Fig. 4. Fgf8 is a chemoattractant for trochlear motor axons in vitro. (A) Diagramatic representation of the experiment. (B) MHB explants cultured for 48 hours together with an Affi-gel bead soaked in Fgf8b. Trochlear axons turn and grow towards the FGF beads. (C) Trochlear axons are not attracted towards a control bead soaked in PBS. (D) Staining for neurofilament to reveal all axons present shows that growth towards an Fgf8 source is not a general feature of axons from r1 neurons. (E) Turning influences of Fgf8 occur both within and outside the explant. The original trajectory of the trochlear axons extends from the top left corner of the picture (out of focus because of the thickness of the explant tissue above it). As they approach the periphery of the explant, axons turn slightly towards the Fgf8 beads, however, further reorientation towards the closest bead (i) occurs outside the explant (arrowheads). Some of the axons stall at that bead, but others (arrows) extend further and appear to be reorienting towards a second, more distant Fgf8 bead (ii).

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