The synchrony and cyclicity of developmental events.

Many of the morphogenetic processes that occur during development in the mouse are based on cyclic events with defined time intervals, as exemplified by somitogenesis (every 2 h), hair cycles (every 25 d), and spermatogenesis (every 35 d). Among these events, somitogenesis is the most dynamic morphogenetic mechanism showing clear cyclicity during embryogenesis and is therefore a good system with which to review the synchronous and cyclic characteristics of developmental pathways. The metameric properties of the somites underpin the segmental properties along the anterior-posterior (AP) axis of the body. The periodicity of somites is controlled by the so-called segmentation clock operating in the presomitic mesoderm (PSM). This tissue contains the somite precursor cells that exist only during embryonic development. Both theoretical and experimental approaches have contributed to the understanding of the mechanism of somite segmentation. This article focuses on how the segmentation clock functions to organize the collective behavior of cells and how this information is translated into the spatial patterning of segmental somites. The interplay between signaling molecules that provides positional information and the transcription factors that respond to such positional cues are critical to the role of the segmentation clock and are discussed.