Disruption of Itsn1 results in corpus callosal agenesis. A, Cresyl violet staining of coronal sections of right-side brain hemispheres from representative WT and Itsn mutant mice (ec, external capsule; ic, internal capsule; or, oriens; cc, corpus callosum; RSGc, retrosplenial granule cortex c). B, Increased magnification of insets shown in yellow boxes in A. The corpus callosum ends in a probst-like bundle. C, Representative confocal image of growth cone from 3 DIV neuronal cultures. Phalloidin labels actin and dotted box regions indicate measured growth cone and axonal regions. D, A total of 29 cultured cells were imaged and both phalloidin and anti-Itsn1 fluorescent units were quantified in growth cones (gray bars) and normalized respectively to axonal levels (black bars). E, Embryonic expression of Itsn1 in cortical plate is detected at E15.5. Comparison of Itsn1 expression in cortical sections of the E11.5, E13.5, E15.5, E18.5, and P14 mouse brain. Fluorescent in situ hybridization using an Itsn1-specific probe revealed no detectable Itsn1 expression at E11.5 and E13.5. Itsn1 expression is detectable by E15.5 and remains on at E18.5. Postnatal brain shows strong Itsn1 expression in the hippocampus. Area surround by a black box in the inset shows region used in magnification. All sections and expression data were provided by the Allen Institute for Brain Science.