Figure 4.

Simulations of mouse retinocollicular map development. (A) and (B) Ten RGC axons from five different retinal locations evenly distributed across the T-N axis of the retina, with examples from three time points during the simulation. The axons shown in each iteration were chosen randomly within each retinal origin from the 300 axons simulated. (A) In simulations run without the branchdensity parameter, branching is initially diffuse across the entire anterior-posterior (A-P) axis of the SC, though slightly biased for the correct TZ, as in vivo (iteration 2). Branching is significantly more diffuse at the start of the simulation compared to the chick simulation. As retinal axons arborize, added repellents on these arbors restricts branching to more topographically correct locations, resulting in the elimination of the RGC axon overshoot. However, though the topographic specificity of branching has increased substantially by iteration 200 a dense TZ does not develop. Many RGC axons still have ectopic arbors outside of the TZ. (B) Simulations run utilizing the branchdensity parameter also have diffuse branching across the SC at early time points. However, at iteration 200, a dense, focused TZ is evident in the correct location with very few ectopic branches remaining. [Color scheme can be viewed in the online issue, which is available at www.interscience.wiley.com.]