Somitogenesis is the process by which somites form. Somites are bilaterally paired blocks of paraxial mesoderm that form along the anterior-posterior axis of the developing embryo in segmented animals. In vertebrates, somites give rise to skeletal muscle, cartilage, tendons, endothelium, and dermis. Overview In somitogenesis, somites form from the paraxial mesoderm, a particular region of mesoderm in the neurulating embryo. This tissue undergoes convergent extension as the primitive streak regresses, or as the embryo gastrulates. The notochord extends from the base of the head to the tail; with it extend thick bands of paraxial mesoderm. As the primitive streak continues to regress, somites form from the paraxial mesoderm by "budding off" rostrally as somitomeres, or whorls of paraxial mesoderm cells, compact and separate into discrete bodies. The periodic nature of these splitting events has led many to say to that somitogenesis occurs via a clock-wavefront model, in which waves of developmental signals cause the periodic formation of new somites. These immature somites then are compacted into an outer layer (the epithelium) and an inner mass (the mesenchyme). The somites themselves are specified according to their location, as the segmental paraxial mesoderm from which they form it itself determined by position along the anterior-posterior axis before somitogenesis. The cells within each somite are specified based on their location within the somite. In addition, they retain the ability to become any kind of somite-derived structure until relatively late in the process of somitogenesis. Signaling Periodicity Once the cells of the pre-somitic mesoderm are in place following cell migration during gastrulation, oscillatory expression of many genes begins in these cells as if regulated by a developmental "clock." As mentioned previously, this has led many to conclude that somitogenesis is coordinated by a "clock and wave" mechanism. In technical terms, this means that somitogenesis occurs due to the largely cell-autonomous oscillations of a network of genes and gene products, which causes cells to oscillate between a permissive and a non-permissive state in a consistently timed-fashion, like a clock. These genes include members of the FGF family, Wnt and Notch pathway, as well as targets of these pathways. The wavefront progress slowly in a posterior-to-anterior direction. As the wavefront of signaling comes in contact with cells in the permissive state, they undergo an epithelial-mesenchymal transition and pinch off from the more posterior pre-somitic mesoderm, forming a somite boundary and resetting the process for the next