COMPARATIVE MORPHOLOGY OF 3 TYPES OF PROJECTION-IDENTIFIED PYRAMIDAL NEURONS IN THE SUPERFICIAL LAYERS OF CAT VISUAL-CORTEX

The morphology and dendritic organization of corticocortical neurons i n the superficial layers of area 18 that project to area 17 were studi ed by intracellular injection of lucifer yellow in the fixed-slice pre paration. This corticocortical population contains primarily standard pyramidal cells, but occasional nonpyramidal, modified, fusiform, star , and inverted pyramidal cells were also seen. Al cell types were pres ent throughout layer 2 and in the upper and middle parts of layer 3. S tandard pyramidal cells were found exclusively in lower layer 3. The m ean somatic area of the area 17 projecting neurons was 251 mu m(2). Th e width of basal dendritic fields was correlated to cell size for stan dard pyramidal cells but not for the other cell types. Next, the morph ology and dendritic organization of the area 17 projecting neurons wer e compared to the pyramidal cells of the local horizontal patch networ ks and of the callosal system. The depth profile of the area 17 projec ting and callosal pyramidal groups was virtually identical, peaking at 400 mu m from the pial surface, whereas the local patch pyramidal gro up peaked at 281 mu m. The local patch, area 17 projecting, and callos al pyramidal cells displayed increasingly larger mean somatic areas an d basilar dendritic field width measurements. The number of basal dend ritic branch points was greatest for callosal cells, and it was indist inguishable between local patch and area 17 projecting neurons. In the tangential plane, circular dendritic fields were observed on all call osal cells, but they were found on only approximately half of the loca l patch and area 17 projecting neurons. The remaining local patch and area 17 projecting neurons displayed mediolaterally and anteroposterio rly elongated basal dendritic fields, respectively. (C) 1996 Wiley-Lis s, Inc.