Directional specificity and patterning of sensory axons in trigeminal ganglion-whisker pad cocultures

In the rodent trigeminal pathway, trigeminal axons invade the developing wh isker pad from a caudal to rostral direction. We investigated directional s pecificity of embryonic day (E) 15 rat trigeminal axons within this periphe ral target field using explant cocultures. E15 trigeminal axons readily gro w into the same age whisker pad explants and form follicle-related patterns along a caudal to rostral direction. They also can grow into this target f rom its lateral aspects. In contrast, they are unable to invade the whisker pad from the rostral (nasal) pole. We did not find any correlation between the distribution of extracellular matrix molecules and trigeminal axon gro wth preferences. We also examined age-related changes in trigeminal axon re sponsiveness to directional cues. E19 trigeminal axone readily,grew into E1 5 whisker pad explants from either the caudal or the rostral pole. These re sults suggest the presence of growth permissive and repulsive cues that gui de sensory axons in the whisker pad. Furthermore, trigeminal axons lose the ir responsiveness to growth inhibitory cues at later stages of development. (C) 2000 Elsevier Science B.V. All rights reserved.