Fig. 8.

Robo2-dependent morphogenesis in trigeminal sensory neurons. Trigeminal sensory neurons from Isl1SS (magenta) and Trpa1b (green) subtypes have similar morphology during early development. In wild-type fish, developmental maturation leads to preferential expression of robo2 in the Isl1SS subtype (marked by ‘+’) but not in the Trpa1b subtype (marked by ‘–’). Robo2, which is likely to be activated by secreted Slit proteins in the hindbrain (yellow oval), inhibits branch growth and synaptogenesis specifically in the Isl1SS subtype. In robo2^–/– fish, both subtypes lack Robo2. The Isl1SS subtype, now relieved of Robo2 inhibition, extends axon branches and forms synapses in the same areas as in the Trpa1b subtype.