Fig. 1. Disruption of retinal axon targeting with exogenously applied GAGs. Lateral view of whole-mount brains at stage 40 showing the trajectories of HRP-filled optic projections. One side of the brain was exposed to GAGs from stage 35/36 to 40, the time when axons first grow from the mid-optic tract into the optic tectum (Tec). (A) Control projection showing HRP-filled axons coursing through the optic tract (Ot) in the diencephalon (Di) and crossing the diencephalon/midbrain boundary (dashed line anteriorly) into the tectum. Note the caudalward bend in the projection in the mid-optic tract. (B,C) Projections exposed to GAGs exhibiting the bypass phenotype. Brains were treated with 100 µg/ml of bovine lung heparin (B) and porcine mucosal HS (C), beginning when axons were in the mid-optic tract. The axons extend normally to the mid-optic tract, then take an aberrant route dorsally in the diencephalon, failing to cross the diencephalon/midbrain boundary, and bypass the tectum. Note the absence of a caudalward bend in the mid-optic tract. Tel, telencephalon; dorsal is up, anterior to the left.