Genetic regulation of somite formation.

Segmentation of the paraxial mesoderm into somites requires a strategy distinct from the division of a preexisting field of cells, as seen in the segmentation of the vertebrate hindbrain into rhombomeres and the formation of the body plan of invertebrates. Each new somite forms from the anterior end of the segmental plate; therefore, the conditions for establishing the anterior-posterior boundary must be re-created prior to the formation of the next somite. It has been established that regulation of this process is native to the anterior end of the segmental plate, however, the components of a genetic pathway are poorly understood. A growing library of candidate genes has been generated from hybridization screens and sequence homology searches, which include cell adhesion molecules, cell surface receptors, growth factors, and transcription factors. With the increasing accessibility of gene knockout technology, many of these genes have been tested for their role in regulating somitogenesis. In this chapter, we will review the significant advances in our understanding of segmentation based on these experiments.