A clock-work somite

Somites are transient structures which represent the most overt segmental f eature of the vertebrate embryo. The strict temporal regulation of somitoge nesis is of critical developmental importance since many segmental structur es adopt a periodicity based on that of the somites, Until recently, the me chanisms underlying the periodicity of somitogenesis were largely unknown. Based on the oscillations of c-hairy1 and lunatic fringe RNA, we now have e vidence for an intrinsic segmentation clock in presomitic cells. Translatio n of this temporal periodicity into a spatial periodicity, through somite f ormation, requires Notch signaling. While the Hox genes are certainly invol ved, it remains unknown how the metameric vertebrate axis becomes regionali zed along the antero-posterior (AP) dimension into the occipital, cervical, thoracic, lumbar, and sacral domains. We discuss the implications of cell division as a clock mechanism underlying the regionalization of somites and their derivatives along the AP axis. Possible links between the segmentati on clock and axial regionalization are also discussed. (C) 2000 John Wiley & Sons, Inc.