Postnatal development of callosal projections in the somatosensory cortex. Layer II/III callosal neurons in the somatosensory cortex were labeled via in utero elecroporation of an EGFP-expressing plasmid, and their callosal axons were traced in the contralateral cortex at different postnatal stages. A–C , Callosal axons appeared on the electroporated side at P2 ( A , arrowhead), crossed the midline (ML) in the corpus callosum at P3 ( B , arrowhead), and extended in the white matter of the contralateral cortex at P5 ( C ). D , E , Callosal axons invaded the contralateral somatosensory cortex. At P6, most callosal axons were located below layer IV ( D ), whereas at P7, a significant number was observed in the superficial cortical layers ( E ). F–J , Callosal axons underwent region- and layer-specific projections during the second postnatal week. F , At P8, callosal axons arborized in layers II–III of the cortex. G , At P9, a dense callosal axon projection was observed at the S1/S2 border, and further axon arborization was seen in S1. H , At P10, callosal axon arborization was increased, with termination mainly in layers II–III of S1 and in layers I–III and V of the S1/S2 border region. I , J , After P12, the callosal projection remained stable as a similar pattern was observed at P60. I–IV indicate cortical layers. SSC, Somatosensory cortex; Hi, hippocampus; S1, primary somatosensory cortex; S2, secondary somatosensory cortex. Scale bar, 500 μm.