During vertebrate development, embryonic segmentation is reflected in the appearance of somites. Somites are bilaterally paired, repeating units of mesodermally-derived tissue that develop along the anterior-posterior axis and flank the notochord. Each somite is composed of two main regions that differ in cellular composition. The cells in the ventral portion undergo an epithelial to mesenchymal transition to form the sclerotome, which gives rise to the vertebrae and ribs. The dorsal region of each somite contains the dermomyotome, which gives rise to skeletal muscle and dermis. Somitogenesis is regulated by the so-called "segmentation clock", which is an oscillation of molecular events driven by FGF, Notch, and Wnt signaling within presomitic mesoderm cells. Defects in somitogenesis are associated with birth disorders such as scoliosis.