Quantitative live imaging reveals PRICKLE1 controls junctional neural tube morphogenesis independent of Planar Cell Polarity.

Neurulation, the process that forms the neural tube - the precursor to the brain and spinal cord - is frequently disrupted in congenital malformations. Primary and secondary neurulation are integrated at a junctional zone, yet the cellular dynamics linking these programs remain unknown. Using high-resolution quantitative live imaging in transgenic quail embryos, we show that the junctional neural tube forms through two coordinated processes: mediolateral convergence and EMT-driven ingression of medial neuroepithelial cells. We demonstrate that PRICKLE1, a core PCP protein, orchestrates these behaviors independently of planar polarity cues. PK1 is enriched at the apical cortex of medial cells, where it drives actomyosin accumulation and apical constriction. This function is essential for both convergence and cell ingression but is uncoupled from classical PCP axis establishment. Our findings redefine the molecular basis of junctional neurulation and implicate impaired EMT as a central cause of localized neural tube defects.