Target specific differentiation of peripheral trigeminal axons in rat-chick chimeric explant cocultures

Avian and rodent trigeminal ganglion (TG) neurons share common features in their neurotrophin requirements and axonal projections between the sensory periphery and the brainstem. In rodents, the whisker pad (WP) is a major pe ripheral target of the infraorbital (IO) nerve component of the TG. The chi ck 10 nerve is much smaller and innervates the maxillary process (MP). In t he embryonic WP, 10 axons course in fascicles from a caudal to rostral dire ction and form terminal plexuses around follicles. In the chick, 10 axons t ravel as a thin bundle to the MP and branch out with no specific patterning . We cocultured E15 rat TG with E5-6 chick MP or chick TG with rat WP expla nts to examine target influences on trigeminal axon growth patterns as visu alized with DiI labeling or neurofilament immunohistochemistry. Chick TG ax ons showed robust growth into WP explants, and the ganglion increased in si ze. Thick bundles of axons traveled between rows of follicles and formed a distinct pattern as they developed terminal arbors around individual follic les. In contrast, rat TG axon growth was sparse in chick MP explants and th e ganglion size reduced over time. Furthermore, rat TG axons did not show a ny patterning in the chick MP. Similar target-specific growth patterns were observed when TG explants were given a choice between chick MP and rat WP explants. Collectively these results indicate that both the chick and rat T G cells respond to similar target-specific peripheral cues in the establish ment of innervation density and patterning in peripheral orofacial targets. (C) 2001 Elsevier Science BY All rights reserved.