Insights into Temporal and Spatial Dynamics of Short Association Fiber Formation in the Human Fetal Brain.

Short association fibers (SAFs) form the local scaffold of cortical connectivity, supporting early functional specialization and marking sites of neurodevelopmental vulnerability. However, their development before birth remains largely unknown. Leveraging advanced fetal diffusion MRI and a histology-validated framework, we present the first in-utero reconstruction of SAF pathways in the human brain. We tracked their volumetric and microstructural developmental trajectories in 243 fetuses spanning a critical period when the brain's connectome is rapidly forming. We found that SAFs emerge before sulcal folding, initially as flat, loosely arranged pathways along the subplate-white matter interface, and later reorganize into coherent U-shaped bundles. Their maturation followed a sensorimotor-to-association gradient, paralleling cortical development. Nonlinear, bundle-specific trajectories captured multiphasic maturation, with subplate dynamics preceding increases in axonal coherence and early myelination. By filling a missing link in lifespan brain connectivity, this study establishes a prenatal reference for cortical wiring and provides tools for investigating the origins of neurodevelopmental disorders.