The striking segmented pattern of the spine is established during embryogenesis when somites are rhythmically added to the posterior of the embryo. Somite formation begins around the third week when the embryo begins gastrulation and continues until all somites are formed.

Their number varies between species: there are 42 to 44 somites in the human embryo and around 52 in the chick embryo. The somites are spheres, formed from the paraxial mesoderm that lies at the sides of the neural tube and they contain the precursors of spinal bone, the vertebrae ribs and some of the skull, as well as muscle, ligaments and skin.

Somitogenesis and the subsequent distribution of somites is controlled by a clock and wavefront model acting in cells of the paraxial mesoderm.

Soon after their formation, sclerotomes , which give rise to some of the bone of the skull, the vertebrae and ribs, migrate, leaving the remainder of the somite now termed a dermamyotome behind.

This then splits to give the myotomes which will form the muscles and dermatomes which will form the skin of the back.

Sclerotomes become subdivided into an anterior and a posterior compartment. This subdivision plays a key role in the definitive patterning of vertebrae that form when the posterior part of one somite fuses to the anterior part of the consecutive somite during a process termed resegmentation.

Disruption of the somitogenesis process in humans results in diseases such as congenital scoliosis. So far, the human homologues of three genes associated to the mouse segmentation clock, MESP2, DLL3 and LFNG , have been shown to be mutated in cases of congenital scoliosis, suggesting that the mechanisms involved in vertebral segmentation are conserved across vertebrates.

In humans the first four somites are incorporated in the base of the occipital bone of the skull and the next 33 somites will form the vertebrae, ribs, muscles, ligaments and skin.

During the fourth week of embryogenesis , the sclerotomes shift their position to surround the spinal cord and the notochord.

This column of tissue has a segmented appearance, with alternating areas of dense and less dense areas. As the sclerotome develops, it condenses further eventually developing into the vertebral body.

Development of the appropriate shapes of the vertebral bodies is regulated by HOX genes. The less dense tissue that separates the sclerotome segments develop into the intervertebral discs.

The notochord disappears in the sclerotome vertebral body segments but persists in the region of the intervertebral discs as the nucleus pulposus.

The nucleus pulposus and the fibers of the anulus fibrosus make up the intervertebral disc. The primary curves thoracic and sacral curvatures form during fetal development.

The secondary curves develop after birth. The cervical curvature forms as a result of lifting the head and the lumbar curvature forms as a result of walking.

The vertebral column surrounds the spinal cord which travels within the spinal canal , formed from a central hole within each vertebra.

The spinal cord is part of the central nervous system that supplies nerves and receives information from the peripheral nervous system within the body.

The spinal cord consists of grey and white matter and a central cavity, the central canal. Adjacent to each vertebra emerge spinal nerves. The spinal nerves provide sympathetic nervous supply to the body, with nerves emerging forming the sympathetic trunk and the splanchnic nerves.

The spinal canal follows the different curves of the column; it is large and triangular in those parts of the column that enjoy the greatest freedom of movement, such as the cervical and lumbar regions, and is small and rounded in the thoracic region, where motion is more limited.

The spinal cord terminates in the conus medullaris and cauda equina. Spina bifida is a congenital disorder in which there is a defective closure of the vertebral arch.

Sometimes the spinal meninges and also the spinal cord can protrude through this, and this is called Spina bifida cystica.

Where the condition does not involve this protrusion it is known as Spina bifida occulta. Sometimes all of the vertebral arches may remain incomplete.

Spondylolisthesis is the forward displacement of a vertebra and retrolisthesis is a posterior displacement of one vertebral body with respect to the adjacent vertebra to a degree less than a dislocation.

Spondylolysis , also known as a pars defect, is a defect or fracture at the pars interarticularis of the vertebral arch. Spinal disc herniation , more commonly called a "slipped disc", is the result of a tear in the outer ring anulus fibrosus of the intervertebral disc , which lets some of the soft gel-like material, the nucleus pulposus , bulge out in a hernia.

Spinal stenosis is a narrowing of the spinal canal which can occur in any region of the spine though less commonly in the thoracic region. The stenosis can constrict the spinal canal giving rise to a neurological deficit.

Pain at the coccyx tailbone is known as coccydynia. Spinal cord injury is damage to the spinal cord that causes changes in its function, either temporary or permanent.

Spinal cord injuries can be divided into categories: complete transection, hemisection, central spinal cord lesions, posterior spinal cord lesions, and anterior spinal cord lesions.

Scalloping vertebrae is the increase in the concavity of the posterior vertebral body. Its concavity is due to the increased pressure exerting on the vertebrae due to a mass.

Internal spinal mass such as spinal astrocytoma, ependymoma, schwannoma, neurofibroma, and achondroplasia causes vertebrae scalloping.

Excessive or abnormal spinal curvature is classed as a spinal disease or dorsopathy and includes the following abnormal curvatures:.

Individual vertebrae of the human vertebral column can be felt and used as surface anatomy , with reference points are taken from the middle of the vertebral body.

This provides anatomical landmarks that can be used to guide procedures such as a lumbar puncture and also as vertical reference points to describe the locations of other parts of human anatomy, such as the positions of organs.

The general structure of vertebrae in other animals is largely the same as in humans. An arch extending from the top of the centrum is called a neural arch, while the haemal arch or chevron is found underneath the centrum in the caudal tail vertebrae of fish , most reptiles , some birds, some dinosaurs and some mammals with long tails.

The vertebral processes can either give the structure rigidity, help them articulate with ribs, or serve as muscle attachment points.

Common types are transverse process, diapophyses, parapophyses, and zygapophyses both the cranial zygapophyses and the caudal zygapophyses.

The centrum of the vertebra can be classified based on the fusion of its elements. In temnospondyls , bones such as the spinous process , the pleurocentrum and the intercentrum are separate ossifications.

Fused elements, however, classify a vertebra as having holospondyly. A vertebra can also be described in terms of the shape of the ends of the centrum.

Centra with flat ends are acoelous , like those in mammals. These flat ends of the centra are especially good at supporting and distributing compressive forces.

Amphicoelous vertebra have centra with both ends concave. This shape is common in fish, where most motion is limited. Amphicoelous centra often are integrated with a full notochord.

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Save Word. Definition of spine. Keep scrolling for more. Synonyms for spine Synonyms backbone , chine , spinal column , vertebral column Visit the Thesaurus for More.

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