Connectional neuroanatomy of U-fibers in the rhesus monkey brain

Technical Summary: Connectional Neuroanatomy of U-fibers in the Rhesus Monkey Brain

Problem Statement
The superficial white matter (SWM), located immediately beneath the gray matter-white matter (GM-WM) border, is estimated to comprise the majority of axons in the cerebral white matter and is critical for cortical information processing. Despite its prominence and increasing attention in diffusion MRI (dMRI) research, the connectional organization of the SWM remains poorly defined. The prevailing view, derived from both brain dissections and dMRI tractography, posits that U-shaped fiber bundles (U-fibers) are ubiquitous, symmetric elements that interconnect adjacent gyri beneath all sulci. However, this model has never been systematically evaluated using experimental tract tracing in the primate brain, leaving a gap in understanding the actual prevalence and organizational principles of these fibers.

Methodology
To address this gap, the authors conducted a systematic analysis of 28 archival macaque tract-tracing cases. The study utilized anterograde tracers injected into gyri adjacent to four major cortical sulci: the intraparietal, central, principal, and superior temporal sulci. These sulci were selected for their clear anatomical correspondences to sulci in the human cerebral cortex. The analysis focused on identifying labeled fibers that left the injection site, passed beneath a sulcus, and terminated on the adjacent gyrus, operationally defining these as U-fibers. The study examined the trajectory, incidence, and termination patterns of these fibers across different sulcal regions.

Key Results
The investigation revealed several organizational principles that challenge the canonical view of U-fibers:

• Limited Prevalence: While short association fibers were present in every case, true U-fibers were observed only in a minority of cases.
• Sulcus-Dependent Incidence: The presence of U-fibers was not uniform across the cortex. They were consistently found beneath the principal and central sulci but were uncommon beneath the intraparietal and superior temporal sulci.
• Trajectory Variability: When present, U-fibers did not follow a uniform trajectory. While some followed a symmetric, orthogonal course relative to the sulcus, others followed oblique trajectories to more distant regions.
• Diversity of Short Association Fibers: The study identified multiple classes of short association fiber bundles beyond the canonical U-fiber. These included bundles terminating on the sulcal bank proximal to the injection and those crossing the fundus of the sulcus to terminate on the opposing bank.
• Structural Distinctions:
  • A consistent 200–300 µm band of white matter beneath cortical layer 6 was identified. This band contained exclusively short association fibers approaching their cortical terminations and was selectively avoided by deeper long cortico-cortical pathways. This feature was present across all examined sulci.
  • U-fibers were found both within this superficial band and in the white matter below it.
  • Deeper fiber bundles were observed adopting a U-shaped course beneath a sulcus without interconnecting adjacent gyri, demonstrating that a U-shaped trajectory alone does not define a U-fiber.

Key Contributions
This study constitutes the first systematic connectional analysis of cortical U-fibers in the major sulci of the non-human primate brain. It provides empirical evidence that the SWM contains multiple classes of short association fiber bundles rather than a single, uniform U-fiber system. The findings refine the operational definition of U-fibers, distinguishing them from other short association fibers and deeper U-shaped trajectories that do not serve inter-gyral connectivity.

Significance
The paper concludes that the white matter beneath the cortex is a highly complex system where the organization of short association fiber bundles reflects the selective connectivity of the specific cortical areas separated by a sulcus, rather than a uniform architecture imposed by the geometry of the overlying sulcus. These results provide an anatomically grounded basis for better defining SWM organization in both anatomical studies and tractography. Furthermore, the findings have potential consequences for the interpretation of dMRI and anatomical studies of white matter, suggesting that current models assuming ubiquitous, symmetric U-fibers may require revision to account for sulcus-specific and trajectory-specific variations.