Heterogeneous growth of the insula shapes the human brain

The human cerebrum consists of a precise and stereotyped arrangement of lobes, primary gyri, and connectivity that underlies human cognition [P. Rakic, Nat. Rev. Neurosci. 10, 724-735 (2009)]. The development of this arrangement is less clear. Current models explain individual primary gyrification but largely do not account for the global configuration of the cerebral lobes [T. Tallinen, J. Y. Chung, J. S. Biggins, L. Mahadevan, Proc. Natl. Acad. Sci. U.S.A. 111, 12667-12672 (2014) and D. C. Van Essen, Nature 385, 313-318 (1997)]. The insula, buried in the depths of the Sylvian fissure, is unique in terms of gyral anatomy and size. Here, we quantitatively show that the insula has unique morphology and location in the cerebrum and that these key differences emerge during fetal development. Finally, we identify quantitative differences in developmental migration patterns to the insula that may underlie these differences. We calculated morphologic data in the insula and other lobes in adults (N = 107) and in an in utero fetal brain atlas (N = 81 healthy fetuses). In utero, the insula grows an order of magnitude slower than the other lobes and demonstrates shallower sulci, less curvature, and less surface complexity both in adults and pro-gressively throughout fetal development. Spherical projection analysis demonstrates that the lenticular nuclei obstruct 60 to 70% of radial pathways from the ventricular zone (VZ) to the insula, forcing a curved migration to the insula in contrast to a direct radial pathway. Using fetal diffusion tractography, we identify radial glial fascicles that originate from the VZ and curve around the lenticular nuclei to form the insula. These results confirm existing models of radial migration to the cortex and illustrate findings that suggest differential insular and cerebral development, laying the groundwork to understand cerebral malformations and insular function and pathologies.SignificanceBuried in the depth of the Sylvian fissure, the insula not only has a flatter, less convoluted surface than the cerebral convexity, but also is the focal point around which the cerebrum is arranged. Existing models of cortical folding do not explain why the insula has a unique morphology, nor its central location in the cerebrum. Here, we quantitatively explore the unique morphology and development of the insula by analyzing in utero fetal and adult brain MRI and diffusion tractography. We show that the insula has both a divergent growth and folding trajectory than the other lobes. Specifically, the insula exhibits much lower surface area expansion, a main force driving cortical folding, compared to other lobes. A curved stream of radially migrating cells originates from the ventricular zone and curve around the lenticular nuclei to reach the insula. Differential expansion between the insula and other lobes results in the formation of the Sylvian fissure and thus defines the interlobar geometry of the human brain.