Postnatal development of the somatosensory thalamocortical projection in rat and rabbit--a combined retrograde transport and stereological comparative study.

A comparative quantitative study of the somatosensory thalamocortical connections in the rat and rabbit, labeled with the fluorescent retrograde tracer Fluoro-Gold (FG), was conducted by means of unbiased stereology. FG was injected into the primary somatosensory cortex of the rat and rabbit in different age groups from P0 to P180 (P-postnatal day). The numerical density of retrogradely labeled the ventroposterolateral (VPL) projection neurons was analyzed. A significant decrease in this parameter was observed during the first two weeks of postnatal life in both studied species. Changes of the neuropil volume and selective elimination of early cortical connections stemming from the VPL may possibly cause this process. A withdrawal of axon collaterals from the expanded cortical sites as well as apoptosis (existing both in the VPL and parietal cortex) contribute to a decrease in the numerical density. Our observations allow us to conclude that the thalamocortical somatosensory connections established before the birth undergo significant quantitative changes in both studied species during the first two weeks of postnatal life and this period seems to be crucial for maturation of the thalamocortical loop.