Sonic hedgehog and the molecular regulation of mouse neural tube closure.

Neural tube closure is a fundamental embryonic event whose molecular regulation is poorly understood. As mouse neurulation progresses along the spinal axis, there is a shift from midline neural plate bending to dorsolateral bending. Here, we show that midline bending is not essential for spinal closure since, in its absence, the neural tube can close by a 'default' mechanism involving dorsolateral bending, even at upper spinal levels. Midline and dorsolateral bending are regulated by mutually antagonistic signals from the notochord and surface ectoderm. Notochordal signaling induces midline bending and simultaneously inhibits dorsolateral bending. Sonic hedgehog is both necessary and sufficient to inhibit dorsolateral bending, but is neither necessary nor sufficient to induce midline bending, which seems likely to be regulated by another notochordal factor. Attachment of surface ectoderm cells to the neural plate is required for dorsolateral bending, which ensures neural tube closure in the absence of sonic hedgehog signaling.