Dynamic Expression oflunatic fringeSuggests a Link betweennotchSignaling and an Autonomous Cellular Oscillator Driving Somite Segmentation

AbstractThe metameric organization of the vertebrate trunk is a characteristic feature of all members of this phylum. The origin of this metamerism can be traced to the division of paraxial mesoderm into individual units, termed somites, during embryonic development. Despite the identification of somites as the first overt sign of segmentation in vertebrates well over 100 years ago, the mechanism(s) underlying somite formation remain poorly understood. Recently, however, several genes have been identified which play prominent roles in orchestrating segmentation, including the novel secreted factorlunatic fringe.To gain further insight into the mechanism by whichlunatic fringecontrols somite development, we have conducted a thorough analysis oflunatic fringeexpression in the unsegmented paraxial mesoderm of chick embryos. Here we report thatlunatic fringeis expressed predominantly in somite −II, where somite I corresponds to the most recently formed somite and somite −I corresponds to the group of cells which will form the next somite. In addition, we show thatlunatic fringeis expressed in a highly dynamic manner in the chick segmental plate prior to somite formation and thatlunatic fringeexpression cycles autonomously with a periodicity of somite formation. Moreover, the murine ortholog oflunatic fringeundergoes a similar cycling expression pattern in the presomitic mesoderm of somite stage mouse embryos. The demonstration of a dynamic periodic expression pattern suggests thatlunatic fringemay function to integrate notch signaling to a cellular oscillator controlling somite segmentation