Somiten

Somiten (somites) are paired blocks of paraxial mesoderm that form along the developing embryo’s axis in vertebrates. They give rise to the axial skeleton (vertebrae and ribs), skeletal muscles of the trunk, and the dermis of the back.

Formation occurs through somitogenesis, a rhythmic, sequential process in which new somites bud off from the anterior border of the presomitic mesoderm. This segmentation is driven by a segmentation clock that produces oscillatory gene expression and a moving wavefront that defines where boundaries form, yielding a regular, cranial-to-caudal series of somites.

Each somite differentiates into three regions: the sclerotome, the dermomyotome, and the myotome. The sclerotome forms the vertebrae and portion of the ribs. The dermomyotome gives rise to the dermatome (dermis of the back) and the myotome (trunk muscles). This subdivision underlies the organization of the axial skeleton and skeletal musculature.

Molecular regulation involves Notch, Wnt, and FGF signaling. The segmentation clock and wavefront interact to establish somite boundaries. Key genes include those driving oscillatory expression (such as Hes/Lfng in some species) and developmental regulators like MESP1/2 that help specify boundary formation.

Clinical relevance is evident because abnormal somitogenesis can lead to vertebral segmentation defects, including congenital scoliosis and vertebral fusions. Somites are a central model in developmental and evolutionary biology, illustrating how a simple, repetitive unit can generate complex body plans.