From somites to vertebral column.

We report on the development and differentiation of the somites with respect to vertebral column formation in avian and human embryos. The somites, which are made up of different compartments, establish a segmental pattern which becomes transferred to adjacent structures such as the peripheral nervous system and the vascular system. Each vertebra arises from three sclerotomic areas. The paired lateral ones give rise to the neural arches, the ribs and the pedicles of vertebrae, whereas the vertebral body and the intervening disc develop from the axially-located mesenchyme. The neural arches originate from the caudal half of one somite, whereas the vertebral body is made up of the adjacent parts of two somites. Interactions between notochord and axial mesenchyme are a prerequisite for the normal development of vertebral bodies and intervening discs. The neural arches form a frame for the neural tube and spinal ganglia. The boundary between head and vertebral column is located between the 5th and 6th somites. In the human embryo, proatlas, body of the atlas segment, and body of the axis fuse to form the axis.