Introduction

One of the most fascinating questions in neurobiology concerns the evolution of the cerebral cortex, a sequence of events that leads to the development of the human cortex and its cognitive abilities (see The Development and Evolutionary Expansion of the Cerebral Cortex in Primates). Genes that control development and growth are privileged targets of evolutionary selection (Raff, 1996), and comparative studies of cortical development may shed light on this process (see Cortical Evolution as the Expression of a Program for Disproportionate Growth and the Proliferation of Areas; Captured in the Net of Space and Time: Understanding Cortical Field Evolution). In the absence of fossil material, however, the evolution of cortical development can only be inferred from comparative studies of modern organisms (Butler and Hodos, 1996).

Studies carried out during the last decades demonstrated that basic, conserved developmental mechanisms pattern the brain in general and the telencephalon in particular (Monuki and Walsh, 2001; Grove and Fukuchi-Shimogori, 2003). Secreted factors (such as Shh, Wnts, Bmps, and Fgfs) establish morphogenetic gradients to which precursors in the neuroepithelial sheet respond by modulating expression of arrays of transcription factors (such as Pax6, Emx1/2, and Tbr1), thereby adapting neuronal cell numbers and types. Different neuronal classes migrate following different routes to colonize various structures.

The cortex is reduced to a periventricular layer in anamniotic vertebrates and increases in size and organization in amniotes. It gains prominence in synapsids, the lineage leading to mammals, and evolves explosively in primates. Despite their obvious importance, however, the molecular events that underlie cortical evolution remain mostly unknown. Work in mice identified several genes with a role in cortical development and presumably evolution (Lambert de Rouvroit and Goffinet, 2001; Monuki and Walsh, 2001). Among them, reelin and its signaling partners may be critical players in the evolution of the mammalian laminar cortical plate (CP) (Bar et al., 2000). In addition, in humans, reelin signaling is essential for cortical foliation (Hong et al., 2000). Comparisons of reelin expression in mammals, reptiles, and birds show that reelin-expressing cells are present in the cortical marginal zone (MZ), from the preplate (PP) stage, in all amniotes, but both the number of positive cells and their level of expression are much higher in mammals than in other lineages. In this article, we will briefly review data on comparative reelin expression during cortical development and discuss some questions that we consider of special neurobiological interest because they would be amenable to study, provided more efforts are invested to provide access to genomic sequences and high-quality embryonic material from multiple species.

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