The motile organ of the axon, the growth cone, exends two types of cellular projections. Long cylindrical projections containing long actin filaments are the filopodia. Filopodia are essential for the extension of the second motile projection, thin, flat lamellae or 'veils' with short branched actin. Veils advance only between pairs of filopodia.
One way to control growth cone advance is to control veil motility. If a local guidance cue suppresses veils on one side of the growth cone, the growth cone will turn away from that cue. Conversely, if a cue induces veils on one side, cytoplasm will subsequently flow into the veil, advancing the growth cone on that side. Control of veil extension is a major guidance mechanism (Oakley and Tosney, 1993; Steketee et al., 1999).
We have discovered an intimate relationship between veil activities and a functionally distinct adhesion, shaft adhesions. These adhesions develop along the shafts of some but not all filopodia. They are functionally distinct from the basal adhesion, at the base of all filopodia, which regulates filopodial rather than veil projection (Steketee and Tosney, 2002; Steketee et al. 2001).
The advance of veils is controlled by shaft adhesions. Veils readily advance along those filopodia that lack shaft adhesions, but rarely extend along those filopodia that displayed shaft adhesions. Experimental manipulation confirms that these adhesions inhibit veil advance. When adhesions are directly reduced using anti-laminin antibodies, the extent of veil advance and the proportion of filopodia lacking shaft adhesions both increase, in accord with a direct relationship.