Postnatal development of thalamocortical projections upon striate and extrastriate visual cortical areas in the cat

The development of visual thalamocortical projections was analyzed quantita tively by comparing, in cresyl violet-stained brain sections of early postn atal (10-17 days) and adult cats, the cell body dimensions and total cell p acking density (CPD) of neuronal populations in different laminae (A, A1 an d C) of the dorsal lateral geniculate (dLGN), medial interlaminar nucleus ( MIN), and in lateral (LPI), intermediate (LPi) and medial (LPm) subdivision s of the lateral posterior complex. Following injections of different fluor escent tracers (FB, NY, EB, RITC) into cortical visual areas 17/18, posteri or medial (PMLS) and posterior lateral (PLLS) lateral suprasylvian and ante rior ectosylvian (AEV), the thalamic distribution and densities of retrogra dely labeled neurons were analyzed. Projection CPDs and ratios of projectio n/total CPDs were determined and compared within the different thalamic com ponents in the kitten and adult cat. A significant decrease in total cell p acking density was observed in the various thalamic components of the adult cat, varying between 43% and 65%, and a marked increase in mean cell body diameter in the A, A1 and C laminae and MIN from kitten to adult (8.4+/-1.8 and 11.8+/-2.8 mum respectively) compared to the LP subnuclei (9.0+/-1.3 a nd 9.1+/-1.5 mum). The ratios of projection/total CPDs decreased significan tly for projections upon areas 17/18 stemming from layers A and Al (20 and 25%, respectively) and from LPi upon both PMLS (34%) and AEV (16%). Thalamo cortical projections observed in the kitten from LPi upon areas 17/18 and f rom the A-laminae upon PMLS were absent in the adult cat. The data indicate that, in comparison to the lateral posterior nucleus, the maturation of ne urons within the dLGN and MIN is incomplete with respect to cell body size during the early postnatal period. In addition, the developmental changes o bserved involve both reductions in the total number of thalamic neurons and a differential loss of cortical projections. The selective elimination of early cortical connections stemming from dorsal lateral geniculate laminae A and A1 and from the intermediate division of the lateral posterior nucleu s may occur through a process of axon collateral withdrawal from the expand ed cortical sites, thereby giving rise to the adult pattern.