MATURATION OF PROJECTIONS FROM OCCIPITAL CORTEX TO THE VENTROLATERAL GENICULATE AND SUPERIOR COLLICULUS IN POSTNATAL HAMSTERS

The development of corticofugal axons from the posterior cortex of young (ages P3-P11, P15 and P22) and adult Syrian hamsters was investigated by anterograde axonal transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). On P3 and P4, 30 h after tracer was deposited in the cortex, the HRP reaction product was observed in the dorsal nucleus of the lateral geniculate body and in the lateral posterior nucleus of the thalamus, but no labeled axons were observed in the ventral nucleus of the lateral geniculate body (LGBv) until P5. Axon arborization, indicated by a granular precipitate scattered throughout the LGBv was light on P6 and robust on P7. Adult-like patterns of cortico-LGBv innervation were present by P11. The time course for the development of corticotectal fibers was similar to that for the cortico-LGBv projection: labeled cortical efferents were present in the pretectum on P5, but these did not enter the optic fiber layer of the superior colliculus (SC) until the following day (P6). Substantial invasion of the superficial gray layer, resulting from possible arborization of cortical axons in the optic fiber layer, occurred on P9. An adult-like distribution was evident by P11. Thus, corticofugal efferents to the LGBv and to the SC follow a similar developmental sequence, with the LGBv afferents maturing slightly earlier. There appears to be, however, a significant difference in the 'waiting periods' for the two systems. Compared to the time course of retinal axon development in both targets, the cortical efferents mature several days later. © 1990.