Target-derived influences on axon growth modes in cultures of trigeminal neurons

Cellular and molecular signals involved in axon elongation versus collateral and arbor formation may be intrinsic to developing neurons, or they may derive from targets. To identify signals regulating axon growth modes, we have developed a culture system in which trigeminal ganglion cells are challenged by various target tissues. Embryonic day 15 (E15) rat trigeminal ganglion explants were placed between peripheral (vibrissa pad) and central nervous system targets. Normally, bipolar trigeminal ganglion cells extend one process to the vibrissa pad and another to the brainstem trigeminal complex. Under coculture conditions, the peripheral processes invade the vibrissa pad explants and form a characteristic circumfollicular pattern. Central processes of E15 ganglion cells invade many, but not all, central nervous system tissues. In isochronic (E15) central nervous system explants such as brainstem, olfactory bulb, or neocortex, these central processes elongate and form a "tract" but have virtually no arbors. However, in more mature targets (e.g., a section from postnatal brainstem or neocortex), they form arbors instead of a tract. We conclude from these observations that whether trigeminal axons elongate to form a tract, or whether they begin to arborize, is dictated by the target tissue and not by an intrinsic developmental program of the ganglion cell body. The explant coculture system is an excellent model for analysis of the molecular basis of neuron-target interactions.