Introduction

Apart from our curiosity about the mechanisms underlying neurite extension and growth cone motility, understanding the cytoskeleton in growth cones is important because it is the ‘final common path’ of action of extrinsic guidance cues (reviewed in: Letourneau & Cypher, 1991; Strittmatter & Fishman, 1991; O'Connor & Bentley, 1993; Bentley & O'Connor, 1994; Lin, Thompson & Forscher, 1994; Tanaka & Sabry, 1995; Challacombe, Snow & Letourneau, 1996a; Letourneau, 1996). Broadly speaking, neurite extension depends upon the integrity of microtubules whereas, in common with motile events in other cell types, the motility of the growth cone, the extension and retraction of filopodia and lamellipodia, is supported by actin microfilaments. Pathfinding, the ability of the growth cone to navigate a route through the embryo to its target cell (see Chapter 3), depends on both of these parts of the cytoskeleton and their associated proteins.

Neurite extension is a special case of cellular motility. When a flbroblast or other motile cell in culture migrates across a substratum it extends a cellular process (lamellipodium) at the leading edge of the cell which attaches to the substratum so that, following traction, the entire cell including the nucleus, can move forward (Bray, 1992). Although there are similarities between neurite extension and cellular translocation, there are two important differences.