Postnatal development of the corticotectal projection in cats.

Although numerous physiological studies have provided compelling evidence for the involvement of the corticotectal projection in the normal development of visual response properties of neurons in the superior colliculus, little information is available on the morphological development of corticotectal axons. Thus, our goal was to determine the postnatal changes characterizing the development of the topography and morphology of corticotectal axon arbors. Topographically restricted injections of Phaseolus vulgaris leucoagglutinin were made into striate cortex to label corticotectal axons and their terminal arbors. Following injections of similar size and location in kittens and adult cats, a similar, localized region of the superior colliculus was labeled at all ages. However, while present in the appropriate topographic location in colliculus, the corticotectal projection revealed a greater tangential distribution in kittens than adults. Corticotectal terminal zones underwent a twofold decrease in tangential area during the first 8 weeks after birth. From corticotectal terminal zones in kittens, extended many fine collaterals that ended as growth cones and radiated up to 1 mm from the focus of the terminal zone. By 8 weeks after birth, these immature collaterals were no longer observed, and the corticotectal terminal zone was indistinguishable from those in 12-week-old kittens and adult cats. Corticotectal axon arbors became more specialized in the first 8 weeks after birth; both en passant and terminal swellings increased in diameter, and terminal swellings increased in number, although the total number of swellings per unit length of axon remained relatively stable. The number of axonal branches increased threefold between 1 and 8 weeks after birth. Thus, as the corticotectal projection becomes spatially restricted during development, individual arbors become progressively more intricate with regard to focused collateral branching and the elaboration of complex axonal specializations.