Reelin mRNA expression during embryonic brain development in the chick

The expression of reelin mRNA was studied during embryonic brain developmen t in the chick, by using in situ hybridization. Reelin was highly expressed in the olfactory bulb and in subpial neurons in the marginal zone of the c erebral cortex. In the diencephalon, the ventral division of lateral genicu late nuclei and perirotundal. nuclei were strongly positive. High levels of expression were associated with some layers of the tectum and with the ext ernal granule cell layer of the cerebellum. A more moderate signal was dete cted in the septal nuclei, hyperstriatal fields, retina, habenular nuclei a nd hypothalamus, in same reticular nuclei of the mid- and hindbrain, and in the spinal cord. Little or no expression was observed in the cortical plat e, Purkinje cells, or the inferior olivary complex. Comparison with reelin expression during mammalian and reptilian brain development reveals several evolutionarily conserved features that presumably define a homology. In ad dition, significant differences are noted, particularly in telencephalic fi elds. Most importantly, the developing chick cortex does not exhibit high l evels of reelin expression in subpial Cajal-Retzius cells characteristic of the mammalian brain. These observations are compatible with an action of r eelin on adhesion and/or of nucleokinesis at the level of target cells. The y further suggest that, whereas the telencephalon of birds and archosaurs e volved primarily from dorsal ventricular ridge derivatives in which reelin is probably secondary, the increase in number of reelin-positive cells, and amplification of reelin expression played a key part in the evolution of t he cortex in the synapsid lineage leading to mammals. J. Comp. Neurol. 422: 448-463, 2000. (C) 2000 Wiley-Liss, Inc.