Fig. 10. How microtubule and Rac 1-dependent actin might contribute to rapid growth on laminin. (A) On a polyamine or fibronectin substrate, there is an incremental advance of the leading edge of the growth cone by polymerization onto the existing actin network, well ahead of the microtubules. (B) An additional mechanism that could contribute to normal growth on laminin. Microtubules penetrate filopodia (1). Multiple microtubules penetrate filopodia and peripheral regions (2). Rac 1-dependent F-actin colocalized with bundled microtubules produces `splashes' of veil at the distal edge (3) and facilitates rapid advancement.