Failure of callosal innervation is detected at P8, and still present in adulthood. (A-F) Layer II/III somatosensory callosal neurons project densely to the S1/S2 border region of the contralateral somatosensory cortex at P8 in control brain (A, B), but transection of the contralateral (C, D) or ipsilateral (E, F) ION at P2 greatly reduces callosal innervation of the S1/S2 border region at P8. (G-L) Dense callosal projection is present in control brain at P120 (G, H), but callosal axons are dramatically reduced after P5 transection of the contralateral (I, J) or ipsilateral ION (K, L). Large triangles point to the S1/S2 border (A-L), small triangles point to callosal axons in the white matter beneath the S1/S2 border, and arrows point to boundary regions between two barrels (A-F). For abbreviation, see Figure 1. Scale bars, 250 μm (A-F) and 350 μm (G-L).