Pyramid-Shape Crossings and Intercrossing Fibers Are Key Elements for Construction of the Neural Network in the Superficial White Matter of the Human Cerebrum.

Structural analysis of the superficial white matter is prerequisite for the understanding of highly integrated functions of the human cerebral cortex. However, the principal components, U-fibers, have been regarded as simple wires to connect adjacent gyri (inter-gyral U-fibers) but have never been thought as indispensable elements of anatomical structures to construct the cortical network. Here, we reported such novel structures made of U-fibers. Seven human cerebral hemispheres were treated with Klingler's method and subjected to fiber dissection (FD). Additionally, tractography using diffusion spectrum imaging (DSI) was performed. Our FD and DSI tractography succeeded disclosing a new type of U-fibers that was hidden in and ran along the white matter ridge of a gyral convolution (intra-gyral U-fibers). They were distinct from inter-gyral U-fibers which paved sulcal floors. Both intra- and inter-gyral U-fibers converged from various directions into junctional areas of white matter ridges, organizing novel anatomical structures, "pyramid-shape crossings". U-fibers to form pyramid-shape crossings also render routes for communication between crossings. There were 97 (mean, range 73-148) pyramid-shape crossings per lateral cortical surface. They are key structures to construct the neural network for intricate communications throughout the entire cerebrum. They can be new anatomical landmarks, too, for the segmentation of the cerebral cortex.