Expression of PLXA3 , PLXA4 , NPN - 1 , and NPN - 2 in the neocortex during corticospinal tract targeting. (a) Diagram of sagittal view of the brain and cross-section of the brainstem and spinal cord representing axon targeting of the corticospinal tract at P0. (b-e) In situ hybridization of PLXA3, PLXA4, NPN-1, and NPN-2. Radioactive (b, c) and non-radioactive (b', b", c', c") in situ hybridization demonstrates that PLXA3 and PLXA4 mRNA is expressed throughout the neocortex at P0. NPN-1 mRNA (d-d") is expressed in deeper layers of the neocortex at P0. Insets in (b'-d') show cortical neurons (arrows) that co-express PLXA3, PLXA4, or NPN-1 with the layer V neuronal marker Ctip2. NPN-2 mRNA (e-e") is not expressed in cortex at P0. (f, g) L1 immunohistochemistry (IH) of the sagittal brain demonstrating the normal course of subcortical projections through the internal capsule of P1 WT and PLXA3/PLXA4-/- mice. (h, i) Sagittal sections of the brain showing the normal course of BDA-labeled subcortical projections from the motor cortex of P25 WT and PLXA3/PLXA4-/- mice. Black arrows indicate BDA-labeled axons descending through the internal capsule. C, caudal; CP, cortical plate; D, dorsal; IC, inferior colliculus; IZ, intermediate zone; MC, motor cortex; Pn, pons; Pyr Dec, pyramidal decussation; R, rostral; SC, superior colliculus; SpC, spinal cord; V, ventral; VC, visual cortex; VZ, ventricular zone. Scale bars: 1,000 μm (b-e); 400 μm (b'-e'); 25 μm (insets in b'-d'); 100 μm (b"-e"); 500 μm (f-i).