Notch signalling is required for cyclic expression of the hairy-like gene HES1 in the presomitic mesoderm

Semitic segmentation provides the framework on which the segmental pattern of the vertebrae, some muscles and the peripheral nervous system is establi shed. Recent evidence indicates that a molecular oscillator, the 'segmentat ion clock', operates in the presomitic mesoderm (PSM) to direct periodic ex pression of c-hairy1 and lunatic fringe (l-fng), Here, we report the identi fication and characterisation of a second avian hairy-related gene, c-hairy 2, which also cycles in the PSM and whose sequence is closely related to th e mammalian HES1 gene, a downstream target of Notch signalling in vertebrat es. We show that HES1 mRNA is also expressed in a cyclic fashion in the mou se PSM, similar to that observed for c-hairy1 and c-hairy2 in the chick. In HES1 mutant mouse embryos, the periodic expression of l-fng is maintained, suggesting that HES1 is not a critical component of the oscillator mechani sm. In contrast, dynamic HES1 expression is lost in mice mutant for Delta1, which are defective for Notch signalling, These results suggest that Notch signalling is required for hairy-like genes cyclic expression in the PSM.