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

The development of visual thalamocortical projections was analyzed quantitatively by comparing, in cresyl violet-stained brain sections of early postnatal (10-17 days) and adult cats, the cell body dimensions and total cell packing density (CPD) of neuronal populations in different laminae (A, A1 and C) of the dorsal lateral geniculate (dLGN), medial interlaminar nucleus (MIN), and in lateral (LPl), intermediate (LPi) and medial (LPm) subdivisions of the lateral posterior complex. Following injections of different fluorescent tracers (FB, NY, EB, RITC) into cortical visual areas 17/18, posterior medial (PMLS) and posterior lateral (PLLS) lateral suprasylvian and anterior ectosylvian (AEV), the thalamic distribution and densities of retrogradely labeled neurons were analyzed. Projection CPDs and ratios of projection/total CPDs were determined and compared within the different thalamic components in the kitten and adult cat. A significant decrease in total cell packing 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 microm respectively) compared to the LP subnuclei (9.0+/-1.3 and 9.1+/-1.5 microm). The ratios of projection/total CPDs decreased significantly for projections upon areas 17/18 stemming from layers A and A1 (20 and 25%, respectively) and from LPi upon both PMLS (34%) and AEV (16%). Thalamocortical projections observed in the kitten from LPi upon areas 17/18 and from 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 neurons within the dLGN and MIN is incomplete with respect to cell body size during the early postnatal period. In addition, the developmental changes observed 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 nucleus may occur through a process of axon collateral withdrawal from the expanded cortical sites, thereby giving rise to the adult pattern.