These discrete responses are interesting for three reasons:
1) They are physiologically relevant. The cells that elicit these responses actually guide axons in the embryo.
2) They are the invariant consequence of contact. Their invariance and simplicity is consistent with the idea that each discrete response is THE direct physiological consequence of filopodial contact with the guidance cue.
3) These responses have another property, discreteness, which makes them particularly valuable as guidance mechanisms. Each of these responses is impressively stereotyped. It does not affect just extension, or engorgement or consolidation: it affects a very precise subset of one activity. For instance, a response can alter only the advance of veils down a filopodium, without changing the initiation of veils on the same filopodium (Steketee and Tosney, 1999). These cues thus have a constant and highly discrete effect on motility, that suffices to steer the growth cone.
The invariance and discreteness of each response also make them highly useful as bio-assays. We are using these assays to dissect out the cell surface, signaling and cytoskeletal mechanisms of guidance.