DIFFERENTIAL SURVIVAL OF CAJAL-RETZIUS CELLS IN ORGANOTYPIC CULTURES OF HIPPOCAMPUS AND NEOCORTEX

Cajal-Retzius (CR) cells are transient, pioneer neurons of layer I of the cortex that are believed to play essential roles in corticogenesis , e.g., in neuronal migration and synaptogenesis. Here we have used ca lretinin immunostaining to study the characteristics, survival, and fa te of CR cells in single organotypic slice cultures of mouse neocortex and hippocampus deprived of their extrinsic afferents. In neocortical explants, CR cells were observed after 1-3 d in vitro (DIV), but they disappeared after 5-7 DIV, which is similar to their time of degenera tion in vivo. The disappearance of CR cells in neocortical slices was prevented by incubation with tetrodotoxin and the glutamate receptor a ntagonist 6-cyano-7-nitroquinoxaline-2,3,-dione but not by 2-amino-5-p hosphonopentanoic acid, suggesting that neuronal activity and non-NMDA glutamate receptors may trigger CR cell death in the neocortex. In co ntrast to the situation in vivo, in which many hippocampal CR cells di sappear at approximately the third postnatal week, CR cells survived i n single hippocampal cultures after long incubation times (31 DIV), wi th their morphology essentially unaltered. In contrast, fewer CR cells were found when hippocampal slices were cocultured with explants from the entorhinal cortex. Because CR cells are transient synaptic target s for entorhinohippocampal afferents, these findings suggest a role fo r entorhinal afferents in the degeneration of CR cells in the hippocam pus. In conclusion, this study shows different survival properties of CR cells in organotypic slice cultures of hippocampus and neocortex, a nd it suggests that different mechanisms are involved in the regulatio n of the process of naturally occurring CR cell death in the two corti cal regions.