Reelin mRNA expression during embryonic brain development in the turtle Emys orbicularis

The expression of reelin messenger ribonucleic acid (mRNA) was studied duri ng embryonic brain development in the turtle Emys orbicularis, by using rad ioactive in situ hybridization. A high expression was consistently found in the olfactory bulb and in a few neurons in the marginal zone and, to a les ser extent, in the subplate of the dorsal and medial cortical sectors. In t he diencephalon, the ventral division of lateral geniculate nuclei and the prospective reticular thalamic nuclei were strongly positive. High reelin s ignal was also associated with some layers of the tectum and with the exter nal granule cell layer of the cerebellum. A more moderate signal was detect ed in the septal nuclei, striatum, dorsal ventricular ridge, retina, habenu lar nuclei, and hypothalamus, and in some reticular nuclei of the midbrain and hindbrain and in ventral spinal cord. The cortical plate, basal forebra in, amygdala, and tegmentum were weakly labeled. When they are compared to reelin expression during mammalian brain development, our data reveal an ev olutionarily conserved canvas of reelin expression and significant differen ces, particularly in developing cortical fields. Most significantly, the de veloping turtle cortex does not display the heavy reelin expression in subp ial Cajal-Retzius cells that is so typical of its mammalian counterpart. Gi ven the key role of reelin in laminar cortical development, our data sugges t that the increase in the number of reelin-producing cells and/or the ampl ification of reelin expression in the cortical marginal zone might have bee n a driving factor during the evolution of the laminated cerebral cortex fr om stem reptiles to mammals, as indicated in previous comparative analyses. (C) 1999 Wiley-Liss, Inc.