The embryonic development of the cortical plate in reptiles: a comparative study in Emys orbicularis and Lacerta agilis.

From the earliest stage of its ontogenesis, the mammalian cerebral cortex displays a remarkable cytoarchitectonic organization, with its neurons oriented radially within the cortical plate (CP). It is not known whether this radial organization of cortical neurons is characteristic of every cerebral cortex or whether it reflects a progressive phylogenetic acquisition. In order to study this question, the embryonic development of the cortex has been examined in reptiles, where it is the most primitive. Two species, Emys orbicularis and Lacerta agilis, representative of the two principal reptilian orders (chelonians and squamates), have been studied with histological methods. Golgi impregnation, and electron microscopy. Very similar patterns of cell proliferation, migration, maturation, and synaptogenesis have been observed. However, important species differences are present in the cellular organization of the cortical plate. Whereas in Emys the structure of the cortical plate is rudimentary, in Lacerta it appears well developed and quite reminiscent of its mammalian counterpart. Preliminary comparisons with embryological preparations of Sphenodon and Crocodilus niloticus show that the organization of the cortical plate displays significant variations among the different reptilian groups. The present results suggest that the radial organization of cortical neurons is not an all or nothing phenomenon but has been acquired independently and is thus a case of homoplasy, probably due to convergence (Northcutt, 81). Several possible implications of these findings are discussed and a working hypothesis based on the role of radial glial cells in the formation of cytoarchitectonic patterns (Rakic, '80) is presented.