ABSTRACT
Towards the end of the third week of gestation, the mesoderm around the neural tube and notochord breaks up into a series of segmental structures called somites. At the beginning of the fourth week, the cells of the ventral and medial wall of the somite change their appearance and start moving towards either side of the neural tube where they form the mesenchymal sclerotome. Towards the end of the fourth week, these start to fuse in a cephalocaudal direction to form primitive hemivertebrae which fuse across the midline at 6 weeks to form the vertebral bcxiies of the spine. The dorsolateral mesenchyme differentiates into a hemilamina and pedicle so that at birth, there are three ossification centres, one in each hemilamina and one in the vertebral body. These fuse at the end of the first year of life. Failures of development, segmentation, migration or fusion during this process may give rise to congenital anomalies.
