English: Advances in techniques for labeling axon tracts and circuits. (A) Carbocyanine dye labeling in the developing brain. DiI-labeled callosal axons are shown in red (arrow in A). Labeling was performed on fixed embryonic day 17 mouse brain. (image courtesy of Dr Celine Plachez, University of Maryland). (B) Labeling of neurons in the brain of a “brainbow” mouse. Different neurons are visualized with different hues of color generated by Cre/loxP recombination in transgenic mice (image courtesy of Dr Jeff Lichtman and Dr Tamily Weissman, Harvard University). (C) Diffusion-weighted (30 directions) magnetic resonance image acquired at 16.4 Tesla—colormap demonstrating commissural tracts in a midsagittal view. Based on their orientation, commissural fibers have been color-coded in red, including the corpus callosum (arrow in C) and anterior commissure (arrowhead in C). (D and E) are tractography images of high angular resolution imaging (HARDI/q-ball). In D, regions of interest (ROI) were selected across the brain, with axon tracts shown that pass through the midline. E demonstrates a more selective placement of ROI’s, one at the midline within the anterior commissure (arrowheads in E depict both the anterior and posterior arms of the anterior commissure that pass through the ROI at the midline), and one in the hindbrain at the midline within the middle cerebellar peduncle and pontine transverse fibers (arrowhead in E). Images in C–E courtesy of Dr Nyoman Kurniawan and Dr Randal Moldrich (The University of Queensland). Scale bar in E = 400 µm in A, 80 µm in B, 2 mm in C and E and 1.35 mm in D.