined at the fiber bundle which separates the two subdivisions. The position of the bands in the pulvinar nucleus varied with the locus of the application in the striate cortex. While no labeled cells were seen in the body of the pulvinar nucleus of tree shrew, small labeled neurons were found in the external medullary lamina forming the capsule of the pulvinar nucleus. These cells were continuous with a larger population of labeled cells in the lateral intermediate nucleus. In both species, labeled cells were also found in the intralaminar nuclei (particularly the paracentral nucleus) and in the dorsal-caudal portion of the claustrum. In the claustrum, few unlabeled neurons were present within the zone containing labeled cells. In conclusion, layer I os striate cortex appears to be the site of convergence of several projection systems originating from principal and intralaminar thalamic nuclei as well as the claustrum. The significance of this overlap is discussed in terms of the total cortical extent of each system.