Paraxial mesoderm

The paraxial mesoderm arises from epiblast cells migrating via the primitive node and the cranial part of the primitive streak. It extends from the primitive streak to the prechordal plate and at first is unsegmented. It remains unsegmented cranially to the otic vesicle and it becomes completely segmented into somites caudally to the otic vesicle. The unsegmented part of the paraxial mesoderm (region of the somitomeres) contributes to the formation of the base of the skull and to formation of the striated muscles of the head. The transformation of paraxial mesoderm into somites provides the basis for the metameric subdivision or segmentation of the body.
The development of somites begins at S9. This occurs along a cranio-caudal gradient (4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4-5 coccygeal somite pairs). By S13, more than 30 pairs of somites are present. The somites are rectangular shaped, and consist of a pseudo-stratified epithelium with a central cavity (somitocoele). They have this form for a short time only. Cells migrate ventro-medially from the ventro-medial wall of the somites. They form the sclerotome and provide an essential contribution to the axial skeleton. The remaining dorso-lateral wall is transformed into the dermomyotome. The medial part of the dermomyotome gives rise to the myotome, and the lateral part to the dermatome. Striated musculature originates from the myotomes, and subcutaneous tissue from the dermatomes.