Old wares and new: five decades of investigation of somitogenesis in Xenopus laevis.

Somites are regular repeated structures formed in pairs on either side of the anterior-posterior axis of developing vertebrate embryos which give rise to all skeletal muscle of the body, the axial skeleton, the tendons and the dorsal dermis. Beginning in the middle of last century, somite formation has been extensively studied in the South African clawed frog (Xenopus laevis) using traditional embryological techniques. Recently, modern molecular methods have been applied to this system, producing substantial insights into the underlying molecular mechanisms driving these morphological events. In this review I discuss these new results in the context of the early embryological observations, looking at all levels of the process of somite formation, from the initial prepatterning of the presomitic mesoderm to the morphomechanical events required for the separation of each somite from the precursor tissue.