The segmentation clock: converting embryonic time into spatial pattern.

In most animal species, the anteroposterior body axis is generated by the formation of repeated structures called segments. In vertebrate segmentation, a specialized mesodermal structure called the somite gives rise to skeletal muscles, vertebrae, and some dermis. Formation of the somites is a rhythmic process that involves an oscillator--the segmentation clock--driven by Wnt and Notch signaling. The clock ticks in somite precursors and halts when they reach a specific maturation stage defined as the wavefront, established by fibroblast growth factor and Wnt signaling. This process converts the temporal oscillations into the periodic spatial pattern of somite boundaries. The study of somite development provides insights into the spatiotemporal integration of signaling systems in the vertebrate embryo.