Regressive events in the postnatal development of association projections in the visual cortex.

In newborn kittens, neurones in area 17 of the visual cortex projecting to area 18 are distributed in bands of uniform density across the superficial layers (laminae II, III and the upper part of IV) and the deep layers (V and VI). During weeks 2 and 3 postnatal, the cells of origin of this association pathway become mainly restricted to discrete, dense clusters, approximately 600 microns from centre to centre, in the upper layers, with intervening zones free of association cells, as in the adult cat. We have used retrogradely transported, long-lasting neuronal markers to investigate this developmental refinement of the pattern of cortico-cortical connections. The results, reported here, indicate that axonal retraction plays a significant part in the maturation of the clustered organization of superficial layer neurones projecting to area 18, but that cell death may also be a factor in the elimination of the inappropriate projection from the deep laminae.