Role of Mechanical Factors in the Morphology of the Primate Cerebral Cortex
Figure 3
Example of Three Types of Trajectories Linking Prefrontal Cortices
(A) Low-power darkfield photomicrograph of a coronal section through a middle level of the prefrontal cortex of a rhesus monkey brain showing an injection of HRP-WGA in area 9 (white area, “is”). Several projection sites can be seen (arrows). One projection with many labeled neurons is seen in the adjacent part of lateral area 9 (top, thick arrows); another projection is seen inferiorly in area 14 (medium arrows); a light projection is found in ventral area 46 below the principal sulcus (thin arrows), and another projection is seen in area 11. The dashed lines drawn in the white matter (appearing blue in darkfied) represent the shortest possible trajectory of axons in three of the projection sites. Axons linking lateral area 9 with the injection site (top) can take a fairly straight course. Labeled axons leaving the injection site are visible at this level (pink fibers in white matter), and some cross directly through layer VI (star). Axons linking area 14 with the injection site take a mildly deflected course through the white matter. Some labeled axons are seen exiting the injection site (pink fibers in white matter, below the depths of the cingulate [Cg] sulcus), and course inferiorly along the retrograde transport route to projection neurons. Axons linking ventral area 46 with the injection site travel below the principal sulcus (which is deep at this level) and must take a curved course.
(B) High magnification of a site from an adjacent section to the one depicted in (A), showing that some axons (gold) coursing below the injection site take a straight course through the deep layers of the cingulate cortex. The approximate limit of the white matter is denoted by the large arrow (bottom left). The tissue was photographed under dark field illumination to show labeled axons in the white matter (gold, to the left of the large arrow), as well as axons coursing among neurons (red, some indicated by arrows), photographed under bright field illumination. The two photographs were merged.
(C) Higher magnification of the mildly deflected projection in area 14 (from boxed area C in [A]) showing labeled neurons under brightfield illumination (blue pyramids). The labeled neurons at the top are in layer III, the ones at the bottom left corner are in layer V.
(D) Higher magnification of the curved projection from ventral area 46 (boxed area D from [A]), shows a few labeled neurons in layer III (blue) under brightfield illumination. The section was counterstained with neutral red.
Scale bars: (A) = 1 mm; (B–D), 100 μm. Orientation axes: medial, to the right; dorsal, on top.
Cg, cingulate; MO, medial orbital; P, principal; Ro, rostral.