Fig. 4. Adult O/E3 mutants exhibit ORN projection defect. (A) The ORN projection to the OB was visualized by O/E3-directed GFP fluorescence in heterozygous and O/E3^GFP/GFP mice. The absence of ORN axons on the dorsal surface of O/E3^GFP/GFP OB can be seen in the whole-mount view and in coronal sections. Similar results were obtained in five independent mice of each genotype. Similar to the observation in O/E2 mutant animals, the OBs of adult O/E3^GFP/GFP mice are slightly smaller and rounder in shape than that of their heterozygous littermates. (B) OMP immunohistochemistry of coronal sections through the OB of O/E3 heterozygous and homozygous neonates showing the ORN projection patterns to the OB. Sections from both anterior and posterior OB are shown. In both animals, the ORN axons cover the entire OB. However, stronger anti-OMP immunoreactivity was seen on the dorsal and lateral OB of the O/E3^GFP/GFP neonate, and thinning of the external plexiform layer on the dorsal aspect of O/E3^GFP/GFP OB was observed, indicating a possible prelude to the adult phenotype. In all panels, dorsal is towards the top and medial is towards the right.