Fig. 2. Abnormal development of the corpus callosum and hippocampal commissure in COUP-TFI^null brains. Coronal sections through the telencephalon of wild-type and COUP-TFI^null embryos at the stages indicated and immunostained for calretinin (A,A') and L1 (B-E'). (A-B') At E18.5 corpus callosum fibres cross the midline in wild-type embryos (A,B), but remain ipsilateral in null mutants (A',B'). Note how the cingulate cortex and the cingulum bundle are ventrally extended in the null mutants (arrows in A',B'). This phenomenon is quantified in F, where the length of the cingulate cortex is compared to the total DV length of the forebrain. (C'-E') Serial rostrocaudal sections of E18.5 embryos indicate a milder phenotype in 3/19 null mutants. Arrowheads in C',D' point to partial crossing. (E') In more caudal sections the mutants lack a morphologically distinct corpus callosum and hippocampal commissure (arrow). The arrowheads in the insets C'-E' indicate defasciculated fibres stopping abruptly at the midline. Scale bars: 200 µm. cb, cingulum bundle; cc, corpus callosum; fo, fornix; hc, hippocampal commissure; Lc, length of the cingulate cortex; Lt, total DV length of the forebrain; NULL, COUP-TFI^null embryos; WT, wild-type embryos.