Much remains to be done, but the tools are at hand to provide further understanding of the molecular mechanisms underlying somitogenesis. Early stages of chick somite development. Christ B, Ordahl CP Institute of Anatomy, University of Freiburg, Germany. We report on the formation and early differentiation of the somites in the avian embryo. The somites are derived from the avian embryo.
Somites develop from the paraxial mesoderm and constitute the segmental pattern of the body.
They are formed in pairs by epithelialization, first at the cranial end of the paraxial mesoderm, proceeding caudally, while new mesenchyme cells enter the paraxial mesoderm as a consequence of gastrulation. The newly formed somite consists of an epithelial ball of columnar cells enveloping mesenchymal cells within a central cavity, the somitocoel. Each somite is surrounded by extracellular matrix material connecting the somite with adjacent structures. The competence to form skeletal muscle is a unique property of the somites and becomes realized during compartmentalization, under control of signals emanating from surrounding tissues. Compartmentalization is accompanied by altered patterns of expression of Pax genes within the somite.