TARGET-DERIVED INFLUENCES ON AXON GROWTH MODES IN CULTURES OF TRIGEMINAL NEURONS

Cellular and molecular signals involved in axon elongation versus coll ateral and arbor formation may be intrinsic to developing neurons, or they may derive from targets. To identify signals regulating axon grow th modes, we have developed a culture system in which trigeminal gangl ion 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 tr igeminal ganglion cells extend one process to the vibrissa pad and ano ther to the brainstem trigeminal complex. Under coculture conditions, the peripheral processes invade the vibrissa pad explants and form a c haracteristic circumfollicular pattern. Central processes of E15 gangl ion cells invade many, but not all, central nervous system tissues. In isochronic (E15) central nervous system explants such as brainstem, o lfactory bulb, or neocortex, these central processes elongate and form a ''tract'' but have virtually no arbors. However, in more mature tar gets (e.g., a section from postnatal brainstem or neocortex), they for m arbors instead of a tract. We conclude from these observations that whether trigeminal axons elongate to form a tract, or whether they beg in to arborize, is dictated by the target tissue and not by an intrins ic developmental program of the ganglion cell body. The explant cocult ure system is an excellent model for analysis of the molecular basis o f neuron-target interactions.