Somite formation in the chicken embryo.

Somites are epithelial blocks of paraxial mesoderm that define the vertebrate embryonic segments. They are responsible for imposing the metameric pattern observed in many tissues of the adult such as the vertebrae, and they give rise to most of the axial skeleton and skeletal muscles of the trunk. Due to its easy accessibility in the egg, the chicken embryo has provided an ideal model to study somite development. Somites were first described in the chicken embryo by Malpighi in the 17th century, soon after the invention of the microscope. Most of the major concepts relating to somite segmentation and differentiation result from studies performed in the chicken embryo (Brand-Saberi and Christ, 2000). In this review, we will discuss how studies on somites in avian embryos have contributed to our understanding of key developmental processes such as segmentation, control of bilateral symmetry or axis regionalization.