Differentiation of the chick retinotectal topographic map by remodeling in specificity and refinement in accuracy.

To understand the development of the retinotopic map, differentiation of the topographic map was quantitatively examined in the chick. Labeling the retinal ganglion cell (RGC) axons anterogradely with the local injections of DiI revealed the relative anteroposterior positions of their growth cones (GCs) on the tecta as a function of the nasotemporal positions of the injected sites in the retinae, which allowed a graphic representation of the map. The topographic map was depicted by combination of two parameters: specificity which indicates strictness of the topographic relationship between locations of the RGC bodies and their GCs on the tectum, and accuracy which indicates an extent of the GC displacement on the tectum. A crude projection with low specificity emerged at embryonic day 11 (E11). The initial crude projection was remodeled into the inaccurate map with high specificity by E13; thereafter, it was refined to the accurate map with higher specificity by E15. The results suggest that the elements of the guidance mechanism operate stage by stage through the formation of the crude projection, the remodeling in specificity, and the refinement in accuracy to establish the final topographic map.