Callosal projection is affected by unilateral but rescued by bilateral lesion of whisker hair follicles. (A) Layer II/III callosal axons of control pups project densely to the contralateral S1/S2 border cortex. (B, C) Unilateral lesion of the hair follicles on either side at P4 also impairs callosal innervation of the S1/S2 border region. (D) Bilateral lesion of whisker hair follicles at P4 does not obviously affect the callosal projection. (E) Cortical invasion index (See Methods); *P < 0.05, **P < 0.01. Large triangles point to the S1/S2 border, and small triangles point to callosal axons in the white matter beneath the S1/S2 border. For abbreviation, see Figure 1. Scale bar, 250 μm.