Abstract
Primary cilia are centriole-derived sensory organelles found in most vertebrate cells including neurons. In the mouse neocortex, the primary cilia of pyramidal neurons are found to orient predominantly toward the pia, reflecting a reverse movement that occurs during postnatal neuronal repositioning. This study compared cilia orientation in principal excitatory neurons in the cerebral cortex across turtles, mice, and macaques to identify patterns to infer mechanisms of cortical evolution. We first developed custom MATLAB Apps to facilitate the fast identification and statistical analyses of cilia orientation. We found that generally the primary cilia of principal neurons in sparse inside-out laminated regions, including the macaque and mouse neocortex, macaque CA1, mouse entorhinal cortex and neighboring regions, and mouse piriform cortex layer III, orient toward the pia. In contrast, primary cilia in compact laminae of these species, including the macaque and mouse dentate gyrus (DG), macaque CA3, mouse piriform cortex layer II, and turtle lateral cortex manifest opposite orientations, positioning perpendicular to the laminae. These data suggest that over the course of cortical evolution, primary cilia in principal neurons evolve from initially having no preferred orientation to becoming increasingly oriented toward the pial surface. We propose a working model for cortical evolution: the placement of principal neurons progresses from minimal migration in lower vertebrates to forward migration in higher species, and ultimately to pronounced reverse soma movement in higher inside-out laminated cortices, driven by the accumulation of large neuronal populations in the outer layers after completing radial migration.