somite

A somite is a paired, segmentally arranged block of paraxial mesoderm that forms along the head-to-tail axis of a vertebrate embryo. Somites arise from the presomitic mesoderm in a process called somitogenesis, which is coordinated by a segmentation clock and a wavefront of signaling activity. In humans, about 40 to 44 somites form during early development, establishing the segmented pattern of the trunk.

Each somite differentiates into a ventromedial sclerotome and a dorsolateral dermomyotome. The sclerotome gives rise to the vertebral bodies and the ribs and contributes to surrounding cartilage and intervertebral structures. The dermomyotome splits into the dermatome, which forms the dorsal dermis, and the myotome, which generates skeletal muscle of the back, trunk, and limbs. Spinal nerves segmentally innervate each somite, reflecting the longitudinal organization of the axial skeleton and musculature.

Formation involves a genetic clock of oscillating gene expression interacting with signaling gradients from the tail bud. Notch, Wnt, and FGF pathways coordinate the timing and location of somite boundaries, with the anterior border forming as wavefront thresholds are reached. In many species, HOX gene expression along the axis provides regional identity to somites, contributing to cervical, thoracic, lumbar, and sacral differentiation.

Clinical relevance notes that defects in somite formation or patterning can lead to vertebral segmentation anomalies, such as congenital scoliosis or vertebral fusions, highlighting the role of somitogenesis in proper musculoskeletal and axial development.