The vertebra (plural: vertebrae) is the fundamental segmental unit of the vertebral column (also known as the spine).
Vertebrae, apart from those that are atypical, have a similar basic structure which can be described as an anterior vertebral body and a posterior neural (or vertebral) arch. These basic characteristics vary depending on the function of each individual vertebra.
The vertebral body is the large anterior cylindrical portion that is predominantly responsible for bearing the weight of the spine and body above it. The size of the vertebral bodies increases down the spine as the size and weight of the body it has to support above it increase. Each vertebra articulates with the vertebrae above and below it via an intervertebral disc.
The neural arch is comprised of the bone posterior to the vertebral body which has several individual components that are fused to form a ring (the vertebral foramen) that encloses the spinal canal. The components are:
- the pedicles which are short, thick bilateral processes that protrude posteriorly from the posterolateral corner of the vertebral body forming the lateral walls of the vertebral foramen. They join with the lateral aspects of the laminae posteriorly. The superior and inferior margins of each pedicle are slightly curved and form the vertebral notches, which when an adjacent vertebra is joined, create the intervertebral (or neural) foramina which transmit the nerve roots and associated vessels
- the laminae are bilateral, flattened plates that extend posteromedially from the posterior margin of the pedicles, to meet in the midline forming the posterior wall of the vertebral foramen
Seven processes arise from the neural arch, that provides articular support and attachments for ligaments and muscles:
- bilateral transverse processes project posterolaterally from where the pedicles and laminae fuse
- bilateral inferior articular processes are lined with synovial cartilage and project posteroinferiorly from where the pedicles and laminae fuse, medial to the base of the transverse processes. They contribute to the facet joint below
- bilateral superior articular processes are lined with synovial cartilage and project posterosuperiorly from where the pedicles and laminae fuse, medial to the base of the transverse processes. They contribute to the facet joint above
- a single spinous process project posteriorly from the laminae in the midline
- in the lumbar spine, a span of bone exists between the superior and inferior articular processes bilaterally, which is known as the pars interarticularis. In the axial plane, this lies between the pedicle and the lamina on each side.
- In the cervical spine, from C3-C7, bilateral uncinate processes are found along the lateral aspect of the superior vertebral bodies
There are 24 vertebrae stacked on top of each other separated by intervertebral discs. In addition, there are five sacral vertebrae which are fused into a single bone, the sacrum. Four small caudal rudimentary vertebrae fuse to form the coccyx. The vertebrae are grouped together based on similar osteology and regional relations which are dependent on function:
- cervical spine (7 vertebrae) are the most mobile
- thoracic spine (12 vertebrae) contribute to the thoracic cage
- lumbar spine (5 vertebrae) are the biggest and strongest, with the least movement
- sacrum (5 fused vertebrae)
- coccyx (4 fused rudimentary vertebrae)
Each vertebra forms in the embryo around the notochord from three primary ossification centers: an anterior midline centrum which forms most of the vertebral body and two bilateral posterolateral centers which form the two halves of the neural arch and the portion of the vertebral body posterior to the costal elements. The neurocentral joint is where these ossification centers meet in the posterior vertebral body. Occasionally the two neural arch centers do not fuse in the midline, which is known as an unfused spinous process. Individual primary centers start to ossify at different ages, but in general terms, ossification starts in the latter half of the embryonic period and the centers begin to fuse around the time of birth.
During puberty, five secondary ossification centers develop in each vertebra. The secondary centers on the tips of the transverse and spinous processes contribute to the length of these. In the developing vertebral body, two ring or annular epiphyses form, one above and one below the ossifying centrum. Failure of a ring epiphysis to unite may occur, known as a limbus vertebra. Again, individual secondary centers ossify at different ages, but in general terms, fusion is complete at 25 years old.
For more specific detail, see ossification centers of the vertebral column.