During neurulation, polarised cell-shape changes at hinge points – specialised regions of the neural plate – help convert the neural plate into a tube. But how are these cell-shape changes regulated? To answer this question, Seema Agarwala and co-workers have been studying neural tube closure in the chick midbrain (see ). They identify a cell cycle-dependent bone morphogenetic protein (BMP) activity gradient in the anterior neural plate and show that it is required for ventral midline hinge point formation and neural tube closure. BMP signalling, they report, regulates the polarised cell behaviours associated with neural tube closure by modulating epithelial apicobasal polarity in tandem with the cell cycle. Because cell-cycle progression in the neural plate is asynchronous, BMP-mediated polarity modulation induces shape changes in only some neural plate cells, whereas their neighbours retain apicobasal polarity. This mosaic and dynamic modulation of polarity, the researchers propose, provides the neural plate with the flexibility to allow folding while retaining its epithelial integrity.
