Examples of the Relationship of Projection Density to Axonal Trajectory

<div><p>(A) Low-power brightfield photomicrograph of a coronal section through the posterior prefrontal cortex of a rhesus monkey brain, showing the halo of an injection of HRP-WGA in ventral area 46, below the principal sulcus (dark area). Green arrows point to a projection site in area 46 in the depths of the lower bank of the principal sulcus (dark blotch). Another projection is found in the upper bank of the lower limb of the arcuate (“A”) sulcus (dark blotch, red arrow); axons from these projection sites can take a mildly deflected or straight course to the injection site.</p><p>(B) Top: higher magnification under darkfield illumination of the depths and lower bank of the principal sulcus in the same section shows that the projection in ventral area 46 has many labeled neurons (gold, green arrows), and labeled axons (pink, white arrowhead) as they leave the halo of the injection site. Note that the dense projection abruptly terminates as the sulcus curves towards the upper bank of the principal sulcus, towards dorsal area 46. The magnified site at the lower right inset (blue arrow, brightfield illumination) shows only a few scattered labeled neurons in dorsal area 46 (white arrowheads), taken from the region with the corresponding blue arrows in (A, brightfield) and (B, darkfield). In contrast, the “straight” projection in the arcuate sulcus has many labeled neurons (lower left inset in [B], red arrow; darkfield illumination). The section was counterstained with neutral red.</p><p>(C) Low-power darkfield photomicrograph of a coronal section through the caudal prefrontal cortex of a rhesus monkey brain showing the halo of the injection site in area OPAll/OPro (white area, bottom, center), and a resultant robust projection site of labeled neurons (white) arranged in a columnar pattern in area 25 (white arrows). The labeled fibers (white) around the putamen (Put) and in the internal capsule (interposed between the caudate and putamen, white) link the injection site with subcortical structures.</p><p>(D) The columns of labeled neurons are shown at higher magnification. The inferiorly situated broad column of neurons (thick arrow) has a fairly straight course from the injection site, while the one above is deflected. Axons leaving the halo of the injection site are visible at this level (white fibers in white matter, which appears blue under darkfield illumination).</p><p>Scale bars: (A), (C), 1 mm; (B), (D), 0.5 mm. Orientation axes: Medial, is to the left; dorsal, at the top.</p>