Fig. 2.

Topographical organization of the retinotectal system and the graded expression of guidance cues and receptors. Anterior tectum receives the input from temporal axons, posterior tectum receives input from nasal axons. Within the tectum the earliest graded expression is that of En1/2, which shows a high expression in the posterior and a low expression in the anterior tectum. Later different Eph As and Ephrins—the member depends on the species—are expressed in counter gradients along the tempero-nasal axis of the retina and the anterior–posterior axis of the tectum. RGM shows an anterior-to posterior raising gradient in the tectum and its receptor, Neogenin, a countergradient in the retina. Another temporal to nasal gradient is formed by the A1R receptor in the retina. Along the dorsoventral axis of retina and mediolateral in the tectum are countergradients of EphB and EphrinB as well as Wnt its receptor Frizzeled (Frz)