somitogenesis

Somitogenesis is the developmental process by which segmented blocks of paraxial mesoderm, called somites, form sequentially along the anterior-posterior axis of vertebrate embryos. Somites give rise to the vertebrae, ribs, and portions of skeletal muscle and dermis, establishing the segmented arrangement of the axial skeleton.

Formation is explained by the segmentation clock and wavefront model. In the presomitic mesoderm (PSM), certain developmental genes in Notch, Wnt, and FGF signaling pathways are expressed in cyclic waves, creating the segmentation clock. Concurrently, opposing gradients—high posterior FGF/Wnt signaling and anterior retinoic acid—define a moving wavefront. When clock oscillations reach the wavefront, a boundary is specified and separated to form a new somite; the clock then resets in newly formed posterior PSM.

Within a somite, progenitor cells differentiate into the sclerotome, which forms vertebrae and ribs, and the dermomyotome, which contributes to the dermis and skeletal muscles. This periodic segmentation underlies the segmented architecture of the vertebral column and associated musculature.

Disruptions of somitogenesis can lead to congenital segmentation defects, such as congenital scoliosis, rib anomalies, and vertebral malformations. Mutations affecting Notch, FGF, or Wnt signaling can disturb the clock or wavefront, giving rise to vertebral segmentation disorders such as spondylocostal dysostosis in humans.

Although conserved across vertebrates, the timing and number of somites vary between species, reflecting differences in developmental pace and axis elongation.