REELIN MESSENGER-RNA EXPRESSION DURING MOUSE-BRAIN DEVELOPMENT

Using in situ hybridization, expression of the mRNA for reelin, the ge ne most probably responsible for the reeler trait, was studied during mouse brain development, from embryonic day 13 to maturity. The highes t revel of expression was found in Cajal-Retzius neurons, while a high signal was also seen in the olfactory bulb, the external granular lay er of the cerebellum and, particularly at early developmental stages, in hypothalamic differentiation fields, tectum and spinal cord. A mode rate to low level of expression was found in the septal area, striatal fields, habenular nuclei, some thalamic nuclei, particularly the late ral geniculate, the retina and some nuclei of the reticular formation in the central field of the medulla. Paradoxically, no reelin expressi on was detected in radial glial cells, the cortical plate, Purkinje ce lls, inferior olivary neurons and many other areas that are characteri stically abnormal in reeler mutant mice. Together with other prelimina ry studies, the present observations suggest that the action of reelin is indirect, possibly mediated by the extracellular matrix. Most of t he data can be explained by supposing that reelin is a cell-repulsive molecule which prevents migrating neurons from invading reelin-rich ar eas, and thus facilitates the deployment of radial glial cell processe s and the formation of early architectonic patterns.