Fig. 5. Ectopic Fgf8 redirects trochlear motor axons in vivo. (A) Diagrammatic representation of the experimental manipulation. (B-G) Flat-mounted hindbrains of HH25 chick embryos stained for SC1 antigen following bead implantation at HH11-12. In all cases anterior is towards the top of the image. Individual motor nuclei are labelled where present on the image: III (oculomotor); IV, trochlear; V, trigeminal (stains only weakly for SC1) and VI, facial. Deflected and misrouted axons or fascicles are indicated by arrows and, where visible, the bead implant is indicated by the red arrowheads. (B) Control embryo showing that implanted PBS beads have no effect on trochlear nerve axon pathways. (C-G) Implanted FGF-soaked beads result in 4 classes of axonal defect. (C) Type 1 phenotype: axons extend from the floor plate dorsally but posterior axons have lost their anterior trajectory towards the isthmus and follow a direct dorsal path. (D) Type 2 phenotype: the most anterior fascicle has turned through 90° within r1 and extends towards the ectopic source of Fgf8 at the r1/2 boundary. (E,F) Low and high magnification images showing the Type 3 phenotype. The trochlear nerve is bisected; the arrow indicates fascicles that have turned towards the ectopic source of Fgf8. (G) Ectopic structures with the morphology of an ectopic isthmic region in posterior r1 induced by an Fgf8 bead are associated with ectopic SC1-positive axons that grow towards the bead.

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