Postnatal development of the corticotectal projection from the visual cortex of the mouse

The postnatal development of the corticotectal projection was investigated by injecting the axon tracer DiI into the visual cortex of mouse pups. It was found that DiI-labeled axons arrive at the ipsilateral superior colliculus and enter the optic nerve layer of this structure on postnatal days 3 and 4 (P3-P4). These corticotectal axons extend into the caudal end of the superior colliculus on P4 and give off small collateral branches that ascend vertically to the superficial gray layer. During the first two postnatal weeks, the collateral branches do not form a demarcated terminal zone, but rather diffusely spread within the superficial gray layer of the superior colliculus. These collateral branches continue to dichotomize and form a bright terminal zone within the superficial gray layer on P11. The terminal zone decreases in size during the second and third postnatal weeks, and appears to be of the same size when compared with the adult counterpart by P19. The terminal zone of the corticotectal axons from the visual cortex is established by P19. In parallel with the maturation of the terminal zone of the corticotectal projection, the distal segment of the corticotectal axons is lost during the second postnatal week. We conclude that the growing tips of the corticotectal axons do not strictly project to their future terminal zone within the superior colliculus, and 'misdirected' axons are eliminated during the early postnatal period.