SPROUTING OF THE VISUAL CORTICOCOLLICULAR TERMINAL FIELD AFTER REMOVAL OF CONTRALATERAL RETINAL INPUTS IN NEONATAL RABBITS

The morphological changes occurring in the visual corticocollicular pr ojection following removal of the contralateral retina (within the fir st 48 h of postnatal life) were studied using New Zealand rabbits. At 45-50 days after lesion, the corticocollicular terminal field was exam ined by anterograde transport of Phaseolus vulgaris leucoagglutinin, w hich was applied iontophoretically in the central region of the contra lateral striate cortex. In contrast to normal intact rabbits of the sa me age, the corticocollicular terminal field was markedly enlarged in experimental animals. In the centre of the field we found abundant obl ique fibres which sent out blanches. These collateral fibres coursed o ver long distances, parallel to the pial surface, in the stratum zonal e and in the upper part of the stratum griseum superficiale. The prese nce of these fibres, together with an increased density of synaptic bo utons at more superficial levels of the sprouted terminal field, sugge st that corticocollicular fibres tended to occupy territories left vac ant when retinocollicular axons degenerated after enucleation. The hig h density and extensive distribution of these corticocollicular fibres may he due to the continued growth of the fibres, which occupy an ext ensive territory during the early postnatal stages and which, under no rmal circumstances are retracted during the process of postnatal matur ation. Despite the expansion of the field occupied by corticocollicula r synapses, its centre coincided topographically with the field centre s in normal animals, indicating the existence of intrinsic positional cues that persisted after enucleation and determined the arrangement o f visual cortical afferents. This model, which involves substantial ch anges in terminal field organization, should prove useful in elucidati ng the cellular and molecular processes underlying regeneration and pl asticity in the visual system.