Fig. 2. Trochlear axons project towards and extend within the isthmic organiser. (A-F) Flat mount preparations of chick hindbrains stained for the SC1 antigen. In all cases anterior is towards the top of the image. (A) HH25 chick hindbrain. Trochlear axons project dorsally forming 3-4 main fascicles. (B) Short, SC1-positive trochlear axons (arrows) are first weakly detected at HH18 extending from the anterior cells of the nucleus. (C) By HH19 pioneer axons have reached the roof plate. Axon extension appears to proceed in an anteroposterior wave with axons extending from more anterior cell bodies reach the roof plate before those emerging from posteriorly positioned cell bodies within r1. (D) At HH20 anteriorly positioned cell bodies have reached the roof plate extending dorsally within the isthmus, perpendicular to the floor plate. By contrast, axons from more posterior neurons follow a route that leads them both anteriorly towards the isthmus and dorsally towards the roof plate. (E,F) At HH25 axons are still just forming from posterior trochlear motor neurons and their growth cones are visible at higher magnification (arrows, F). (G-J) Chick hindbrain at HH25 stained for both Isl1/2 (G; green) and SC1 (H; red) and these images are combined in I and J with the trochlear nucleus (boxed in I) being shown at higher magnification in J.