In vivo evidence for trigeminal nerve guidance by the cement gland in Xenopus.

In Xenopus embryos growth cones of the mandibular trigeminal nerve contact cells located in the posterior end of the cement gland. We report three lines of evidence suggesting that cues from the posterior cells of the target influence the behavior of trigeminal neurites. First, target ablation in vivo results in failure of axons to stop and to arborize. Second, 180 degree rotation in the anteroposterior axis of the target results in anterior innervation of the cement gland. Third, ectopic cement gland implantation in the path of neurites migration leads to ectopic innervation. These results provide evidence for short-range cues (< 50 microns) in the posterior part of the cement gland controlling the stopping and the branching of the mandibular trigeminal nerve. Furthermore, we show that an ectodermic explant expressing follistatin mimics the natural target in homing trigeminal neurites. We finally propose to use this novel in vivo assay to isolate the trigeminal nerve target-recognition molecule(s).