Maintaining the distinct serrated alignment of CR-positive L5a neurons requires sensory input. CR-positive neurons in L5a send long processes into L4 at P3 (A, B, arrows). Until P4, the CR-positive L5a neurons display the distinct serrated aligning pattern, and their processes form septa-like structures (C, D). At P8, the septa-like structures become more distinct (I, J). The expression pattern of CR is maintained until P15 (K). At P30, the CR expression in L5a disappears, and the barrels in L4 become more distinct. After ION transection at P2, the expression level of CR rapidly declines at P3 (E, F). From P4 to P15, the processes of the CR-positive neurons become shorter; the distinct serrated aligning pattern is disrupted; and no septa-like structures of the CR-positive processes are observed (G, H, M, N, and O). By P30, the expression of CR in L5a disappears, and only scattered CR-positive neurons are observed throughout the cortex (P). The asterisks in C, I and L denote barrels and prospective barrels. The arrows in B, D, J, and K denote processes that extend to L4 to form septa-like structures. Scale bar: 100?m.