Reelin expression during embryonic brain development in lacertilian lizards

The expression of reelin mRNA and protein was studied during embryonic brai n development in the lacertilian lizards L. viridis and L. galloti, by usin g radioactive in situ hybridization and immunohistochemistry. At all stages studied, high reelin expression was consistently found in the olfactory bu lb, in the lateral cortex, and in neurons of the marginal zone and subplate of medial and dorsal cortical sectors. In the dorsal ventricular ridge (DV R), reelin expression was confined to deeply located, large cells which wer e more abundant in the caudal than the rostral part of the DVR. In the dien cephalon, the ventral lateral geniculate complex and the perirotundal were strongly positive, whereas other nuclei were mostly negative. High reelin s ignal was associated with some layers in the tectum, with the torus semicir cularis, cerebellar granule cell layers, and the ventral horn of the spinal cord. A more moderate signal was detected in the septal nuclei, striatum, retina, habenular nuclei, preoptic and periventricular hypothalamic compone nts, and in reticular nuclei of the mid- and hindbrain. The medial and dors al cortical plate and Purkinje cells were reelin-negative but expressed dis abled-1 (Dab1) mRNA. When they are compared with reelin expression during m ammalian brain development, our data reveal an evolutionarily conserved can vas of reelin expression, as well as significant differences, particularly in developing cortical fields. The developing lizard cortex differs from th at of turtles, birds, crocodiles, and mammals in that it displays heavy ree lin expression not only in neurons of the marginal zone that might be homol ogous to mammalian Cajal-Retzius cells, but also in subplate neurons. This difference in the pattern of reelin expression suggests that the elaborate radial organization of the lacertilian cortical plate, somewhat reminiscent of its mammalian counterpart, results from evolutionary convergence. Our d ata lend support to the hypothesis that the reelin signaling pathway played a significant role during cortical evolution. (C) 1999 Wiley-Liss, Inc.