Chemotropic effect of specific target epithelium in the developing mammalian nervous system.

Developing nerve fibres are guided to their targets by specific directional cues which are thought to be expressed in the tissues along the route and may involve the extracellular matrix. Another possibility, that directional cues emanate from the target itself, is consistent with the recent demonstration of homing behaviour by ectopic retinal ganglion axons and our previous demonstration that early trigeminal neurites grow directly to their virgin peripheral target in vitro. Here we show that this chemotropic effect is precisely limited to the trigeminal system; trigeminal ganglion neurites grow directly to their own target field but not to the adjoining field, normally innervated by the geniculate ganglion; furthermore, the trigeminal field does not influence the growth of geniculate neurites. Also, when trigeminal ganglia are co-cultured with isolated tissue layers of their target, neurites grow only towards the epithelial and not the mesenchymal component. These findings suggest that trigeminal epithelium is specified to attract correct innervation and that pathway mesenchyme, in which preformed guidance cues have been postulated, may provide favourable conditions for nerve fibre growth but not govern its direction.