Abstract
Somites are transient structures which represent the
most overt segmental feature of the vertebrate embryo.
The strict temporal regulation of somitogenesis is of
critical developmental importance since many segmental
structures adopt a periodicity based on that of the
somites. Until recently, the mechanisms underlying the
periodicity of somitogenesis were largely unknown.
Based on the oscillations of c-hairy1 and lunatic fringe
RNA, we now have evidence for an intrinsic segmentation
clock in presomitic cells. Translation of this temporal
periodicity into a spatial periodicity, through somite
formation, requires Notch signaling. While the Hox
genes are certainly involved, it remains unknown how
the metameric vertebrate axis becomes regionalized
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 segmentation clock and axial regionalization
are also discussed.