Reelin-immunoreactive neurons in the adult vertebrate pallium

Reelin, an extracellular matrix protein, plays a crucial role in cortical development. By using Reelin-immunohistochemistry in different vertebrates (fish, amphibians, reptiles, and mammals : insectivores, odontocetes, rodents, carnivores and man) we show here that Reelin is also expressed by a variety of neurons in the adult pallium. In the everted telencephalon of the zebrafish, Reelin-positive neurons are widely distributed over the dorsal pallium. In land vertebrates, the most consistent and evolutionary conserved location of Reelin-expressing neurons is in the cell-sparse molecular layer associated with laminated cortical organization. We describe an additional heterogeneous population of Reelin-positive neurons outside the molecular layer, the location and distribution of which are more variable, and which may reflect major evolutionary changes in cortical architecture. In squamate reptiles, the Reelin-negative main cell layer is flanked by a superficial and a deep plexiform layer which both contain Reelin-expressing neurons. In mammals, Reelin-positive interneurons are dispersed throughout layers II--VI; the human neocortex is particularly poor in Reelin-positive interneurons. Reelin is also expressed by large stellate and modified pyramidal neurons in layer II of the mammalian entorhinal cortex, and in the superficial lateral cortex of lizards. Examination of this cell population (layer II Pre-alpha) in human brains of different age groups points to a decrease in Reelin-expression in the course of adult life.