Development of the occipital corticotectal projection in the hamster.

Anterograde and retrograde labelling with the carbocyanine dye, Di-I, was used to assess the development of the visual cortical projection to the superior colliculus (SC) in pre- and postnatal hamsters. Posterior cortical axons arrive in the SC on postnatal (P-) day one (the first 24 hours after birth = P-0) and begin to arborize in the superficial laminae (the stratum griseum superficiale [SGS] and stratum opticum [SO]) within one day after they enter the tectum. Over succeeding days, the density of the projection increases and numerous labelled fibers are visible throughout the depth of the SGS and SO. Beginning on P-6, there is a decrease in the density of labelled fibers in the upper SGS and by P-10, the laminal distribution of the occipital corticotectal pathway appears adult-like. Anterograde tracing with Di-I also revealed the presence of a few corticotectal fibers that crossed the midline in both the SC and posterior commissures to terminate mainly in the superficial tectal laminae contralateral to the injection site. Crossed corticotectal fibers were visible in hamsters aged between P-3 and P-12. Retrograde tracing with Di-I in hamsters killed between P-3 and P-12 demonstrated that both the ipsilateral and crossed corticotectal projections arose exclusively from pyramidal cells in developing lamina V.