Maturation of thalamic inputs into the associative cortical areas during postnatal ontogenesis in cats

Thalamic afferents into the sensorimotor and parietal cortex (area 7) were studied using the method of retrograde axonal transport of horseradish peroxidase (HRP) in kittens of three age groups. Following HRP injection into the sensorimotor cortex of 1- to 3-day-old kittens, labeled cells were found in the following nuclei: ventral posterolateral (VPL), ventral posteromedial (VPM), medialis dorsalis (MD), centrum medianum (CM), and centralis lateralis (CL). In 9- to 10-day-old kittens the number of labeled cells and density of reaction product within the cells was found to increase markedly. In 21- to 30-day-old kittens, besides the above-mentioned nuclei, labeled cells were also revealed in n. ventralis lateralis (VL). A similar trend was seen following HRP injections into area 7. In 1- to 3-day-old kittens, labeled cells were found in n. lateralis posterior (LP), in n. ventralis anterior (VA), and in Pulvinar (Pulv). Both in the 9- to 10-day-old and the 21- to 30-day-old kittens, labeled cells were also demonstrated in n. lateralis dorsalis (LD) and CL. For both of the cortical areas the number of labeled cells and the density of labeling within the cells increase during postnatal ontogenesis. This may be attributed to the growing number of axon terminals. In general, our results suggest that differences in the thalamic connections of single cortical areas can be distinguished during the postnatal ontogenesis of the brain.