Dynamic expression and essential functions of Hes7 in somite segmentation

The basic helix-loop-helix (bHLH) gene Hes7, a putative Notch effector, enc odes a transcriptional repressor. Here, we found that Hes7 expression oscil lates in 2-h cycles in the presomitic mesoderm (PSM). In Hes7-null mice, so mites are not properly segmented and their anterior-posterior polarity is d isrupted. As a result, the somite derivatives such as vertebrae and ribs ar e severely disorganized. Although expression of Notch and its ligands is no t affected significantly, the oscillator and Notch modulator lunatic fringe is expressed continuously throughout the mutant PSM. These results indicat e that Hes7 controls the cyclic expression of lunatic fringe and is essenti al for coordinated somite segmentation.