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.

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.

[8] In addition, some cellular inter-dependency has been displayed in studies concerning the protein Sonic hedgehog (Shh) in somitogenesis.

Studies indicate the importance of pathways involving Eph receptor and the Ephrin family of proteins, which coordinate border formation, in this process.

[10][11] Regarding the paraxial mesoderm from which somites form, fate mapping experiments at the blastula stage show pre-somitic mesoderm progenitors at the site of gastrulation, referred to as the primitive streak in some organisms, in regions flanking the organizer.

For instance, exposure of pre-somitic mesoderm to Bone morphogenetic proteins (BMPs) ventralizes the tissue, however in vivo, BMP antagonists secreted by the organizer (such as Noggin and chordin) prevent this and thus promote the formation of dorsal structures.

[13][14] Others have suggested that the inhibition of BMP signaling by Noggin, a Wnt target gene, suppresses the epithelial-to-mesenchymal transition necessary for the splitting off of somites from the bands of pre-somitic mesoderm and thus terminates somitogenesis.

[16] Other studies suggest termination may be due to an imbalance between the speed of somite formation and growth of the pre-somitic mesoderm extending into this tail region.