somites

Somites are blocks of paraxial mesoderm that segment along the head-to-tail axis of a developing vertebrate embryo. They form from the presomitic mesoderm as the embryo elongates, in a periodic process called somitogenesis that is synchronized with neurulation.

Formation is governed by a segmentation clock and a wavefront: cyclic expression of several signaling pathways, notably Notch, Wnt, and FGF, creates a clock that ticks as cells mature and form boundaries; a gradient of maturation factors moves anteriorly to interact with the clock, producing discrete somites. The gene MESP2 is a key regulator of boundary formation.

Each somite later differentiates into a sclerotome (which gives rise to vertebrae and rib cartilage) and a dermomyotome (which generates the dermis of the dorsal body and skeletal muscle precursors). The dermomyotome itself splits into the dermatome and myotome; myotomes form axial and limb muscles, while dermatomes contribute to the dermis of the back.

In the head region, precursors called somitomeres exist rather than full somites; somitogenesis in the trunk leads to a segmented axial skeleton in most vertebrates.

Disruptions in somitogenesis can cause vertebral and rib malformations and other congenital segmentation defects. Somite formation is a well-studied model of vertebrate development, illustrating how timing, signaling gradients, and genetic regulatory networks translate an elongated embryo into a segmented body plan.