Specific neurotrophic interactions between cortical and subcortical visual structures in developing rat: in vitro studies.

We investigated the influence of different subcortical structures on the survival of specific populations of occipital cortex neurons developing in vitro. Explants of embryonic day 14-15 (E14-15) rat cortex were cultured for 5 days with explants of either diencephalon or optic tectum or another occipital cortex explant. Stereological analysis of the explants revealed that after 5 days in vitro (5 DIV) all the cortical explants contained equal proportions of healthy neurons, glia, neuropil, and degenerating profiles, regardless of the culturing conditions. In order to determine if different neuronal populations survived preferentially in the cortical explants as a result of the presence of potential target or afferent structures, we used HRP filling and 3H-thymidine labeling techniques. Specific differences in the morphology of the cells and their time of origin are found in the cortical explants. In the cortical explants cocultured with diencephalon (Cx + D) the cortical cells that survive tend to be round with small cross-sectional areas and have few neurites. These cells are generated late in the culturing period. The surviving cortical neurons in the cortex plus tectum (Cx + T) cultures are larger--many with a pyramidal-shaped soma and several neurites. These cells are generated earlier in vitro. The cortex cultured with other cortex (Cx + Cx) gives values intermediate to the Cx + D and Cx + T cultures. The results of these experiments suggest that there are diffusible trophic factors that arise from subcortical structures that selectively support the survival of neuron populations in the developing neocortex.