Planar-cell-polarity (PCP) signalling confers polarity on cells within an epithelium, and contributes to the correct fusion of neural folds during neural tube closure in higher vertebrates. Mutations in mouse PCP genes cause severe neural tube defects - such as are also seen in 10% of human neural tube defects. Surprisingly, whether convergent extension (CE) is involved in such defects is unknown. Now, on , Andrew Copp and colleagues reveal that CE is defective in the axial mesoderm and neuroepithelium of mice with mutant PCP genes, such as loop-tail (Lp), before the onset of neurulation. Wild-type cells occur at the midline of chimeric Lp mice, indicating that this process is cell autonomous. Unlike in other vertebrates, CE in mice depends on downstream RhoA, but not JNK signalling. Other findings and those reported here should lead to the search for more candidate genes that underlie human neural tube defects. (For more on PCP in vertebrates, see for a review by Yanshu Wang and Jeremy Nathans.)
