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Gross Anatomy Notes - The Spine Matthew Wood's Lecture Notes E-learning -
The-Spine.html Textbooks - Essential Clinical Anatomy, Oxford Handbook of Medical Sciences, McMinn's Atlas of Human Anatomy Development of the Spine

* Vertebral column is formed from the paraxial mesoderm - the SOMITES.

* These segmented structures give rise to three key cell groups during development; the sclerotome (which will give rise to the bones of the vertebral column), the myotome (goes on to forms muscles) and the dermatome (goes on to form aspects of the dermis).

* Somites are particularly important in allowing the spinal nerves to bud off from the spinal cord in an organised fashion. The nerves are only able to pass through certain aspects of the somites. Axons are channelled through the ROSTRAL (anterior) parts of the somites; the caudal parts are inhibitory, as shown by the immunocytochemical slide on the right. This key inhibition ensures that mixed spinal nerves emerge from the column in a segmented fashion.

* Each individual vertebra forms from the caudal and cranial halves of two adjacent sclerotomes. The failure of some sclerotome cells to migrate can lead to lateral scoliosis, due to the formation of hemivertebrae.

* In the primitive spinal column, a fissure develops between each sclerotome; this will be the location for the intervertebral discs.

* The intervertebral discs are composed of two elements; the cartilaginous annulus fibrosus, which forms a tough coat around the gel-like nucleus pulposus. The latter is the only structure in the adult body which is directly derived from axial mesoderm - the NOTOCHORD. Clinical correlate - scoliosis Very common spinal defect, caused by the formation of unstable hemivertebrae. Normally, two centres fuse to form each vertebral body; a failure of sclerotome cells to accrue and develop properly on one side causes this abnormality, which worsens with weight-bearing and further development. The hemivertebrae cannot support the weight above them in the upright position, causing lateral curvature of the spine.

The adult spinal regions show distinct curvature. There are two different ways in which the spine is curved:

* Lordosis - the region shows posterior concavity - i.e. the cervical, lumbar and sacral regions.

* Kyphosis - the region shows posterior convexity - i.e. the thoracic (and coccygeal) region. In newborns, however, typical spinal curvature is rather different. The neonate spine shows kyphosis ONLY. Note that there are several different clinical abnormalities related to the curvature of the spine. Hunchbacks display excessive kyphosis, whilst swaybacks are overly lordotic. The spine should never have lateral curvature. Those who suffer from scoliosis have this lateral abnormality which can have serious complications. Spinal structure

* Complex column that is made up of muscles, ligaments and synovial joints.

* Two primary functions. It protects the delicate spinal cord from damage and it provides FLEXIBLE support for the head and the trunk.

* Each vertebra has two key components - a body (sits anteriorly and is the weight-bearing vertebral structure) and an arch (sits posteriorly and is the site of attachment for most ligaments, muscles and articulating bones). Transverse and spinous processes are also observed in almost all vertebrae. Vertebrae

* Vertebral column consists of THIRTY THREE different vertebrae.

* Highly complex developmental patterning and communication needed to arrange these discrete vertebrae in their correct columnar orientation - why not just opt for a single long bone that could serve the same primary function (protection of the spinal cord)?

* Many vertebrae allow a great deal of movement which can occur in different directions. Protection allied with motility is highly advantageous.

* Seven cervical vertebrae lie at the top of the vertebral column. C1 and C2 (atlas and the axis) are highly specialised for their functions. Cervical vertebrae are capable of flexion, extension and some lateral rotation.

* The thoracic region of the spinal cord consists of TWELVE vertebrae. Their structures allow rotation only.

* The five lumbar vertebrae are largely weight-bearing, which explains their relative thickness. They allow some lateral rotation, as well as flexion and extension.

* The sacral vertebrae lie below the lumbar region. The sacrum is formed by the fusion of the five sacral vertebrae. Note that there are four anterior and four posterior foramina in the sacrum which allow spinal nerves to access the legs.

* The coccyx, meanwhile, is a vestigial "tail-bone" that is composed of 3-5 fused vertebrae. These vertebrae have no bodies and no functions.

* All of the vertebrae, no matter where they are located in the column, share common features. Their subtle structural variations, however, is what allows them to carry out their discrete and important functions. Cervical vertebrae

* There are seven different cervical vertebrae, which are found at the top of the spinal column. However, they are often subdivided into C1 and C2 and C3-7. This is due to the fact that the first two cervical vertebrae, the atlas and the axis, have very specialised roles and atypical structures.

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