Fig. 1

a-d: Embryology of vertebral bodies and intervertebral discs: Illustration (1–6) demonstrating the steps in development of vertebral bodies from sclerotomes. The sclerotomes migrate around the neural tube and the notochord forming the vertebral bodies, arches, transverse and spinous processes. Chondrification centres appear during the 6th week of development within a mesenchymal template, which enclose the notochord and developing neural tube. The notochord expands and develops into the nucleus pulposus of intervertebral disks. The six stages of vertebral development include: (1) gastrulation and formation of the somitic mesoderm and notochord, (2) condensation of the somitic mesoderm into somites, (3) formation of dermomyotomes and sclerotomes, (4) formation of membranous somites and re-segmentation with definitive vertebral formation, (5) vertebral chondrification and (6) vertebral ossification. b : Ossification of subaxial cervical, thoracic and lumbar vertebrae: This follows chondrification and typically occurs after 9 weeks of gestation. One dorsal and one ventral primary ossification centre forms for each vertebral body; these unite to create a centrum, which develops into three primary loci at the conclusion of embryonic development. One of these loci eventually develops into the vertebral body while the other two develop into one half of the eventual vertebral arch. c: Development of C1 (atlas): The atlas develops from three centres of ossification separated by cartilaginous synchondroses—one for body and two for lateral masses. The sequence of appearance and fusion of the ossification centres as noted in the illustration. d: Development of the C2 (axis): Six centres of ossification separated by four cartilaginous synchondroses. Sequence of appearance and fusion of the ossification centres as noted in the illustration