Figure 21–72 Internal architecture  of a neuronal growth cone, as seen in culture on a flat substratum. the growth cone forms as an expansion of the tip of the growing axon. (a) Image by interference-contrast microscopy.
(B) Immunostaining to show microtubules (green). (c) Immunostaining to show actin filaments (red). (D) Diagram of the cytoskeletal machinery. Filopodia form and push forward by assembly of actin filaments at the leading edge of the growth cone. Microtubules stabilize the directional decisions made by the actin-rich protrusions. Filopodia adhering to the flat substratum contract and pull the growth cone forward. (Images by chi-hung Lin, paul Forscher Laboratory, Yale University, New haven, ct.)
If commissural growth cones are attracted to the floor plate, why do they cross it and emerge on the other side, instead of staying in the attractive territory? And having crossed it, why do they never cross back again? The answers lie in a change in the responsiveness of the growth cones during their journey. As the growth cones cross the midline, they lose sensitivity to Netrin and become sensitive instead to a signal protein called Slit (see Figure 21–74). Slit is also produced by the floor plate, but it has the opposite effect to that of Netrin: it repels the growth
Figure 21–74 The guidance of commissural axons. (a) the pathway taken by commissural axons in the embryonic spinal cord of a vertebrate.
(B) attraction to the midline. the growth cone is first attracted to the floor plate by Netrin, which is secreted by the floor-plate cells and acts on the receptor Dcc in the axonal membrane. (c) repulsion from the midline after crossing it. as the growth cone crosses the floor plate, Slit comes into play: it binds to its receptors robo1 and robo2 and acts as a repellent to keep the growth cone from re-entering the floor plate. In addition, it blocks responsiveness to the attractant Netrin. Before crossing the midline, the commissural neurons express robo3.1, an alternative splice form of robo3 that is related to robo proteins but blocks Slit signaling. as neurites cross the midline, robo3.1 is lost and growth cones become responsive to Slit and are repelled from the midline.
cones, preventing them from re-entering the midline territory. The responses of the growth cone depend on the receptors that it expresses: as commissural neurons approach the floor plate, the Slit receptors are kept inactive by an inhibitory protein (Robo3.1) in the same membrane, allowing the commissural axons to grow to the midline without being repelled. Robo3.1 is lost as the growth cones cross the midline; now the growth cones become sensitive to repulsion by Slit and are thereby prevented from crossing back to the other side. At the same time, signals from the Slit receptors interfere with those from the Netrin receptors, making the growth cones deaf to the signal that attracted them to the floor plate initially. A similar mechanism, using similar proteins, seems to govern midline crossing of commissural axons in other animals, including flies and worms.
The guidance of commissural axons illustrates how axons rarely navigate directly to their targets. Instead, they use intermediate targets, or guideposts, and switch their sensitivities as they move from one local guidepost to the next, steering their way through a complex environment to a far-away destination.
the Formation of Orderly Neural Maps Depends on Neuronal Specificity
In many cases, neurons of a similar type are laid out in a broad array of different positions, but send out axons that come together for their journey and arrive at the target region in a tight bundle. There the axons disperse again, to terminate at different sites in the target territory. This they do in an orderly way, creating a regular mapping from one territory to another—a neural map.