Paraxial mesoderm forms paired segments called somites that are located along the dorsal aspect of the human embryo. The somites in the cervical, thoracic and lumbosacral regions are all innervated by 31 pairs of segmental spinal nerves that arise from the ectoderm (neural tube and crest). When somites appear to 'migrate' during the process of differential growth, they take their nerve supply with them. Each innervated somite pair subdivides further into two parts: sclerotome and dermatomyotome. The fusion of two or more contiguous myotomes forms the axial and appendicular skeletal muscles, while individual dermatomes become part of the skin (dermis). Sclerotomes form the definitive axial skeleton.
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