Contralateral ION transections arrests target selection of layer II/III callosal neurons in the contralateral cortex. Slices from P10 mouse brains counterstained with Hoechst (red) showing somatosensory callosal neurons expressing EGFP (green) in their cell bodies (middle panels) and axon terminals in the opposite hemisphere (right-hand panels). (A) Layer II/III callosal axons, labeled by in utero electroporation of an EGFP expression construct at 15.5 dpc, project extensively into the contralateral S1/S2 border cortex. (B, C) Projection to the cortex is arrested in the white matter beneath the S1/S2 border when afferent sensory input from the ION is excluded from P2 (B) and P5 (C). (D) When ION is excluded from P7, more callosal axons enter the cortex by P10. (E) When ION is excluded from P9, however, callosal axons densely project to the border regions by P25. (F) Area of GFP immunofluorescence in the superficial (I-IV) and deep (V-VI) layers of S1/S2 border (large boxed area) and white matter (small boxed area) was measured, and the cortical invasion index was calculated as the area of axons in the cortex normalized to that in the white matter. (G) Cortical invasion index (see Methods); *P < 0.05, ** P < 0.01. Large triangles point to the S1/S2 border, small triangles point to callosal axons in the white matter beneath the S1/S2 border, and arrows point to boundary regions between two barrels. Pr5, principal sensory trigeminal nucleus; VPM, ventral posteriomedial thalamic nucleus; I-VI, cortical layers. Scale bars, 250 μm (A-D) and 300 μm (E).