MORPHOLOGICAL SPECIFICATION OF TRIGEMINAL NEURITES DEPENDS ON TARGET FIELDS

Primary sensory neurons bridge the sensory periphery to the central ne rvous system (CNS) via their two axonal processes. The morphological p atterning of the teminals of each process in its respective target is unique. Whether the differences between peripheral and central axons r esult from an intrinsic developmental program of the ganglion cell bod y, or from target-derived signals is not known. To explore this issue, we have used an explant coculture system in which embryonic (E15) tri geminal ganglion explants were placed between a vibrissa pad and a bra instem explant, but the explants were rotated 180 degrees relative to their normal orientation. In other experiments, individual ganglia wer e placed between two vibrissa pad explants or between two slices taken through the brainstem. The cultures were fixed after several days and ganglion cell processes were labeled with the lipophilic tracer DiI. Results of the ganglion rotation experiments suggest that trigeminal a xons which would be directed centrally in vivo can regenerate into per ipheral targets, and peripheral axons can grow into CNS tissue. Simila rly, in cocultures with two peripheral or two central targets, bath pr ocesses of trigeminal ganglion cells can simultaneously invade vibriss a pad explants or project into brainstem slices. Moreover, in all cocu ltures the differentiation of each set of processes is specific to the target innervated by it. These results show that the axons of embryon ic sensory neurons are not selective in their choice of targets, and t hat their morphological patterning is dictated by target-derived signa ls.