Radial and tangential neuronal migration pathways in the human fetal brain: anatomically distinct patterns of diffusion MRI coherence.

Corticogenesis is underpinned by a complex process of subcortical neuroproliferation, followed by highly orchestrated cellular migration. A greater appreciation of the processes involved in human fetal corticogenesis is vital to gaining an understanding of how developmental disturbances originating in gestation could establish a variety of complex neuropathology manifesting in childhood, or even in adult life. Magnetic resonance imaging modalities offer a unique insight into anatomical structure, and increasingly infer information regarding underlying microstructure in the human brain. In this study we applied a combination of high-resolution structural and diffusion-weighted magnetic resonance imaging to a unique cohort of three post-mortem fetal brain specimens, aged between 19 and 22 post-conceptual weeks. Specifically, we sought to assess patterns of diffusion coherence associated with subcortical neuroproliferative structures: the pallial ventricular/subventricular zone and subpallial ganglionic eminence. Two distinct three-dimensional patterns of diffusion coherence were evident: a clear radial pattern originating in ventricular/subventricular zone, and a tangentio-radial patterns originating in ganglionic eminence. These patterns appeared to regress in a caudo-rostral and lateral-ventral to medial-dorsal direction across the short period of fetal development under study. Our findings demonstrate for the first time distinct patterns of diffusion coherence associated with known anatomical proliferative structures. The radial pattern associated with dorsopallial ventricular/subventricular zone and the tangentio-radial pattern associated with subpallial ganglionic eminence are consistent with reports of radial-glial mediated neuronal migration pathways identified during human corticogenesis, supported by our prior studies of comparative fetal diffusion MRI and histology. The ability to assess such pathways in the fetal brain using MR imaging offers a unique insight into three-dimensional trajectories beyond those visualized using traditional histological techniques. Our results suggest that ex-vivo fetal MRI is a potentially useful modality in understanding normal human development and various disease processes whose etiology may originate in aberrant fetal neuronal migration.