baffled mutants have the most severe deficit in thalamocortical innervation. (A) Compared to a wild-type E18.5 forebrain (left), a baffled mutant littermate forebrain (right) has a slightly smaller cortex (ctx) and olfactory bulbs (OB), and shows severely reduced beta-galactosidase signal in the cortex, indicating decreased TCA innervation. mb, midbrain. (B, C) Coronal views of the cut face of the rostral halves of control and baffled forebrains reveal that baffled TCAs are disorganized in the lateral vTel, and some appear stalled in thickened bundles near the corticostriatal boundary (red arrow). Only a small number have extended in the cortex (black arrowheads). Dashed line indicates cut edge of brain surface. (D, E) Dye tracing with DiI crystal placements in dorsal thalamus (dTh) was done in control and baffled mutant fixed P0 brains. Coronal vibratome sections at the level of the internal capsule show that in the baffled mutant, dye-labeled axons are disorganized in lateral vTel (E, red arrow), and are not seen in the cortex at this level. (F, G) Coronal cryosections of P0 control and baffled mutant brains were immunostained for L1-CAM antibody. In mutant sections (G), axons appear disorganized (arrows), and in thicker bundles (arrowheads). (H, I) Sagittal thin sections of lateral cortex near the corticostriatal boundary were stained with hematoxylin (purple) and eosin (pink). The axonal layer (intermediate zone, iz) of the control E17.5 cortex (H) contains evenly dispersed thin axon fascicles (pink within bracketed zone), while the baffled cortex (I) intermediate zone appears disorganized and contains striking abnormally large axon bundles (red arrowheads). Rostral is to the left. cp, cortical plate; mz, marginal zone. Scale bar, 250 μm.