Role of competitive interactions in the postnatal development of X and Y retinogeniculate axons

The cat's retinogeniculate pathway is largely composed of X and Y axons, which represent two distinct neuronal streams organized in parallel. Our earlier data, summarized in the previous paper, suggest that the postnatal development of retinogeniculate axon arbors is characterized by competitive interactions between the X and Y axons. Thus, during development, X arbors in lamina A or A1 are initially broad or exuberant before the Y arbors begin to develop adultlike arbors; the X arbors then shrink to their adult form as the Y arbors grow and establish their mature complement of connections; monocular lid suture prevents the rapid growth of Y arbors, which in turn prevents the pruning of X arbors; and monocular enucleation at birth allows X arbors from the remaining eye to retain their exuberance although completely confined to their appropriate lamina A or A1, whereas the Y arbors develop aberrant extensions into adjacent, previously denervated laminae. We now provide additional evidence for the role of competition between retinogeniculate X and Y axons during development. The addition of visual deprivation by lid suture of the remaining eye to monocular enucleation at birth causes no apparent change in the morphology of X arbors in laminae A and A1. In contrast, the Y arbors of such cats continue to form extensive translaminar sprouts in the previously denervated laminae despite severely reduced terminations in the lamina A or A1 normally innervated by the remaining eye. We interpret these new data, in conjunction with our earlier data, as follows. If retinogeniculate X and Y arbors complete for synaptic space during postnatal development, terminations of Y axons can be affected by lid suture only in geniculate laminae where terminations of X axons are also present. Thus, Y axon arbors are severely reduced in deprived lamina A or A1 following lid suture whether or not the other eye is removed. Where X arbors are not present, such as in lamina C or the laminae inappropriate for the remaining eye after removal of the other, the lid suture has no obvious effect on development of the Y arbors.