Vertebra

A vertebra (plural: vertebrae) is an irregular bone, made of bone and some hyaline cartilage, that forms the vertebral column (spine) of vertebrates. The basic structure of a vertebra can vary, but it always includes a vertebral body (also called the centrum) that supports the weight of the column, and a posterior vertebral arch. This arch is built from two pedicles, two laminae, and seven processes. The vertebral body’s upper and lower surfaces attach to the intervertebral discs. The pedicles create notches that, when vertebrae connect, form the intervertebral foramina—openings that let spinal nerves enter and exit. The vertebral body and arch together enclose the vertebral foramen, the central canal that houses and protects the spinal cord. Vertebrae link together to give the spinal column both strength and flexibility, and the shape of their front and back surfaces determines how much movement is possible. Across vertebrate species, vertebrae are structurally similar, with the biggest differences seen between aquatic and other animals. Vertebrates get their name from these vertebrae.

In humans, the size of each vertebra changes depending on its position in the spine, as well as on factors like load, posture, and disease. The vertebral body is made of cancellous (spongy) bone, covered by a thin layer of compact (cortical) bone; the arch and processes have thicker compact bone. The flattened, rough upper and lower surfaces of the body—called vertebral endplates—attach to the intervertebral discs. These endplates are thickened layers of cancellous bone, with a denser top layer, and they help contain the discs, spread loads evenly, anchor disc collagen fibers, and act as a semi-permeable barrier for water and solute exchange. The vertebral arch consists of two pedicles (short, thick processes extending backward from the body) and two laminae (broad plates that project backward and inward from the pedicles to complete the arch and form the rear border of the vertebral foramen). The laminae’s rough upper surfaces attach to the ligamenta flava, which connect adjacent vertebrae from the second cervical vertebra downward. Above and below each pedicle are shallow vertebral notches; when vertebrae articulate, these notches align to form the intervertebral foramina, which allow spinal nerves and blood vessels to pass through.

Seven processes project from each vertebra: one spinous process, two transverse processes, and four articular processes. The spinous process extends backward and downward from the junction of the laminae, serving as an attachment point for muscles and ligaments. The two transverse processes project laterally from each side, where the lamina meets the pedicle, between the superior and inferior articular processes; they also attach muscles and ligaments, including the intertransverse ligaments. In thoracic vertebrae, each transverse process has a facet that articulates with the tubercle of a rib, and each side of the thoracic vertebral body has a facet for the head of a rib. In lumbar vertebrae, the transverse process is sometimes called the costal or costiform process because it corresponds to a rudimentary rib that does not develop in the lumbar region. The superior and inferior articular facet joints on each side of the vertebra limit the range of movement; these facets are connected by a thin part of the arch called the pars interarticularis.

Vertebrae are named after the region of the spine they occupy. In humans, there are typically thirty-three vertebrae: seven cervical, twelve thoracic, five lumbar, five fused sacral vertebrae (forming the sacrum), and four coccygeal vertebrae.