Editing Reelin (section)

==Evolutionary significance==
[[File:Cajal-Retzius cell drawing by Cajal 1891.gif|left|thumb|440px|[[Cajal-Retzius cell]]s, as drawn by Cajal in 1891. The development of a distinct layer of these reelin-secreting cells played a major role in brain evolution.]]
[[File:Schematic illustration of differences in neuronal specification and migration patterns between the mammalian and avian pallium.png|thumb|left|250px|Neuronal development: mammals (left) and avians (right) have different patterns of reelin expression (pink). Nomura T. et al., 2008.<ref name="pmid18197264" />]]
Reelin-DAB1 interactions could have played a key role in the structural evolution of the cortex that evolved from a single layer in the common predecessor of the [[amniote]]s into multiple-layered cortex of contemporary mammals.<ref name="pmid11137154" /> Research shows that reelin expression goes up as the cortex becomes more complex, reaching the maximum in the human brain in which the reelin-secreting Cajal-Retzius cells have significantly more complex axonal arbour.<ref name="pmid16519657" /> Reelin is present in the telencephalon of all the vertebrates studied so far, but the pattern of expression differs widely. For example, [[zebrafish]] have no Cajal-Retzius cells at all; instead, the protein is being secreted by other neurons.<ref name="pmid11173219" /><ref name="pmid12124768" /> These cells do not form a dedicated layer in amphibians, and radial migration in their brains is very weak.<ref name="pmid11173219" />

As the cortex becomes more complex and convoluted, migration along the radial glia fibers becomes more important for the proper lamination. The emergence of a distinct reelin-secreting layer is thought to play an important role in this evolution.<ref name="pmid18197264" /> There are conflicting data concerning the importance of this layer,<ref name="pmid16410414" /> and these are explained in the literature either by the existence of an additional signaling positional mechanism that interacts with the reelin cascade,<ref name="pmid16410414" /> or by the assumption that mice that are used in such experiments have redundant secretion of reelin<ref name="pmid17132178" /> compared with more localized synthesis in the human brain.<ref name="pmid9671277" />

Cajal-Retzius cells, most of which disappear around the time of birth, coexpress reelin with the [[HAR1]] gene that is thought to have undergone the most significant evolutionary change in humans compared with chimpanzee, being the most "evolutionary accelerated" of the genes from the [[human accelerated regions]].<ref name="pmid16915236" /> There is also evidence of that variants in the DAB1 gene have been included in a recent selective sweep in Chinese populations.<ref name="pmid17542651" /><ref name="NYT1" />