During mammalian embryonic development, excitatory neurons form on the inside of the developing brain and then move outwards in a process known as radial migration. This means that younger neurons are found in the outer, or ‘superficial’, layers of the cerebral cortex, while older neurons are found in the inner, or ‘deep’, layers. However, this isn’t the end of the story; brain development continues postnatally, with these neurons undergoing some final repositioning in the weeks following birth. Here, Xuanmao Chen and colleagues investigate the mechanisms underpinning this postnatal neuron positioning in mice. Using immunostaining on postnatal brain slices, they find that, in compact layers of the mouse brain, such as the hippocampus, the orientation of primary cilia on neurons changes during postnatal development, ultimately resulting in cilia on younger neurons pointing in opposite directions from those on older neurons. In contrast, cilia on neurons in looser layers, such as the neocortex, predominately point in the same direction. Guided by this clue, the authors collect multiple lines of evidence showing that the cell bodies of superficial layer neurons in both the neocortex and the hippocampus move backwards against the direction of radial migration during postnatal repositioning. They show that this ‘reverse movement’ of neurons can promote gyrification (the process that creates folds on the brain’s surface). Overall, this study identifies reverse movement as a postnatal repositioning process that refines cerebral cortex structure and might offer insights into how human brain architecture evolved.
