Fig. 4.

Examples of broader gray matter seeding in sulci. Despite the larger seeding regions, the track lines are restricted by the dense U-fiber bundles in the superficial white matter. (A) Larger seed regions in four prominent sulci. (B) Track lines from these seed regions remain restricted to the local sulcus environment and fail to produce labeling elsewhere in the cerebral cortex despite the known long-range projections. (C) Visualization of the white matter distribution of the track line spreading for each seed in A, together with myelin-stained histological slide corresponding to two of the seeds. (D) Magnified view of myelin-stained histological section in the vicinity of the arcuate sulcus featured in the histological section in C. Stained fibers surrounding the sulcus are generally parallel to the WGB, whereas those at the adjacent gyrus are primarily radial in their orientation. This difference is presumed to give rise to the gyral bias associated with gray matter seeding. AS, arcuate sulcus; CS, central sulcus; LS, lateral sulcus; PMc, caudal premotor; PS principal sulcus.