Involvement of reelin and Cajal-Retzius cells in the developmental formation of vertical columnar structures in the cerebral cortex: evidence from the study of mouse presubicular cortex.

The mammalian cerebral cortex is organized into horizontal and vertical arrays of neurons and their fiber connections that form anatomically and physiologically distinct laminar and columnar compartments. However, the developmental mechanism(s) underlying this dichotomous pattern remains a mystery. We provide anatomical evidence suggesting that reelin, a diffusible protein produced and secreted by Cajal-Retzius cells, is involved in the developmental formation of the vertical cell structures in the mouse presubicular cortex, the unique site where the vertical columnar arrays of cortical plate neurons and their dendritic branches are clearly identified during the early postnatal period. Our results also suggest that reelin plays a role in the formation of these vertical structures by acting as an inhibitory or stop signal for cortical plate neurons and their dendritic extensions. In addition to having perturbed horizontal laminae, reeler mutant mice, lacking reelin, display disruption of these vertical structures. Based on the present findings, we hypothesize that reelin and Cajal-Retzius cells regulate the developmental formation of not only horizontal laminations, but also vertical columnar structures in the cerebral cortex.