The skeleton is of mesenchymal origin and the most of its parts ass through, first a membranous and then a cartilaginous stage before they become ossified.
The vertebral column is derived from the slcerotomes of somites. Each slcerotome divides into three parts-cranial, middle and caudal. A vertebra is formed by fusion of caudal part of one sclerotome and the cranial part of next sclerotome. Therefore it is intersegmental in position. The middle part of the sclerotome forms an intervertebral disc which is therefore segmental in position.
The calls of each sclerotome get converted into loose mesenchyme. This mesenchyme migrates medially and surrounds the notochord. The mesenchyme then extends backwards on either side of the neural tube and surrounds it. Extensions of this mesenchyme also take place laterally in the position to be subsequently occupied by the transverse processes and ventrally in the body wall.
The mesenchyme derived from each somite can be seen as distinct segment. The mesenchymal cells of each segment are at first uniformly distributed. However, the cells become condensed in a region that runs transversely across the middle of the segment. This condensed region is called the perichordal disc. Above and below it there are less condensed parts. The body (centrum) of each vertebra is formed by the fusion of the adjoining, less condensed parts of two segments. The perichordal disc becomes the intervertebral disc.
The neural arch, the transverse processes and the costal elements are formed in the same way as the body. The interspinous and intertransverse ligaments are formed in the same manner as the intervertebral disc. The notochord disappears in the region of the vertebral bodies. In the region of the intervertebral discs the notochord becomes expanded and forms the nucleus pulposus.
A typical vertebra is ossified from three primary centres,One in each half of vertebral arch and one in the centrum (body). Centre in arches appear at the roots of the transverse process, ossification spreading backwards into laminae and spines, forwards into pedicels and posterolateral parts of the body. Laterally into pedicals and posterolateral parts of body.Laterally into transverse process and upward and downwards into articular processes. Centres in vertebral arches are said to appear first in upper cervical vertebrae in the ninth to tenth week and then in successively lower vertebrae, reaching lower lumbar levels in the twelfth week. The body’s major part, the centrum, ossifies from a primary centre dorsal to the notochord. During early post natal years the centrum is connected to each half neural arch by neurocentral joint. In thoracic vertebrae costal facets on bodies are posterior to neurocentral joints.
Until puberty the upper and lower surfaces of bodies and apices of transverse and spinous processes are cartilaginous, then five secondary centres appears, one in the apex of each transverse and spinous process and two annular epiphyseal righs for circumferential parts of upper and lower surfaces of body.
Exceptions to this pattern of ossification occur in the first, second and seventh cervical canal and in the lumbar vertebrae.
Lumbar vertebrae have two additional centres for mamillary processes. In the fifth lumbar pair of scale like epiphysis usually appear on the tips of costal elements.