When body segmentation goes wrong.

The segmented or metameric aspect is a basic characteristic of many animal species ranging from invertebrates to man. Body segmentation usually corresponds to a repetition, along the anteroposterior (AP) axis, of similar structures consisting of derivatives from the three embryonic germ layers. In humans, segmentation is most obvious at the level of the vertebral column and its associated muscles, and also in the peripheral nervous system (PNS). Functionally, segmentation is critical to ensure the movements of a rod-like structure, such as the vertebral column. The segmented distribution of the vertebrae derives from the earlier metameric pattern of the embryonic somites. Recent evidence from work performed in fish, chick and mouse embryos indicates that segmentation of the embryonic body relies on a molecular oscillator called the segmentation clock, which requires Notch signaling for its proper functioning. In humans, mutations in genes required for oscillation, such as Delta-like 3 (DLL3), result in abnormal segmentation of the vertebral column, as found in spondylocostal dysostosis syndrome, suggesting that the segmentation clock also acts during human embryonic development.