Target influences on the morphology of trigeminal axons

Axons grow in two stages: First, they exhibit rapid, target-directed extension; then they begin to collateralize and elaborate terminal arbors in their targets. To investigate possible regulatory influences on these phases of axon growth, we have used an in vitro paradigm in which we cocultured embryonic or postnatal rat trigeminal ganglion explants with isochronic, heterochronic, and/or heterotypic targets. Cultures were fixed after 5 days and ganglion cell processes were labeled with DiI. Trigeminal processes were able to regenerate into several peripheral targets as well as into CNS explants from trigeminal or nontrigeminal regions of the brain. In peripheral tissues, the processes showed target-specific growth patterns. In CNS tissue, the type of growth (unbranched extension versus collateralization/arbor formation) varied markedly with the explant age: If trigeminal ganglia were harvested at a time (E15) when their axons would be elongating in the embryo and cocultured with isochronic tissues, their processes had a simple morphology, were loosely bundled, and reconstituted a distinct fiber tract, mimicking their in vivo growth pattern. If, challenged by more mature tissue, axons of E15 ganglion cells formed discrete arbors. Finally, if trigeminal ganglia were harvested at an age (E20, PND 5) when their axons had already formed arbors in the brain and induced to innervate younger (E15) targets, their axons reverted back to the elongation stage. These results demonstrate that the target environment sets specific, developmentally regulated constraints on the patterns of growth manifested by primary sensory axons.