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Somites are transient, segmentally organized mesodermal structures. In the vertebrate embryo, the somites contribute to multiple tissues, including the axial skeleton, skeletal and smooth muscles, dorsal dermis, tendons, ligaments, cartilage and adipose tissue. The somites also determine the migration paths of trunk neural crest cells and spinal nerve axons.
As the primitive streak regresses and the neural folds begin to gather at the center of the embryo, the paraxial mesoderm separates into blocks of cells called somites. These structures are formed by budding off as epithelial spheres from the cranial end of the unsegmented paraxial mesoderm that lies on either side of the neural tube.
Initially called somitomeres, they then mature into compact individual somites. The total number of somites formed is species-specific (38-39 in humans, 50 in chickens, 65 in mice) and is used as an indicator of embryonic developmental stages. Once formed, the epithelial somite is patterned rapidly into distinct compartments that subsequently give rise to distinct cell lineages. In response to signals from surrounding tissues, the ventral portion of the epithelial somite de-epithelializes to form the mesenchymal sclerotome, whereas the dorsal portion, or dermomyotome, remains an epithelial sheet. As the somite matures, cells delaminate from the dermomyotome edges and migrate underneath to form the myotome, a third compartment, which is located between the dermomyotome and sclerotome. The sclerotome is the source of the axial skeleton, the myotome contains skeletal muscle precursor cells, and the dermomyotome contributes to the dorsal dermis and skeletal muscle.
The paraxial mesoderm, epithelial somites and somatic compartments can be further subdivided into distinct medial and lateral components. The ventromedial sclerotome gives rise to the vertebral bodies, intervertebral discs, neural arches and proximal ribs; the dorsomedial sclerotome gives rise to the spinous process; and the ventrolateral sclerotome gives rise to the distal portion of the ribs—with possible distal rib contribution from the lateral dermomyotome. The behavior of the ventrolateral dermomyotome is dictated by the axial level.
At limb bud levels, ventrolateral edge dermomyotome cells delaminate and migrate into the lateral plate mesoderm where they differentiate to form limb and limb girdle muscles. At interlimb levels, cells at the ventrolateral edge or lip of the dermomyotome translocate underneath to produce the hypaxial myotome. The ventrolateral dermomyotome and myotome invade the lateral plate mesoderm together as a somitic bud, which gives rise to the body wall and abdominal muscle. The somites also house a subset of scleraxis-expressing cells, which constitute the tendon progenitor cells.