Figure 6. Segregation of DV axons in Superior Colliculus at PND1.

The day after birth (PND1), the majority of RGC axons have invaded the SC, but the future target zone of a focally labeled population of RGC axons is not yet discernible. Nonetheless, the mediolateral extent of the presumptive target zone is already defined by the mediolateral extent of the invading axons. A. Example of a superior colliculus labeled by a doubly injected retina (dorsal green, ventral red) at PND1. Overlying cortex partly obscures the rostral colliculus at the white arrow. Axons labeled after focal injection into dorsal (B) or ventral (C) retina travel in lateral or medial SC, respectively. Black arrows indicate the average center of mass of these distributions. Axons from dorsal retina are more lateral (P<0.001) than axons from ventral retina. D. Example of PND1 SC after a dorsal retinal injection illustrates that axons are quite confined in the mediaolateral plane, but stretch nearly the entire rostrocaudal extent of the SC. E. Example of a PND1 SC after a ventral injection, showing again that axons are confined mediolaterally, but extend nearly the entire rostrocoaudal length of the SC. F. Quantification of the difference between the mediolateral and rostrocaudal extent of axon label after confined dorsal or ventral injections into retina. The mediolataral spread (width at ¾ height) of axon label in the SC is much less (P<0.001) than the rostrocaudal spread. Medial is to the right, caudal is up in all panels. 500 μm scale bar in A applies to all panels.